Physiologic
Regulation
of Body Energy Storage
Grover C. Pitts Both new and published data (rats, mice, and human beings) on three parametersfat mass, fat-free body mass (FFBM), and total body mass in some cases-are evaluated. Steady state values of the parameten are analyzed for changes in response to specific perturbing agents and for their frequency distributions. Temporal sequences of values on individuals are ex-
amined for evidence of regulatory responses. The results lead to the hypothesis that the FFBM is regulated, but probably not as a unit, and that mass of fat is regulated with a high priority near the range extremes but with a much lower priority in the mid-range. Properties and advantages of such a mechanism are discussed.
T
HE POSSIBILITY that body energy storage is regulated has been in contention for a number of years. Kennedy’ proposed that food intake in the rat is controlled to maintain near-constancy of body lipid stores. Liebelt et al.* reported apparent regulation of fat stores in the mouse. Passmore concluded from general considerations that body weight must be regulated in man. By contrast, the participants in a symposium on energy balance4 could not agree on whether such regulation occurs at all in man. Occasionally, investigators assume the reality of regulation and refer to the “set-point” of body weight or body fat content. Garrow’ (p 211) reviewed the evidence for and against a setpoint mechanism and proposed a “buffer” mechanism as a substitute. Many other authors have discussed such regulation tangentially, but convincing support for it has never been marshaled for any species. In short, the absence of a consensus probably reflects the inadequacies of existing data. In the approach employed here we have emphasized two factors: (1) The specific body component that most directly reflects energy balance, stored fat,* has been evaluated separately, if possible, by body composition methods that separate fat mass from the FFBM. (2) The evaluation of two classes of data: steady state values from the literature and temporal sequences of values obtained on individuals. The steady state values were obtained mostly by post mortem analyses, and were analyzed statistically for their sensitivity to specific perturbing agents and for their frequency distributions. The results of this comprehensive approach suggest that body energy storage is regulated, but with some differences from the well-known systems of regulation.
*We define “fat” operationally as that which is extracted by petroleum ether, the method employed by most of the investigators referenced here. The material so obtained is a mixture in which triglycerides predominate. That which remains is our fat-free body mass. From the Department of Physiology, School of Medicine, University of Virginia, Charlottesville. Va. Receivedfor publication June 20. 1977. Supported in part by NASA research grants NGR47-005-213 and NSG 2225. Address reprint requests to Grover C. Pitts, Department of Physiology, School of Medicine. University of Virginia, Charlottesville, Va. 22901. o 1978 by Grune & Stratton, Inc. 0026-0495/78/2704-0008$02.00/O
Metabolism, Vol. 27, No. 4 (April), 1978
469
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GROVER
MATERIALS
C. PITTS
AND METHODS
Many physiologic parameters are controlled by the organism. That is. they are adjusted on a continuing basis to any level within the limits of viability as needed to serve the immediate economy of the body. Other regulated parameters are normally maintained within a narrow optimal range.6 A regulated parameter is typically coupled with one or more controlled parameters (effecters).’ Thus, if body energy storage is regulated, it must result from appropriate adjustments in the effecters’ food consumption and energy expenditure. Failure to make the distinction between regulation and control continues to cause confusion in the literature. The steady state values available for analysis were largely sample means for body fat content and FFBM reported in the literature and obtained with standard body composition techniques by indirect in vivo methods on man and direct terminal methods on other species. Because data on individuals were available in only one study, the sample means were treated as data units. In a few cases we derived the desired data from those published by very simple calculations. The eligibility of published steady state values for inclusion in our analyses was established with the following criteria: (I) Each sample had to be of adults to rule out growth as a variable. We have defined as adults laboratory rats 24+ wk old, laboratory mice 12+ wk. human beings 18+ yr. and wild mammals meeting the published weight and dimension descriptions for adults of each species. (2) Each sample must have been maintained ad lib. on a nutritionally balanced diet. (3) Each sample had to be intact prior to sacrifice. The sole exception was the use (Fig. 2) 01 rats with hypothalamic lesions. (4) The duration of the specific regimens had to insure that the values quoted represented steady states rather than transients. (5) Each sample had to provide values for both fat mass and FFBM. A comparison of the frequency distributions of these two components was desired and this precaution insured that the two data sets represented the same constellation of known and unknown variables. Data from 25 published papers plus two unpublished ones from this laboratory qualified. These 27 papers provided the II4 separate statistical samples that are plotted one or more times in Figures I. 2, and 3. Of these, 53 were samples of rats,‘-‘* 28 of men and women.” ‘9 I7 of mice.n.30
60
z Y
0
40
E
4 J 0
20
E
/
ORAL TEMP.
1 1 ,
0 0
(F")
zoo 400 SAMPLE MEAN (g)
Fig. 1. (A) Frequency distribution of statistical samples of rats, the sexes plotted separately. Both fat and FFBM are expressed in mass units. Each point represents the percentage frequency of occurrence within an abscissa band 40 g wide. Rats that had genetic or experimental obesity were omitted. (B) The frequency distribution of FFBM of male rats (from 1A) compared with that for resting systolic blood pressure of men 20-24 yr of age and resting oral temperature of men 20-25 yr of age. The respective abscissae are stacked below the figure. The common ordinate has the same units as in I A.
BODY ENERGY
STORAGE
-30
REGULATION
471
? /’
4
RATS MAN M,CE
I
(N=52) (N=28) (NZ,7,
.------
1 60
0
MEAN
S&E
BOd°FAT
(%)
Fig. 2. The frequency distribution (by species) of mean percentage body fat in statistical samples of rats, mice, and human beings. Males and females are combined in the curve for each species. Each point represents the percentage of samples occurring within an abscissa band 4% wide. The abscissa locations of samples with experimental or genetic obesity are indicated at upper right. Hypothal, rats with hypothalamic obesity; ob ob, hyperglycemic mice; GTG, mice with gold thioglucose obesity.
50 A-WILD
LABORATORY
zmB E h
SPECIES
B----.INBRED, II?
RATS
CONTROLLED
: ENVIRONMENT
C-RANDOM-BRED,
CHOW
D---RANDOM-BRED,
HIGH FAT DIET
(UNPUB.)
DlEl
30 -
5 0 W 30h L
10 -
‘_-I
j
1
Ir---_____l ?_l
I
oMEAN
BODY
FAT (%I
Fig. 3. A comparison of the frequency distributions of body fat in four subpopulations. The bar for subpopulation B is based on the frequency distribution of individuals. The other three bars are based on the frequency distribution of sample means. Ai3’ B (unpub.); C7-“~3’~3z (and unpub.); D.6.&9.ll-tW~
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C. PITTS
(and unpublished) and 16 of wild mammals.31 Sample size varied from 3 to 98 with most failing in the 5 to 10 range. The data obtained with the above criteria had been tested by the respective investigators for statistical significance of the difference between the perturbed mean and the control mean. These results were used in our evaluation of the effects of specific perturbing agents. The wild mammal subpopulation plotted in Figure 3 represents a special set of steady state values. With the exception of the shrews, the other 15 species (mostly rodents and squirrels) are plant eaters and were collected during the season when their natural foods occurred in greatest abundance. Thus, their dietary regimens may reasonably be regarded as ad lib. They were taken with spring traps that killed at capture. Post mortem examinations revealed lean individuals but with no suggestion of emaciation. Our frequency distribution criterion of regulation was based upon the statistical distribution of multiple measurements of a single parameter under a variety of circumstances. If the parameter is perturbed by various physiologic or environmental factors during the measurements (e.g., the rate of total body thermogenesis), the frequency distribution is likely to be scattered with little central tendency. But if regulatory mechanisms oppose perturbation and confine the parametric value within an optimal range (e.g., deep body temperature), a narrow distribution may nevertheless result. While the individual samples used are homogeneous with respect to the experimental variables of interest to the particular investigator, collectively they represent a great variety of controlled and uncontrolled variables (diet, amount and kind of exercise, investigator, location of laboratory, etc.). In the absence of a quantitative evaluation applicable to the diverse circumstances encountered, we have visually judged the position of the distributions on a continuous scale ranging from a narrow symmetrical (“bell-shaped”) distribution to one that is broadly scattered with no discernible peak. This method has been found useful in two similar studies.32’33 The methods employed in the temporal sequences of values on individuals are usually selfexplanatory and have been presented in the source references. RESULTS Among the available data on specific perturbing agents, factors that decreased body fatness include forced exercise,y,‘2,34 voluntary exercise (unpublished), force-feeding,35,36 surgical stress3’ and chronic acceleration.‘5,‘6 Factors producing consistent and predictable fat changes but which cannot be collectively characterized in a few words include age,34s38 circannual rhythms,3y,40 changes in daily 1ight:dark ratio,j’ environmental temperature,4’ dietary composition,‘3~‘4 apparent palatability of nutritionally complete diets,424 and social factors.45.46 Apparently, few perturbations cause an increase in body fatness, but one that does is ad lib consumption of high-fat diets.“.14 Thus, body fatness appears to be highly labile and moves to a new steady state in response to nearly any change in regimen. With respect to the FFBM, the following factors have been studied: forced exercise,34q36 ad lib exercise (unpublished), force feeding,36 circannual rhythms40 daily 1ight:dark ratio, 37 chronic acceleration ‘5~‘6and diet.13 Only chronic acceleration was shown to alter FFBM in the adult, the others being without effect. Chronic acceleration has undergone no physiologically significant change during the period in which life evolved and hence there would be no opportunity, or selective pressure, to evolve a system for defense against changes in it. The implication is that acceleration alters FFBM because it is not defended by a regulatory system, whereas the other perturbing factors have no effect because regulatory defenses exist. To apply our frequency distribution criterion of regulation we have plotted the available sample means in Figures 1, 2, and 3. Figure 1A shows the frequency distribution of sample means for FFBM and fat mass in our predom-
BODY ENERGY
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inant species, the rat. The curves for fat have maxima at approximately 60 g, but 12% of the values in each sex occur far to the right. By contrast, the distributions for FFBM are much narrower and less skewed. In Figure lB, the distribution of the male FFBM from Figure 1A is placed on an ordinate common with distributions for two parameters generally regarded as regulated-resting systolic blood pressure in young men 20-24 yr old4’ and resting oral temperature in medical students 20-25 yr old.48 In comparing the curves, two things should be kept in mind. First, the curves for blood pressure and oral temperature represent the distribution of individual values, whereas the FFBM curve represents the distribution of sample means. Second, the blood pressures and oral temperatures were measured under standardized conditions, whereas the sample means for FFBM represent wide variations in experimental circumstances. Uncertainty about how to quantitatively compare the respective abscissae restricts us to a comparison of the general shapes. However, this suggests that regulation may play a role in the distribution of rat FFBM, but there is considerable room for doubt in the case of body fat. The ordinate and abscissa units in Figure 2 are dimensionless and theoretically equally applicable to either sex and to a variety of species, within limits. In the figure, we have combined the sexes and represented the three species separately. The distributions are very broad and strongly skewed to the right. However, the peaks suggest that each species has a “preferred” level of fatness roughly in the lo%-20% range. This plot suggests, more clearly than in Figure 1, a deviation from what is expected of regulated parameters. In the figure, note that all samples with experimental or genetic obesity are located above 40% fatness. However, intact apparently normal rats and human beings occur in the same high range, suggesting that hypothalamic lesions do not boost animals to levels of fatness above what is physiologically available to most individuals. This interpretation is in accord with the findings of Mu et a1.42 but not with Keesey and Powley. 43 The only mice above 40% fatness were those with gold thioglucose obesity or the obob gene. Whether normal mice might range this high would require a larger sample for a satisfactory answer. In Figure 3 are plotted frequency distributions in percentage body fat for four subsamples as identified there. The overlapping abscissa ranges extend from nearly 0’?-56% fatness. This figure demonstrates that subpopulations individually may show relatively narrow frequency distributions, but collectively suggest a broad continuum. Figure 3 also provides an opportunity to compare the frequency distribution of individual rats (subsample B) with that of sample means for rats on the same diet (subsample C). The general similarity of the two distributions suggests that the comparison of a distribution of means with two distributions of individuals made in Figure 1B is reasonable. The data available as temporal sequences of values on individuals present a special problem. The parameter measured in nearly every case is live mass, and we must infer the extent of separate participation by fat and FFBM. The above observed lability of body fat mass and stability of FFBM and the results of other studies’4*‘6,34 allow the rather secure inference that in an adult on a generally ad lib. regimen with a nutritionally complete diet, changes in mass of stored fat can almost fully account for observed changes in live mass. This inference has also been verified for rats with hypothalamic lesions.43
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PITTS
In these temporal sequences, live mass may show a progressive and prolonged change. Where such changes are rectilinear, the absence of regulatory adjustments is obvious. For example, in a laboratory study by Spiegel of human feeding responses,49 6 of 15 subjects were classified as nonregulators of body mass, and the nonregulator presented in detail (Spiegel’s Figure 2) showed a rectilinear loss of - 7% of his total mass over a 2 l-day period. Figure 4 presents previously unpublished data on one rat. This individual, always appearing healthy and well-groomed, underwent a rectilinear loss of - 25% of her mass in 10 days. A closely similar response has been reported by Cohn and Josephj5 (their Fig. 6). A further observation is that if such rectilinear changes go far enough, they eventually evoke a regulatory response with an apparently exponential recovery curve resembling the “homing” typical of recovery from interrupted growth.” Upon reaching a mass of 190 g, the animal in Figure 4 abruptly began eating. and its mass homed on a growth curve in the 250-300 g range. Postmortem analysis revealed a mid-range body fat value of 7.4%. The rat observed by Cohn and Joseph35 showed a closely similar response. At sacrifice it had 8.3q, body fat. In each of these cases, it is difficult to escape the conclusion that the level of body mass (i.e., body fat) eventually established had a low priority, since each rat passed completely through the regulatory range and belatedly returned to it. Conceivably, the rarity in the literature of such findings could represent a failure of reporting. An atypical response of the type presented in Figure 4 might often be rejected as the result of intervention by an unknown and uncontrolled variable, for example, a systemic infection. In contrast with the seemingly casual regulation described, rats with hypothalamic lesions, which produce lean, hypophagic43 or fat hyperphagic animals,” show vigorous regulation of body mass (i.e., body fat).43 The common feature of these two groups is probably that altered appetite or satiety has changed the body fatness to higher or lower levels where regulatory defense against even more extreme deviations has an enhanced priority. Numerous investigators have monitored effector mechanisms potentially involved in the regulation of energy storage. Food consumption, when averaged for periods of a week or more, is usually altered in a direction appropriate to
Fig. 4.
Body mass,
tion
on
tary
running
rat,
the
one wheel
female in
activity,
rat.
Activity
a vertical
first
becoming
wheel
and food consumpmode
was
propelled
available
volunby
on day
the 0.
BODY ENERGY
STORAGE
475
REGULATION
the maintenance of energy balance. 5 Several studies now support the conclusions that prolonged hyperphagia or hypophagia are associated with an involuntary increase or decrease, respectively, of energy dissipated as heat.5152-54In general, these two effecters behave as though they are participating in a system with negative feedback, but one cannot tell whether they subserve regulation of body fat or FFBM or both. DISCUSSION
Any hypothesis for the regulation of energy storage must explain these findings: (1) The FFBM is quite stable while fat mass is altered by all classes of perturbations that have been tested. (2) The frequency distribution of steady state values for FFBM is typical of regulated parameters while that for fat mass is not. (3) The two effector mechanisms, food consumption and involuntary dissipation of energy as heat, respond as though they are participating in a negative feedback system directed at energy storage. (4) Animals with hypothalamic lesions, where a change in hunger or satiety pushes the individual against one or the other extreme level of fatness, show vigorous regulation of fatness. Intact animals in the mid-range of fatness occasionally show progressive rectilinear mass changes, continuing over extended periods, that appear unopposed by regulatory processes. Points 1,2, and 3 suggest that FFBM is regulated, 1 and 2 that fat mass is not regulated, and 4 that fat mass is regulated but with a priority that changes in different ranges of fatness. The above observations lead us to the following hypothesis of energy storage regulation. The fat-free compartment, included in the hypothesis because it represents stored energy that becomes available during advanced inanition, is quite constant in the absence of growth. But since regulation of FFBM as a unit is improbable, and perhaps disadvantageous, because of its heterogeneity, its major components (muscle, bone, etc.) are probably regulated individually, and the observed near-constancy of the whole is the result of these several interacting systems of regulation. The total body fat mass is regulated, but with a changing priority that is a function of the actual level of fatness. In the mid-range of fatness, the priority is very low and the regulatory mechanisms may be readily overruled by a variety of competing factors. However, the priority for regulation becomes higher as extreme values of fatness are approached. The most clearly defined difference between the suggested regulation of fat mass and the known regulation of other parameters is in the breadth of the optimal range. This is not a qualitative difference, since regulation in general is probably directed at optimal ranges rather than specific optimal values. Various changes, some regulated and some not, occur within the broad optimal range of body fatness and account for the observed lability in body fatness. They further substantiate Adolph’s warning55 that the living organism is characterized by complex interactions based on established priorities between multiple systems of regulation. A low priority, such as we would assign to maintaining fatness constant so long as it is within the optimal range, means that the system would probably yield at nearly every conflict with another system.
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Broad-optimum regulation has substantial advantages. For example, ageassociated changes in fatness do not require a regulatory explanation so long as they do not approach extreme values; mechanisms exerting independent control may make appropriate adjustments in body protein unimof protein intake56,57 paired by resulting changes in fat mass; increases in fatness in preparation for hibernation, migration, etc., will not encounter insurmountable interference from the fat regulatory system. Garrow’s “buffer” system for control of fatness’ involves no feed-back loop and minimizes changes without correcting them. In support, human data are cited,5 and it is apparently intended to apply only to man. We doubt that human fat regulation differs, in principle, from that of most mammals, and the evidence presented above for the existence of feedback loops and for corrections made in altered fatness (particularly in rats with hypothalamic lesions) explains our preference for the present hypothesis. Energy balance probably comprises several distinguishable processes, including: control of the release of chemical energy so that it matches physical work; control of the proportionate energy contributions from endogenous stores of glycogen, fat, and protein; adjustment of energy metabolism to changes in physiologic condition (e.g., growth, gestation, lactation, egg-laying, inanition) as well as regulation of the mass of body fat. It appears unlikely that a single system could control or regulate all of these. Other regulatory systems involved could operate by principles different from any proposed to date. Broad-optimum regulation does not constitute a unique explanation of reported findings. However, it provides a satisfactory (and hopefully a heuristic) framework for the relevant data as viewed from a new perspective. REFERENCES I. Kennedy GC: The role of depot fat in the hypothalamic control of food intake in the rat. Proc R Sot Lond (Biol) 140:578-592, 1953 2. Liebelt RA, Vismara L, Liebelt AG: Autoregulation of adipose tissue mass in the mouse. Proc Sot Exper Biol Med 127:458-462, 1968 3. Passmore R: The regulation of body weight in man. Proc Nutr Sot 30:122-127, 1971 4. Vigy M, Apfelbaum M, Miller DS, et al (eds): Energy Balance in Man. General Discussion. Paris, Masson et Cie, 1973, p 346 5. Garrow JS: Energy Balance and Obesity in Man. New York, American Elsevier, 1974 6. Brobeck JR: Exchange, control and regulation, in Yamamoto WS, Brobeck JR (eds): Physiological Controls and Regulators. Philadelphia, W.B. Saunders, 1965, p 1 7. Babirak SP, Dowell RT, Oscai LB: Total fasting and total fasting plus exercise: Effects on body composition of the rat. J Nutr 104:452457. 1974 8. Bates MW, Nauss SF. Hagman NC, et al: Fat metabolism in three forms of experimental obesity. Am J Physiol 180:301-303, 1955
9. Hanson DL, Lorenzen JA, Morris AE. et al: Effects of fat intake and exercise on serum cholesterol and body composition of rats. Am J Physiol213:3477353, 1967 IO. Lemonnier D: Experimental obesity in the rat by high-fat diets, in Vague J (ed): Physiopathology of Adipose Tissue. Amsterdam, Excerpta Medica Foundation, 1969, p 197 Il. Montemurro DG, Stevenson JAF: Survival and body composition of normal and hypothalamic obese rats in acute starvation. Am J Physiol 198:757-761, 1960 12. Oscai LB, Holloszy JO: Effects of weight changes produced by exercise, food restriction. or overeating on body composition. J Clin Invest 48:2124-2128, 1969 13. Peckham SC, Entenman C, Carroll HW: The influence of a hypercaloric diet on gross body and adipose tissue composition in the rat. J Nutr 77:187-197, 1962 14. Pitts GC, Bull LS: Exercise, dietary obesity and growth in the rat. Am J Physiol 232: R38-R44, 1977
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15. Pitts GC, Bull LS, Oyama J: Effect of chronic centrifugation on body composition in the rat. Am J Physiol223:1044~1048, 1972 16. Pitts GC, Bull LS, Oyama J: Regulation of body mass in rats exposed to chronic acceleration. Am J Physiol228:714-717, 1975 17. Schemmel R, Mickelsen 0, Fisher L: Body composition and fat depot weights of rats as influenced by ration fed dams during lactation and that fed rats after weaning. J Nutr 103: 4777487, 1973 18. Schemmel R, Michelsen 0, Tolgay Z: Dietary obesity in rats: Influence of diet, weight, age and sex on body composition. Am J Physiol 216:373-379, 1969 19. Barnard DL, Ford J, Garnett ES, et al: Changes in body composition produced by prolonged total starvation and refeeding. Metabolism 18:564-569, 1969 20. Bernstein LM. Johnston LC, Ryan R, et al: Body composition as related to heat regulation in women. J Appl Physiol 9:241-256, 1956 21. Baling EA. Taylor WL, Entenman C, et al: Total exchangeable potassium and chloride and total body water in healthy men of varying water and fat content. US Nav Radio1 Def Lab Tech Rep 313, 1959 22. Brockett JE, Brophy EM, Koniski F. et al: Influence of body size, body fat, nutrient intake and physical fitness on the energy expenditure of walking. US Army Med Nutr Lab Rep No 177. 1956 23. Broiek J: Changes of body composition in man during maturity and their nutritional implications. Fed Proc 11:784-793, 1952 24. McMurrey JD, Baling EA. Davis JM, et al: Body composition: Simultaneous determination of several aspects by the dilution principle. Metabolism 7:651-667, 1958 25. Osserman EF, Pitts GC, Welham WC, et al: In vivo measurements of body fat and body water in a group of normal men. J Appl Physiol 2:633-639, 1950 26. Paiizkova J, Eiselt E: A further study on changes in somatic characteristics and body composition of old men followed longitudinally for 8-10 years. Human Biol43:318-326, 1971 27. von Dobeln W: Human standard and maximal metabolic rate in relation to fatfree body mass. Acta Physiol Stand 37 (Suppl 126). 1956 28. Welch BE, Riendeau RP, Crisp CE, et al: Relationship of maximal oxygen consumption to various components of body composition. J Appl Physiol 12:395-398, 1958 29. Young CM, Blondin J, Tensuan R. et al:
477
Body composition studies of “older” women, thirty to seventy years of age. Ann NY Acad Sci 110:589~607, 1963 30. Keil LC: Changes in growth and body composition of mice exposed to chronic centrifugation. Growth 33:83-88, 1969 3 1. Pitts GC, Bullard TR: Some interspecific aspects of body composition in mammals, in Body Composition In Animals And Man, Nat1 Acad Sci Pub1 1598, 1968, p 45-70 32. Gasnier A, Mayer A: Recherches sur la regulation de la nutrition. Ann Physiol Physicochim Biol 15:145-214, 1939 33. Schmidt-Nielsen K, Dawson WR: Terrestrial animals in dry heat: Desert reptiles, in Handbook of Physiology. Adaptation to the Environment (section 4). Washington, D. C., American Physiological Society, 1964, p 467480 34. Pitts GC. Bull LS, Hollifield G: Physiologic changes in composition and mass of total body adipose tissue. Am J Physiol 22 1:961-966, 1971 35. Cohn C, Joseph D: Influence of body weight and body fat on appetite of “normal” lean and obese rats. Yale J Biol Med 34:598607, 1962 36. Pitts GC, Bull LS, Wakefield JA: Exercise with force feeding in the rat. Am J Physiol 2271341-344, 1974 37. Pitts GC, Bullard TR, Tremor JR, et al: Rat body composition: Sensor implantation and lighting effects. Aerospace Med 40:4 17-420, 1969 38. Keys A, Broiek J: Body fat in adult man. Physiol Rev 33:2455325, 1953 39. Odum EP. Connell CE: Lipid levels in migrating birds. Science 123:892-894. 1956 40. Odum EP, Rogers DT. Hicks DL: Homeostasis of the nonfat components of migrating birds, Science 143:1037-1039, 1964 41. Barnett SA, Widdowson EM: Organweights and body-composition in mice bred for many generations at -3°C. Proc R Sot London (Biol) 162:502-516, 1965 42. Mu JY, Yin TH, Hamilton CL, et al: Variability of body fat in hyperphagic rats. Yale J Biol41:133-142, 1968 43. Keesey RE, Powley TL: Hypothalamic regulation of body weight. Am Sci 63:558-565. 1975 44. Corbit JD, Stellar E: Palatability, food intake, and obesity in normal and hyperphagic rats. J Comp Physiol Psycho1 58:63-67. 1964 45. Garn SM, Clark DC, Guire KE: Growth, body composition and development of obese and lean children, in Winick M (ed): Childhood
478
Obesity,
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New York,
John
Wiley & Sons.
1975,
P 23 46. Miller RE, Mirsky IA, Gaul WF, ct al: Hyperphagia and polydipsia in socially isolated Rhesus monkeys. Science 165:1027-1028, 1969 47. Lasser RP, Master AM: Observation of frequency distribution curves of blood pressure in persons aged 20 to 106 years. Geriatrics 14: 345360. 1959 48. Ivy AC: What is normal or normality? Q Bull Northwestern U Med School 18~22-32, ID”” L 7-T*
49. Spiegel TA: Caloric regulation of food intake in man. J Comp Physiol Psycho1 84:2437, 1973 50. Tanner JM: Regulation of growth in size in mammals. Nature 199:845-850, 1963 51. Hoebel BG. Teitelbaum P: Weight regulation in normal and hypothalamic hyperphagic rats. J Comp Physiol Psycho1 61: I89- 193, 1966
52. Apfelbaum M, Bostsarron J. Lacatis D: Effect of caloric restriction and excessive calorie intake on energy expenditure. Am J Clin Nutr 24:140551409, 1971 53. Miller DS, Mumford P: Gluttony. I. An experimental study of overeating low- or highprotein diets. Am J Clin Nutr 20:1212-1222, 1967 54. Miller DS, Payne PR: Weight maintenance and food intake. J Nutr 782555262, 1962 55. Adolph EF: Physiological Regulations. Lancaster, Pa, Jacques Cattell, 1943 56. Musten B, Peace D, Anderson GH: Food intake regulation in the weanling rat: Selfselection of protein and energy. J Nutr 104:563572, 1974 57. Rozin P: Are carbohydrate and protein intakes separately regulated? J Comp Physiol Psycho1 65:23-29. 1968
Regulation
of Body Energy Storage
Grover C. Pitts Both new and published data (rats, mice, and human beings) on three parametersfat mass, fat-free body mass (FFBM), and total body mass in some cases-are evaluated. Steady state values of the parameten are analyzed for changes in response to specific perturbing agents and for their frequency distributions. Temporal sequences of values on individuals are ex-
amined for evidence of regulatory responses. The results lead to the hypothesis that the FFBM is regulated, but probably not as a unit, and that mass of fat is regulated with a high priority near the range extremes but with a much lower priority in the mid-range. Properties and advantages of such a mechanism are discussed.
T
HE POSSIBILITY that body energy storage is regulated has been in contention for a number of years. Kennedy’ proposed that food intake in the rat is controlled to maintain near-constancy of body lipid stores. Liebelt et al.* reported apparent regulation of fat stores in the mouse. Passmore concluded from general considerations that body weight must be regulated in man. By contrast, the participants in a symposium on energy balance4 could not agree on whether such regulation occurs at all in man. Occasionally, investigators assume the reality of regulation and refer to the “set-point” of body weight or body fat content. Garrow’ (p 211) reviewed the evidence for and against a setpoint mechanism and proposed a “buffer” mechanism as a substitute. Many other authors have discussed such regulation tangentially, but convincing support for it has never been marshaled for any species. In short, the absence of a consensus probably reflects the inadequacies of existing data. In the approach employed here we have emphasized two factors: (1) The specific body component that most directly reflects energy balance, stored fat,* has been evaluated separately, if possible, by body composition methods that separate fat mass from the FFBM. (2) The evaluation of two classes of data: steady state values from the literature and temporal sequences of values obtained on individuals. The steady state values were obtained mostly by post mortem analyses, and were analyzed statistically for their sensitivity to specific perturbing agents and for their frequency distributions. The results of this comprehensive approach suggest that body energy storage is regulated, but with some differences from the well-known systems of regulation.
*We define “fat” operationally as that which is extracted by petroleum ether, the method employed by most of the investigators referenced here. The material so obtained is a mixture in which triglycerides predominate. That which remains is our fat-free body mass. From the Department of Physiology, School of Medicine, University of Virginia, Charlottesville. Va. Receivedfor publication June 20. 1977. Supported in part by NASA research grants NGR47-005-213 and NSG 2225. Address reprint requests to Grover C. Pitts, Department of Physiology, School of Medicine. University of Virginia, Charlottesville, Va. 22901. o 1978 by Grune & Stratton, Inc. 0026-0495/78/2704-0008$02.00/O
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C. PITTS
AND METHODS
Many physiologic parameters are controlled by the organism. That is. they are adjusted on a continuing basis to any level within the limits of viability as needed to serve the immediate economy of the body. Other regulated parameters are normally maintained within a narrow optimal range.6 A regulated parameter is typically coupled with one or more controlled parameters (effecters).’ Thus, if body energy storage is regulated, it must result from appropriate adjustments in the effecters’ food consumption and energy expenditure. Failure to make the distinction between regulation and control continues to cause confusion in the literature. The steady state values available for analysis were largely sample means for body fat content and FFBM reported in the literature and obtained with standard body composition techniques by indirect in vivo methods on man and direct terminal methods on other species. Because data on individuals were available in only one study, the sample means were treated as data units. In a few cases we derived the desired data from those published by very simple calculations. The eligibility of published steady state values for inclusion in our analyses was established with the following criteria: (I) Each sample had to be of adults to rule out growth as a variable. We have defined as adults laboratory rats 24+ wk old, laboratory mice 12+ wk. human beings 18+ yr. and wild mammals meeting the published weight and dimension descriptions for adults of each species. (2) Each sample must have been maintained ad lib. on a nutritionally balanced diet. (3) Each sample had to be intact prior to sacrifice. The sole exception was the use (Fig. 2) 01 rats with hypothalamic lesions. (4) The duration of the specific regimens had to insure that the values quoted represented steady states rather than transients. (5) Each sample had to provide values for both fat mass and FFBM. A comparison of the frequency distributions of these two components was desired and this precaution insured that the two data sets represented the same constellation of known and unknown variables. Data from 25 published papers plus two unpublished ones from this laboratory qualified. These 27 papers provided the II4 separate statistical samples that are plotted one or more times in Figures I. 2, and 3. Of these, 53 were samples of rats,‘-‘* 28 of men and women.” ‘9 I7 of mice.n.30
60
z Y
0
40
E
4 J 0
20
E
/
ORAL TEMP.
1 1 ,
0 0
(F")
zoo 400 SAMPLE MEAN (g)
Fig. 1. (A) Frequency distribution of statistical samples of rats, the sexes plotted separately. Both fat and FFBM are expressed in mass units. Each point represents the percentage frequency of occurrence within an abscissa band 40 g wide. Rats that had genetic or experimental obesity were omitted. (B) The frequency distribution of FFBM of male rats (from 1A) compared with that for resting systolic blood pressure of men 20-24 yr of age and resting oral temperature of men 20-25 yr of age. The respective abscissae are stacked below the figure. The common ordinate has the same units as in I A.
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? /’
4
RATS MAN M,CE
I
(N=52) (N=28) (NZ,7,
.------
1 60
0
MEAN
S&E
BOd°FAT
(%)
Fig. 2. The frequency distribution (by species) of mean percentage body fat in statistical samples of rats, mice, and human beings. Males and females are combined in the curve for each species. Each point represents the percentage of samples occurring within an abscissa band 4% wide. The abscissa locations of samples with experimental or genetic obesity are indicated at upper right. Hypothal, rats with hypothalamic obesity; ob ob, hyperglycemic mice; GTG, mice with gold thioglucose obesity.
50 A-WILD
LABORATORY
zmB E h
SPECIES
B----.INBRED, II?
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CONTROLLED
: ENVIRONMENT
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30 -
5 0 W 30h L
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Fig. 3. A comparison of the frequency distributions of body fat in four subpopulations. The bar for subpopulation B is based on the frequency distribution of individuals. The other three bars are based on the frequency distribution of sample means. Ai3’ B (unpub.); C7-“~3’~3z (and unpub.); D.6.&9.ll-tW~
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(and unpublished) and 16 of wild mammals.31 Sample size varied from 3 to 98 with most failing in the 5 to 10 range. The data obtained with the above criteria had been tested by the respective investigators for statistical significance of the difference between the perturbed mean and the control mean. These results were used in our evaluation of the effects of specific perturbing agents. The wild mammal subpopulation plotted in Figure 3 represents a special set of steady state values. With the exception of the shrews, the other 15 species (mostly rodents and squirrels) are plant eaters and were collected during the season when their natural foods occurred in greatest abundance. Thus, their dietary regimens may reasonably be regarded as ad lib. They were taken with spring traps that killed at capture. Post mortem examinations revealed lean individuals but with no suggestion of emaciation. Our frequency distribution criterion of regulation was based upon the statistical distribution of multiple measurements of a single parameter under a variety of circumstances. If the parameter is perturbed by various physiologic or environmental factors during the measurements (e.g., the rate of total body thermogenesis), the frequency distribution is likely to be scattered with little central tendency. But if regulatory mechanisms oppose perturbation and confine the parametric value within an optimal range (e.g., deep body temperature), a narrow distribution may nevertheless result. While the individual samples used are homogeneous with respect to the experimental variables of interest to the particular investigator, collectively they represent a great variety of controlled and uncontrolled variables (diet, amount and kind of exercise, investigator, location of laboratory, etc.). In the absence of a quantitative evaluation applicable to the diverse circumstances encountered, we have visually judged the position of the distributions on a continuous scale ranging from a narrow symmetrical (“bell-shaped”) distribution to one that is broadly scattered with no discernible peak. This method has been found useful in two similar studies.32’33 The methods employed in the temporal sequences of values on individuals are usually selfexplanatory and have been presented in the source references. RESULTS Among the available data on specific perturbing agents, factors that decreased body fatness include forced exercise,y,‘2,34 voluntary exercise (unpublished), force-feeding,35,36 surgical stress3’ and chronic acceleration.‘5,‘6 Factors producing consistent and predictable fat changes but which cannot be collectively characterized in a few words include age,34s38 circannual rhythms,3y,40 changes in daily 1ight:dark ratio,j’ environmental temperature,4’ dietary composition,‘3~‘4 apparent palatability of nutritionally complete diets,424 and social factors.45.46 Apparently, few perturbations cause an increase in body fatness, but one that does is ad lib consumption of high-fat diets.“.14 Thus, body fatness appears to be highly labile and moves to a new steady state in response to nearly any change in regimen. With respect to the FFBM, the following factors have been studied: forced exercise,34q36 ad lib exercise (unpublished), force feeding,36 circannual rhythms40 daily 1ight:dark ratio, 37 chronic acceleration ‘5~‘6and diet.13 Only chronic acceleration was shown to alter FFBM in the adult, the others being without effect. Chronic acceleration has undergone no physiologically significant change during the period in which life evolved and hence there would be no opportunity, or selective pressure, to evolve a system for defense against changes in it. The implication is that acceleration alters FFBM because it is not defended by a regulatory system, whereas the other perturbing factors have no effect because regulatory defenses exist. To apply our frequency distribution criterion of regulation we have plotted the available sample means in Figures 1, 2, and 3. Figure 1A shows the frequency distribution of sample means for FFBM and fat mass in our predom-
BODY ENERGY
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inant species, the rat. The curves for fat have maxima at approximately 60 g, but 12% of the values in each sex occur far to the right. By contrast, the distributions for FFBM are much narrower and less skewed. In Figure lB, the distribution of the male FFBM from Figure 1A is placed on an ordinate common with distributions for two parameters generally regarded as regulated-resting systolic blood pressure in young men 20-24 yr old4’ and resting oral temperature in medical students 20-25 yr old.48 In comparing the curves, two things should be kept in mind. First, the curves for blood pressure and oral temperature represent the distribution of individual values, whereas the FFBM curve represents the distribution of sample means. Second, the blood pressures and oral temperatures were measured under standardized conditions, whereas the sample means for FFBM represent wide variations in experimental circumstances. Uncertainty about how to quantitatively compare the respective abscissae restricts us to a comparison of the general shapes. However, this suggests that regulation may play a role in the distribution of rat FFBM, but there is considerable room for doubt in the case of body fat. The ordinate and abscissa units in Figure 2 are dimensionless and theoretically equally applicable to either sex and to a variety of species, within limits. In the figure, we have combined the sexes and represented the three species separately. The distributions are very broad and strongly skewed to the right. However, the peaks suggest that each species has a “preferred” level of fatness roughly in the lo%-20% range. This plot suggests, more clearly than in Figure 1, a deviation from what is expected of regulated parameters. In the figure, note that all samples with experimental or genetic obesity are located above 40% fatness. However, intact apparently normal rats and human beings occur in the same high range, suggesting that hypothalamic lesions do not boost animals to levels of fatness above what is physiologically available to most individuals. This interpretation is in accord with the findings of Mu et a1.42 but not with Keesey and Powley. 43 The only mice above 40% fatness were those with gold thioglucose obesity or the obob gene. Whether normal mice might range this high would require a larger sample for a satisfactory answer. In Figure 3 are plotted frequency distributions in percentage body fat for four subsamples as identified there. The overlapping abscissa ranges extend from nearly 0’?-56% fatness. This figure demonstrates that subpopulations individually may show relatively narrow frequency distributions, but collectively suggest a broad continuum. Figure 3 also provides an opportunity to compare the frequency distribution of individual rats (subsample B) with that of sample means for rats on the same diet (subsample C). The general similarity of the two distributions suggests that the comparison of a distribution of means with two distributions of individuals made in Figure 1B is reasonable. The data available as temporal sequences of values on individuals present a special problem. The parameter measured in nearly every case is live mass, and we must infer the extent of separate participation by fat and FFBM. The above observed lability of body fat mass and stability of FFBM and the results of other studies’4*‘6,34 allow the rather secure inference that in an adult on a generally ad lib. regimen with a nutritionally complete diet, changes in mass of stored fat can almost fully account for observed changes in live mass. This inference has also been verified for rats with hypothalamic lesions.43
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PITTS
In these temporal sequences, live mass may show a progressive and prolonged change. Where such changes are rectilinear, the absence of regulatory adjustments is obvious. For example, in a laboratory study by Spiegel of human feeding responses,49 6 of 15 subjects were classified as nonregulators of body mass, and the nonregulator presented in detail (Spiegel’s Figure 2) showed a rectilinear loss of - 7% of his total mass over a 2 l-day period. Figure 4 presents previously unpublished data on one rat. This individual, always appearing healthy and well-groomed, underwent a rectilinear loss of - 25% of her mass in 10 days. A closely similar response has been reported by Cohn and Josephj5 (their Fig. 6). A further observation is that if such rectilinear changes go far enough, they eventually evoke a regulatory response with an apparently exponential recovery curve resembling the “homing” typical of recovery from interrupted growth.” Upon reaching a mass of 190 g, the animal in Figure 4 abruptly began eating. and its mass homed on a growth curve in the 250-300 g range. Postmortem analysis revealed a mid-range body fat value of 7.4%. The rat observed by Cohn and Joseph35 showed a closely similar response. At sacrifice it had 8.3q, body fat. In each of these cases, it is difficult to escape the conclusion that the level of body mass (i.e., body fat) eventually established had a low priority, since each rat passed completely through the regulatory range and belatedly returned to it. Conceivably, the rarity in the literature of such findings could represent a failure of reporting. An atypical response of the type presented in Figure 4 might often be rejected as the result of intervention by an unknown and uncontrolled variable, for example, a systemic infection. In contrast with the seemingly casual regulation described, rats with hypothalamic lesions, which produce lean, hypophagic43 or fat hyperphagic animals,” show vigorous regulation of body mass (i.e., body fat).43 The common feature of these two groups is probably that altered appetite or satiety has changed the body fatness to higher or lower levels where regulatory defense against even more extreme deviations has an enhanced priority. Numerous investigators have monitored effector mechanisms potentially involved in the regulation of energy storage. Food consumption, when averaged for periods of a week or more, is usually altered in a direction appropriate to
Fig. 4.
Body mass,
tion
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rat,
the
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rat.
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and food consumpmode
was
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the 0.
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the maintenance of energy balance. 5 Several studies now support the conclusions that prolonged hyperphagia or hypophagia are associated with an involuntary increase or decrease, respectively, of energy dissipated as heat.5152-54In general, these two effecters behave as though they are participating in a system with negative feedback, but one cannot tell whether they subserve regulation of body fat or FFBM or both. DISCUSSION
Any hypothesis for the regulation of energy storage must explain these findings: (1) The FFBM is quite stable while fat mass is altered by all classes of perturbations that have been tested. (2) The frequency distribution of steady state values for FFBM is typical of regulated parameters while that for fat mass is not. (3) The two effector mechanisms, food consumption and involuntary dissipation of energy as heat, respond as though they are participating in a negative feedback system directed at energy storage. (4) Animals with hypothalamic lesions, where a change in hunger or satiety pushes the individual against one or the other extreme level of fatness, show vigorous regulation of fatness. Intact animals in the mid-range of fatness occasionally show progressive rectilinear mass changes, continuing over extended periods, that appear unopposed by regulatory processes. Points 1,2, and 3 suggest that FFBM is regulated, 1 and 2 that fat mass is not regulated, and 4 that fat mass is regulated but with a priority that changes in different ranges of fatness. The above observations lead us to the following hypothesis of energy storage regulation. The fat-free compartment, included in the hypothesis because it represents stored energy that becomes available during advanced inanition, is quite constant in the absence of growth. But since regulation of FFBM as a unit is improbable, and perhaps disadvantageous, because of its heterogeneity, its major components (muscle, bone, etc.) are probably regulated individually, and the observed near-constancy of the whole is the result of these several interacting systems of regulation. The total body fat mass is regulated, but with a changing priority that is a function of the actual level of fatness. In the mid-range of fatness, the priority is very low and the regulatory mechanisms may be readily overruled by a variety of competing factors. However, the priority for regulation becomes higher as extreme values of fatness are approached. The most clearly defined difference between the suggested regulation of fat mass and the known regulation of other parameters is in the breadth of the optimal range. This is not a qualitative difference, since regulation in general is probably directed at optimal ranges rather than specific optimal values. Various changes, some regulated and some not, occur within the broad optimal range of body fatness and account for the observed lability in body fatness. They further substantiate Adolph’s warning55 that the living organism is characterized by complex interactions based on established priorities between multiple systems of regulation. A low priority, such as we would assign to maintaining fatness constant so long as it is within the optimal range, means that the system would probably yield at nearly every conflict with another system.
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Broad-optimum regulation has substantial advantages. For example, ageassociated changes in fatness do not require a regulatory explanation so long as they do not approach extreme values; mechanisms exerting independent control may make appropriate adjustments in body protein unimof protein intake56,57 paired by resulting changes in fat mass; increases in fatness in preparation for hibernation, migration, etc., will not encounter insurmountable interference from the fat regulatory system. Garrow’s “buffer” system for control of fatness’ involves no feed-back loop and minimizes changes without correcting them. In support, human data are cited,5 and it is apparently intended to apply only to man. We doubt that human fat regulation differs, in principle, from that of most mammals, and the evidence presented above for the existence of feedback loops and for corrections made in altered fatness (particularly in rats with hypothalamic lesions) explains our preference for the present hypothesis. Energy balance probably comprises several distinguishable processes, including: control of the release of chemical energy so that it matches physical work; control of the proportionate energy contributions from endogenous stores of glycogen, fat, and protein; adjustment of energy metabolism to changes in physiologic condition (e.g., growth, gestation, lactation, egg-laying, inanition) as well as regulation of the mass of body fat. It appears unlikely that a single system could control or regulate all of these. Other regulatory systems involved could operate by principles different from any proposed to date. Broad-optimum regulation does not constitute a unique explanation of reported findings. However, it provides a satisfactory (and hopefully a heuristic) framework for the relevant data as viewed from a new perspective. REFERENCES I. Kennedy GC: The role of depot fat in the hypothalamic control of food intake in the rat. Proc R Sot Lond (Biol) 140:578-592, 1953 2. Liebelt RA, Vismara L, Liebelt AG: Autoregulation of adipose tissue mass in the mouse. Proc Sot Exper Biol Med 127:458-462, 1968 3. Passmore R: The regulation of body weight in man. Proc Nutr Sot 30:122-127, 1971 4. Vigy M, Apfelbaum M, Miller DS, et al (eds): Energy Balance in Man. General Discussion. Paris, Masson et Cie, 1973, p 346 5. Garrow JS: Energy Balance and Obesity in Man. New York, American Elsevier, 1974 6. Brobeck JR: Exchange, control and regulation, in Yamamoto WS, Brobeck JR (eds): Physiological Controls and Regulators. Philadelphia, W.B. Saunders, 1965, p 1 7. Babirak SP, Dowell RT, Oscai LB: Total fasting and total fasting plus exercise: Effects on body composition of the rat. J Nutr 104:452457. 1974 8. Bates MW, Nauss SF. Hagman NC, et al: Fat metabolism in three forms of experimental obesity. Am J Physiol 180:301-303, 1955
9. Hanson DL, Lorenzen JA, Morris AE. et al: Effects of fat intake and exercise on serum cholesterol and body composition of rats. Am J Physiol213:3477353, 1967 IO. Lemonnier D: Experimental obesity in the rat by high-fat diets, in Vague J (ed): Physiopathology of Adipose Tissue. Amsterdam, Excerpta Medica Foundation, 1969, p 197 Il. Montemurro DG, Stevenson JAF: Survival and body composition of normal and hypothalamic obese rats in acute starvation. Am J Physiol 198:757-761, 1960 12. Oscai LB, Holloszy JO: Effects of weight changes produced by exercise, food restriction. or overeating on body composition. J Clin Invest 48:2124-2128, 1969 13. Peckham SC, Entenman C, Carroll HW: The influence of a hypercaloric diet on gross body and adipose tissue composition in the rat. J Nutr 77:187-197, 1962 14. Pitts GC, Bull LS: Exercise, dietary obesity and growth in the rat. Am J Physiol 232: R38-R44, 1977
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15. Pitts GC, Bull LS, Oyama J: Effect of chronic centrifugation on body composition in the rat. Am J Physiol223:1044~1048, 1972 16. Pitts GC, Bull LS, Oyama J: Regulation of body mass in rats exposed to chronic acceleration. Am J Physiol228:714-717, 1975 17. Schemmel R, Mickelsen 0, Fisher L: Body composition and fat depot weights of rats as influenced by ration fed dams during lactation and that fed rats after weaning. J Nutr 103: 4777487, 1973 18. Schemmel R, Michelsen 0, Tolgay Z: Dietary obesity in rats: Influence of diet, weight, age and sex on body composition. Am J Physiol 216:373-379, 1969 19. Barnard DL, Ford J, Garnett ES, et al: Changes in body composition produced by prolonged total starvation and refeeding. Metabolism 18:564-569, 1969 20. Bernstein LM. Johnston LC, Ryan R, et al: Body composition as related to heat regulation in women. J Appl Physiol 9:241-256, 1956 21. Baling EA. Taylor WL, Entenman C, et al: Total exchangeable potassium and chloride and total body water in healthy men of varying water and fat content. US Nav Radio1 Def Lab Tech Rep 313, 1959 22. Brockett JE, Brophy EM, Koniski F. et al: Influence of body size, body fat, nutrient intake and physical fitness on the energy expenditure of walking. US Army Med Nutr Lab Rep No 177. 1956 23. Broiek J: Changes of body composition in man during maturity and their nutritional implications. Fed Proc 11:784-793, 1952 24. McMurrey JD, Baling EA. Davis JM, et al: Body composition: Simultaneous determination of several aspects by the dilution principle. Metabolism 7:651-667, 1958 25. Osserman EF, Pitts GC, Welham WC, et al: In vivo measurements of body fat and body water in a group of normal men. J Appl Physiol 2:633-639, 1950 26. Paiizkova J, Eiselt E: A further study on changes in somatic characteristics and body composition of old men followed longitudinally for 8-10 years. Human Biol43:318-326, 1971 27. von Dobeln W: Human standard and maximal metabolic rate in relation to fatfree body mass. Acta Physiol Stand 37 (Suppl 126). 1956 28. Welch BE, Riendeau RP, Crisp CE, et al: Relationship of maximal oxygen consumption to various components of body composition. J Appl Physiol 12:395-398, 1958 29. Young CM, Blondin J, Tensuan R. et al:
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Body composition studies of “older” women, thirty to seventy years of age. Ann NY Acad Sci 110:589~607, 1963 30. Keil LC: Changes in growth and body composition of mice exposed to chronic centrifugation. Growth 33:83-88, 1969 3 1. Pitts GC, Bullard TR: Some interspecific aspects of body composition in mammals, in Body Composition In Animals And Man, Nat1 Acad Sci Pub1 1598, 1968, p 45-70 32. Gasnier A, Mayer A: Recherches sur la regulation de la nutrition. Ann Physiol Physicochim Biol 15:145-214, 1939 33. Schmidt-Nielsen K, Dawson WR: Terrestrial animals in dry heat: Desert reptiles, in Handbook of Physiology. Adaptation to the Environment (section 4). Washington, D. C., American Physiological Society, 1964, p 467480 34. Pitts GC. Bull LS, Hollifield G: Physiologic changes in composition and mass of total body adipose tissue. Am J Physiol 22 1:961-966, 1971 35. Cohn C, Joseph D: Influence of body weight and body fat on appetite of “normal” lean and obese rats. Yale J Biol Med 34:598607, 1962 36. Pitts GC, Bull LS, Wakefield JA: Exercise with force feeding in the rat. Am J Physiol 2271341-344, 1974 37. Pitts GC, Bullard TR, Tremor JR, et al: Rat body composition: Sensor implantation and lighting effects. Aerospace Med 40:4 17-420, 1969 38. Keys A, Broiek J: Body fat in adult man. Physiol Rev 33:2455325, 1953 39. Odum EP. Connell CE: Lipid levels in migrating birds. Science 123:892-894. 1956 40. Odum EP, Rogers DT. Hicks DL: Homeostasis of the nonfat components of migrating birds, Science 143:1037-1039, 1964 41. Barnett SA, Widdowson EM: Organweights and body-composition in mice bred for many generations at -3°C. Proc R Sot London (Biol) 162:502-516, 1965 42. Mu JY, Yin TH, Hamilton CL, et al: Variability of body fat in hyperphagic rats. Yale J Biol41:133-142, 1968 43. Keesey RE, Powley TL: Hypothalamic regulation of body weight. Am Sci 63:558-565. 1975 44. Corbit JD, Stellar E: Palatability, food intake, and obesity in normal and hyperphagic rats. J Comp Physiol Psycho1 58:63-67. 1964 45. Garn SM, Clark DC, Guire KE: Growth, body composition and development of obese and lean children, in Winick M (ed): Childhood
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P 23 46. Miller RE, Mirsky IA, Gaul WF, ct al: Hyperphagia and polydipsia in socially isolated Rhesus monkeys. Science 165:1027-1028, 1969 47. Lasser RP, Master AM: Observation of frequency distribution curves of blood pressure in persons aged 20 to 106 years. Geriatrics 14: 345360. 1959 48. Ivy AC: What is normal or normality? Q Bull Northwestern U Med School 18~22-32, ID”” L 7-T*
49. Spiegel TA: Caloric regulation of food intake in man. J Comp Physiol Psycho1 84:2437, 1973 50. Tanner JM: Regulation of growth in size in mammals. Nature 199:845-850, 1963 51. Hoebel BG. Teitelbaum P: Weight regulation in normal and hypothalamic hyperphagic rats. J Comp Physiol Psycho1 61: I89- 193, 1966
52. Apfelbaum M, Bostsarron J. Lacatis D: Effect of caloric restriction and excessive calorie intake on energy expenditure. Am J Clin Nutr 24:140551409, 1971 53. Miller DS, Mumford P: Gluttony. I. An experimental study of overeating low- or highprotein diets. Am J Clin Nutr 20:1212-1222, 1967 54. Miller DS, Payne PR: Weight maintenance and food intake. J Nutr 782555262, 1962 55. Adolph EF: Physiological Regulations. Lancaster, Pa, Jacques Cattell, 1943 56. Musten B, Peace D, Anderson GH: Food intake regulation in the weanling rat: Selfselection of protein and energy. J Nutr 104:563572, 1974 57. Rozin P: Are carbohydrate and protein intakes separately regulated? J Comp Physiol Psycho1 65:23-29. 1968
