European Journal of
Europ. J. appl. Physiol. 36, 247-254 (1977)
Applied Physiology and Occupational Physiology c by Springer-Verlag 1977
Effect of Glucose Ingestion on Energy Substrate Utilization during Prolonged Muscular Exercise F. Pirnay, M. Lacroix, F. Mosora, A. Luyckx, and P. Lefebvre Institut Provincial E. Malvoz; Department of Atomic and Molecular Physics and Division of Diabetes, Institute of Medicine, Universityof Liege, B-4020 Liege, Belgium
Summary. The distribution of substrates utilized during prolonged exercise was investigated in normal human volunteers with and without ingestion of 100 g exogenous glucose. The energy provided by protein oxidation was derived from urinary nitrogen excretion and the total energy provided by carbohydrates and lipids was calculated from respiratory quotient (RQ)determinations. The contribution of exogenous glucose to the energy supply was determined by an original procedure using "naturally labeled ~3C-glucose" as metabolic tracer. Protein oxidation provided between 1 and 2% of the total energy requirement; this amount was not affected by glucose ingestion. In the absence of exogenous glucose ingestion, carbohydrate were progressively replaced by lipids as source of energy. Exogenous glucose contributed markedly to total carbohydrate oxidation and decreased the percentage of energy derived from lipids. In addition, ingestion of exogenous glucose resulted in a significant economy of endogenous carbohydrates and permitted to prolong the duration of exercise. Key words: Exercise -- Respiratory quotient - Fuels -- Stable isotopes. Introduction
During muscular exercise, the respiratory quotient (RQ) significantly exceeds its value at rest, a finding suggesting that carbohydrates serve as a major energy substrate in that condition. However, more detailed analysis of the respiratory quotient data suggested that the muscle not only uses carbohydrates for its aerobic metabolism, but also oxidizes lipids (Dill et al., 1932; Christensen and Hansen, 1939). New techniques, using radio-active isotopes (Friedberg et al., 1960; Issekutz et al., 1965, 1966; Paul, 1970, 1975) or vascular catheterizations (Carlson and Pernow, 1959; Wahren et al., 1971) have given formal proof, with regard to carbohydrates as well as to lipids, of increased mobilization and oxidation during muscular contractions. The respective contributions of the various substrates utilized is a controversial matter. It depends mainly on intensity and duration of the work (Keul et al., 1972;
248
F. Pirnay et al.
W a h r e n et al., 1975). It is know that the level of body stores, as well as the nature of the food ingested during the preceeding days, clearly modify the energy sources (Bergstrrm et al., 1967). O n the other hand, few investigations have been made on the immediate effect of glucose intake during the actual exercise. Dill et al. (1932) observed that the running time of a dog on a treadmill was much lengthened when it was given glucose. Christensen and H a n s e n (1939) also noted that an exhausted subject can prolong his effort on a bicycle, if he ingests 200 g of glucose. Prolongation of the effort was attributed to the recovery of a normal blood glucose level. The problem of the immediate utilization of ingested glucose is still debated. Our aim was to study the influence of an oral load of 100 g of glucose on the distribution of substrates utilized during prolonged exercise, and to evaluate the possible contribution of the exogenous glucose to the energy supply.
Material and Methods
Principles The energy provided by protein metabolism was calculated by multiplying the quantity of urinary nitrogen by its caloric coefficient. The bladder was emptied before exercise; all the urinary output produced during and immediately after exercise was collected. The amounts of carbohydrates and of lipids oxidized were calculated from the non-proteic RQ values using classical tables (Lusk, 1928). Exogenous glucose consumption was measured using as metabolic tracer a "naturally enriched laC-glucose". Due to isotopic effects during photosynthesis, some plants such as maize and sugar-cane contain slightlyhigher quantities of 13C than other plants (Smith and Epstein, 1971). As a consequence, the animals who feed on these substances, partially reproduce the ~3C enrichment in their expired CO2. Thus, naturally enriched ~3C-glucosecan be used as tracer for metabolic studies in animals and in man (Duchesne et al., 1973; Lacroix et al., 1973; Shreeve, 1973; Lacroix and Mosora, 1975). The ratio of isotopes 13 and 12 of carbon, measured in the expired CO2, enabled us to calculate the proportion of CO2 provided by the ingested glucose. The total amount metabolizedin the body requires, in addition, the determination of the total volume of expired CO2. The details of the method have been recently published (Mosora et al., 1976).
Techniques The expired CO2 and 02 consumption were continuously measured in open circuit. The subject breathed atmospheric air from two Tissot gasometers which determined the volume of ventilated air. He was alternately connected to one gasometer or the other by means of magnetic valves controlled by a time clock. The expired air was then conveyed by a weak resistance expiratory valve, through a mixer, before flowingout. After coldtrapping of the water vapour, a sample of expired air was analyzed for its O2 content, using a paramagnetic analyser (Servomex). COz concentration was measured with an infrared analyzer (Godart Capnograph). A manometer, set in parallel, constantly controlled any variation of pressure, and a flowmeter, fitted to each analyzer, ensured constant flows in the analysis circuits. Both analyzers were calibrated, after measurement, with three gases of known concentration. The respiratory quotient was obtained by the ratio between the expired CO2 and 02 consumption determined continuously and simultaneously.The exogenous glucose utilizedwas measured as from the total volume of expired CO2 and from the 13C/12C ratio, by the technique devised by Mosora et al. (1976).
Glucose Ingestion and Muscular Exercise
249
To assess the 13C/12Cisotopic ratio, a sample of expired air was collected in a rubber bag of about 1 1. The isotopic ratio was measured by means of a high sensitivity mass spectrograph with a double collector (Varian/Mat CH5) which compares peak height ratios of masses 45 and 44 of the CO2 extracted from the tested air with a standard sample of CO2. The heart rate was measured from a thoracic lead of the electrocardiogram. Every 15 rain, blood was withdrawn through a soft catheter set in an antecubital vein for blood glucose determination using the method of Hoffman (1932) adapted to the Technicon Auto Analyzer.
Muscular Exercise The subjects, fasted overnight, walked on a 10% uphill treadmill. Speed was constant and set between 4 and 5 km/h. In these conditions, oxygen consumption averaged about 50% of the individual maximum. The exercise required an 02 consumption of 1.8-2.1 l/min, while the heart rate varied between 144 and 162 beats/min. After a 15 rain exercise adaptation period and without stopping walking, the subjects ingested, in I - 2 min, 100 g of "naturally enriched 13C-glucose"diluted in 400 ml of water. We have used anhydrous glucose prepared from maize (Glucopur, Glucosuries Reunies, Alost, Belgium). Melting point determinations showed that this glucose was partially hydrated and calculations indicated that the amount of pure glucose in the load given was in fact 95 _+ 1 g (mean + SD; 4 determinations).
Subjects Seven volunteer male students, accustomed to physical exercise but without particular training were investigated. They were between 21 and 25 years old, and had a mean body weight close to their ideal body weight: 103 + 2.4% [Metropolitan Life Insurance Company (1959)]. A preliminary oral load test of I00 g of glucose showed, for all subjects, normal glucose tolerance (Wilkerson, 1964) and no glycosuria.
Results The distribution of energy substrates utilized during exercise is shown in Figure 1. Protein oxidation, as evaluated from urinary nitrogen excretion, was very low and provided between 1 and 2% of the total energy. A t the beginning o f the exercise, the contributions of lipids and o f c a r b o h y d r a t e s in the metabolism were almost equal. A s the test was on without glucose ingestion, c a r b o h y d r a t e s were gradually less utilized. Their contribution decreased from 43.9 to 18.7% after 4 h o f exercise. The energy was then mainly supplied by lipid oxidation. The same Figure 1 illustrates, throughout the exercise, the influence exerted by the ingestion o f 100 g o f glucose. Distribution o f energetic substrates utilized for aerobic metabolism was clearly modified. The ingested glucose rapidly participated in oxidations. Fifteen minutes after its ingestion, the expired air was already enriched in 13C. Utilization of exogenous glucose increased during the first hour, reached a m a x i m u m 6 0 - 9 0 min after its ingestion, then progressively decreased. The contribution o f exogenous glucose to energy supply is quantitatively important, since it m a y provide as much as 27.2 + 7.4% of the total energy. The ingestion of glucose simultaneously increased the total energy made available from carbohydrates. After glu-
%
I
pr~
[---] ['pids M end~176 ~ ex~176 carbohydrates
100.
gtucose
-r~
i +
80
2 el_ cl_
~ 60 cn
~ 40c LI
"
I
20-
EXERCISE BEFORE GLUCOSE
0
30
60
90
120
150
180
210
I
I
I
I
I
I
I
I
30
60
90
120
150
180
210
240
TIME {min)
Fig. 1. Respective contributions of the various energy fuels during prolonged muscular exercise. For each period, the left column corresponds to the control test effected without any glucose ingestion while the right column corresponds to the test effected after the oral ingestion of glucose at time 0. The two columns to the far left represent data obtained during the last 5 min of the exercise-adaptation period. Results are exposed as mean + SD (standard deviation). The same three subjects were investigated under both experimental conditions. During the last period (210-240 rain), two out of the three subjects were exhausted when exercising without glucose ingestion. For that period, the left column corresponds to the value recorded in the only subject and is not figured. When given glucose, all three subjects completed the program
o,o
I
pr~
[ipids
end~167176
exogenous
carbohydrates~
gtucose
100-
80(D_ CL
-
60-
+I ++ +++ ~
cI.U
20"
T
T
......... i .......... ,i~!)i!ii! /!:iSS:. ~,:i~i!!iii +~++ X'X-X'3:
......... EXERCISE BEFORE GLUCOSE
i:!:i:!:!:i!:!i.;:;:;:;:;:;:;:;:.: iiiiiiiii
0
30
60
90
I
I
I
I
30
60
90
120
.'2"2"X.X'2. 9...,.........+
120 i 150
X'X-X'X!-X' r.,;.....+... " :-2"X-X! ! ii!iiii : 150 i 180
180 I 210
210
TIME
240
(rain)
Fig. 2. Respective contributions of the various sources of energy during muscular exercise after 100 g glucose intake. Means and standard deviations of seven experiments are shown. The first column to the left represents data obtained during the last 5 min of the exercise-adaptation period
Glucose Ingestion and Muscular Exercise
251
cose ingestion, the contribution of carbohydrates to aerobic metabolism always remained superior to that observed under control conditions. Total carbohydrate oxidation increased temporarily during the first 2 h after glucose intake, before gradually decreasing. The part played by exogenous glucose in energy supply was confirmed in a greater number of subjects whose results are shown in Figure 2. Slight individual variations in the evolution of the phenomena were observed, but the dispersion of the results was rather weak. In all subjects, the ingested glucose participated in cellular oxidations, and permitted total carbohydrate oxidation to provide between 50% (first hour) and 30% (last hour) of the total energy requirements.
Discussion
The respective contributions of the various substrates utilized during long-term muscular exercise, correspond to the results reported in the literature (Keul et al., 1972; Wahren et al., 1975; Paul, 1975). In fasting individuals, the proportion of the energy supplied by lipid oxidation gradually increased in prolonged exercise. In our investigations, it reached more than 80% during the fourth hour of the exercise. This value is in good agreement with the 62% measured by Ahlborg et al. (1974) in the active leg, as well with Paul's results (1975), who reported that lipid contribution may reach 80-90% of the energy requirement. In addition to the duration of the exercise, its intensity modifies the respective contributions of the substrates utilized. Light exercises mobilize greater quantities of lipids, while intense efforts preferentially use carbohydrates (Keul et al., 1972). Lipid participation increased in well trained individuals (Paul, 1975) or after a low carbohydrate diet (Christensen and Hansen, 1939; Bergstr6m et al., 1967). Glucose ingested during exercise modifies the distribution of oxidized substrates. Using "naturally labeled 13C-glucose" we have defined the early an important contribution of ingested glucose in muscular metabolism (Pirnay et al., 1977). The large utilization of exogenous glucose is consistent with the results of Benade et al., (1973) who gave a 14C-sucrose load. These authors reported that the participation can reach 44.2% of the carbohydrate metabolism, and started 10 min after the ingestion of the labeled sucrose. Under other experimental conditions however, Costill et al. (1973) reported that the contribution of exogenous glucose to total carbohydrate oxidation appeared smaller, but these authors restricted their analysis to a 1 h period after the ingestion of 32 g of glucose. Glucose intake induced a parallel increase in total carbohydrate metabolism and a corresponding decrease in lipid oxidation. This modification in fuel utilization is maximal during the second hour, when it reached 10%. The increase in total carbohydrate metabolism, however, remained lower than the amount of energy made available from exogenous glucose, thus sparing endogenous carbohydrates. As shown in Table 1, the quantities of endogenous carbohydrates oxidized during exercise were lower when the subjects were given glucose during the test. This sparing effect was already obvious 30 min after glucose intake and remained significant until the 150th rain. It represented 0.3-0.5 g of glucose per minute. In the first 2 h, cumulative endogenous glucose consumption was evaluated at 118.6 and 79.3 g respectively in control and glucose-given subjects. When the test was prolonged, the
F. Pirnay et al.
252 Table 1. Carbohydrate oxidation Period min
Without glucose ingestion
With glucose ingestion (100 g)
Endogenous glucose
Total
g/30 rain
Exogenous
Cumul (g) g/30 rain
Cumul (g) g/30 min
Endogenous Cumul (g) g/30 rain
Cumul (g)
0 30
M SD
31.9 4.3
31.9 4.3
33.3 5.3
33.3 5.3
4.7 1.9
4.7 1.9
28.6 5.3
28.6 5.3
30 60
M SD
31.2 3.0
63.1 8.4
36.7 5.6
69.9 10.8
16.1 5.0
20.8 6.8
20.5 3.5
49.1 7.4
60 90
M SD
30.4 1.6
93.6 14.2
35.5 6.1
105.4 16.1
19.4 5.2
40.2 11.2
16.1 2.6
65.2 11.4
90 120
M SD
25.0 5.1
118.6 21.1
30.8 7.3
136.2 22.9
16.6 1.9
56.9 12.8
14.2 5.8
79.3 16.2
120 150
M SD
20.9 7.2
139.6 28.7
28.4 5.3
173.6 34.6
12.9 4.2
74.9 9.1
15.5 9.5
98.6 18.4
150 180
M SD
17.7 6.6
157.3 31.4
26.1 2.0
199.8 36.6
9.3 5.8
84.3 5.1
16.8 7.8
115.6 24.6
180 210
M SD
14.3 3.5
171.6 38.2
24.2 3.8
223.8 40.3
6.5 3.6
90.7 4.7
17.6 7.2
133.1 26.2
210 240
M
(13.6)
(185.2)
18.6 6.5
242.4 46.6
4.1 2.0
94.8 4.2
14.5 8.5
147.6 33.6
Results are given as mean (M) + standard-deviation (SD)
m
/100 mi
BLOOD
GLUCOSE
120-
IO0
- .....
w i t h glucose without
glucose
80
60
(I
30
6'0
90
120 150 I~0 2'10 2'/.0 TIME {min)
Fig. 3. Mean blood glucose concentrations of three subjects during exercise with and without glucose intake
influence o f glucose ingestion g r a d u a l l y decreased, but persisted for 3 h. It should be p o i n t e d out t h a t these calculations o f the e n d o g e n o u s c o n t r i b u t i o n o f glucose in e n e r g y supply are a p p r o x i m a t e . I n fact, t h e y neglect other p a t h w a y s o f glucose f o r m a t i o n and b r e a k d o w n , w h i c h m a y be i m p o r t a n t in m u s c u l a r exercise ( Y o u n g et
Glucose Ingestion and Muscular Exercise
253
al., 1967; Wahren et al., 1975). Despite these restrictions, it appears as if the contracting muscles used a lesser amount of their carbohydrate stores. Normally, the muscular glycogen level strongly decreases during long-term effort, and its depletion m a y be the main limiting factor (Bergstrrm et al., 1967; Hultmann et al., 1967, 1971; Costill et al., 1971). Ingestion of glucose seems to delay this limit. In our investigations, it enabled to prolong their exercise two subjects out of three, thereby confirming the results of others (Dill et al., 1932; Christensen and Hansen, 1939; Bergstrrm et al., 1967). Simultaneously, blood glucose level (Fig. 3) gradually decreased in fasting exercising subjects and showed a mean value of 65 mg/100 ml at the end of the test. It constantly remained at a higher level throughout exercise when exogenous glucose was given. Both mechanisms, sparing of endogenous carbohydrates and maintenance of a higher blood glucose level, m a y contribute to enhance tolerance to prolonged effort. Early and high oxidation of exogenous glucose as shown in this study, warrants carbohydrate ingestion during long-term exercise.
Acknowledgements. This work was supported by the Fonds National de la Recherche Scientifique (F.N.R.S.), the Fonds de la Recherche Fondamentale Collective (F.R.F.C.) and the Fonds de la Recherche Scientifique Mgdicale (F.R.S.M.) of Belgium and by a grant-in-aid of Chemic Griinenthal GmbH (Federal Republic of Germany).
References
Ahlborg, G., Felig, P., Hagenfeldt, L.: Substrate turnover during prolonged exercise in man: splanchnic and leg metabolism of glucose free fatty acids, and amino acids. J. clin. Invest. 53, 1080-1090 (1974) Benade, A. J. S., Jansen, C. R., Rogers, G. G., Wyndham, C. H., Strydom, N. B.: The significance of an increased RQ after sucrose ingestion during prolonged aerobic exercise. Pfl/igers Arch. 342, 199--206 (1973) Bergstrrm, J., Hermansen, L., Hultman, E., Saltin, B.: Diet, muscle glycogen and physical performance. Acta physiol, scand. 71, 140--150 (1967) Carlson, L. A., Pernow, B.: Studies of blood lipids during exercise. Arterial and venous plasma concentration of unesterified fatty acid. J. Lab. clin. Med. 53, 833--841 (1959) Christensen, E. H., Hansen, O.: Arbeitsf'~ihigkeitund Ern~ihrung. Skand. Arch. Physiol. 81, 160-172 (1939) Costill, D. L., Bowers, R., Sparks, K., Turner, C.: Muscle glycogen utilization during prolonged running. J. appl. Physiol. 31, 353--356 (1971) Costill, D. L., Benett, A., Branam, G., Eddy, D.: Glucose ingestion at rest and during prolonged exercise. J. appl. Physiol. 34, 764-769 (1973) Dill, D. B., Edwards, H. T., Talbott, J. H.: Studies in muscular activity. VII. Factors limiting the capacity of work. J. Physiol. 77, 49--54 (1932) Duchesne, J., Mosora, F., Lacroix, M., Lefebvre, P., Luyckx, A., Lopez-Habib, G.: Une application clinique d'une nouvelle m&hode biophysique basre sur l'analyse isotopique du CO: exhal+ par l'homme. C.R. Acad. Sci. (Paris) 277D, 2261--2264 (1973) Friedberg, S. J., Harlan, W. R., Jr., Trout, D. L., Estes, E. H., Jr.: The effect of exercise on the concentration and turnover of plasma nonesterified fatty acids. J. olin. Invest. 39, 215-230 (1960) Hoffman, W. S.: A rapid photoelectric method for the determination of glucose in blood and urine. J. biol. Chem. 120, 51-55 (1937) Hultman, E., Bergstrrm, J.: Muscle glycogen synthesis in relation to diet studied in normal subjects. Acta med. scand. 182, 109-117 (1967)
254
F. Pirnay et al.
Hultman, E., Bergstrtm, J., Roch-Norlund, A. E.: Glycogen storage in human skeletal muscle. In: Muscle metabolism during exercise (B. Pernow, B. Saltin, ed.), pp. 273-288. New-York: Plenum 1971 Issekutz, B., Jr., Miller, H. I., Rodahl, K.: Lipid and carbohydrate metabolism during exercise. Fed. Proc. 25, 1415-1420 (1966) Issekutz, B., Jr., Miller, H. I., Paul, P., Rodahl, K.: Aerobic work capacity and plasma FFA turnover. J. appl. Physiol. 20, 293--296 (1965) Keul, J., Doll, E., Kepler, D.: Energy metabolism of human muscle. Basel: Karger 1972 Lacroix, M., Mosora, F., Pontus, M., Lefebvre, P., Luyckx, A., Lopez-Habib, G.: Glucose naturally labeled with carbon-13: use for metabolic studies in man. Science 181, 445-446 (1973) Lacroix, M., Mosora, F.: Variations du rapport 13C/X2C dans le m&abolisme animal. A congress report, in isotope ratios as pollutant source and behaviour indicators. At. En. Ag. Vienna 343-358 (1975) Lusk, G.: The science of nutrition. Philadelphia: Saunders 1928 Metropolitan Life Insurance Company: Statistical Bulletin, (tables 2 and 3). 40 (1959) Mosora, F., Lacroix, M., Pontus, M., Duchesne, J.: Effets de la dtsoxycorticosttrone, du glucagon et de l'insuline sur le rapport isotopique 13C/12C du CO 2 respiratoire chez le rat. Bull. Acad. roy. Belg. Sci. 58, 565-576 (1972) Mosora, F., Lefebvre, P., Pirnay, F., Lacroix, M., Luyckx, A., Duchesne, J.: Quantitative evaluation of the oxidation of an exogenous glucose load using naturally labeled ~3C-glucose. Metabolism 25, 1575-1582 (1976) Paul, P.: FFA metabolism of normal dogs during steady-state exercise at different work loads. J. appl. Physiol. 28, 127-132 (1970) Paul, P.: Effects of long lasting physical exercise and training on lipid metabolism. In: Metabolic adaptation to prolonged physical exercise (H. Howald, J. R. Poortmans, ed.), pp. 156-193. Basel: Birkh/iuser 1975 Pirnay, F., Lacroix, M., Mosora, F., Luyckx, A., Lefebvre, P.: Glucose oxidation during prolonged exercise evaluated with naturally labeled 13C-glucose. J. appl. Physiol. (in press) Shreeve, W. W.: Potential uses of 13C-labeled carbohydrates in the study and diagnosis of diabetes mellitus. Proceed 1st Int. Conf. Stable Isotopes, May, 1973, Argonne, III., USAEC Conf. 730525 Smith, B., Epstein, S.: Two categories of 13C/12Cratios for higher plants. Plant Physiol. 47, 380-384 (1971) Wahren, J., Felig, P., Ahlborg, G., Jorfeldt, L.: Glucose metabolism during leg exercise in man. J. clin. Invest. 50, 2715-2725 (1971) Wahren, J. P., Felig, P., Hagenfeldt, L., Hendler, R., Ahlborg, G.: Splanchnic and leg metabolism of glucose, free fatty acids and amino acids during prolonged exercise in man. In: Metabolic adaptation to prolonged physical exercise (H. Howald, J. R. Poortmans, ed.), pp. 144--153. Basel: Birkh/iuser 1975 Wilkerson, H. L. C.: Diagnosis, oral glucose tolerance tests. In: Diabetes mellitus: Diagnosis and treatment, pp, 31-34. New York: American Diabetes Association 1964 Young, D. R., Pelligra, R., Shapira, J., Adachi, R. R., Skrettin-Gland, K.: Glucose oxidation and replacement during prolonged exercise in man. J. appl. Physiol. 23, 734-741 (1967) Received October 20, 1976
Europ. J. appl. Physiol. 36, 247-254 (1977)
Applied Physiology and Occupational Physiology c by Springer-Verlag 1977
Effect of Glucose Ingestion on Energy Substrate Utilization during Prolonged Muscular Exercise F. Pirnay, M. Lacroix, F. Mosora, A. Luyckx, and P. Lefebvre Institut Provincial E. Malvoz; Department of Atomic and Molecular Physics and Division of Diabetes, Institute of Medicine, Universityof Liege, B-4020 Liege, Belgium
Summary. The distribution of substrates utilized during prolonged exercise was investigated in normal human volunteers with and without ingestion of 100 g exogenous glucose. The energy provided by protein oxidation was derived from urinary nitrogen excretion and the total energy provided by carbohydrates and lipids was calculated from respiratory quotient (RQ)determinations. The contribution of exogenous glucose to the energy supply was determined by an original procedure using "naturally labeled ~3C-glucose" as metabolic tracer. Protein oxidation provided between 1 and 2% of the total energy requirement; this amount was not affected by glucose ingestion. In the absence of exogenous glucose ingestion, carbohydrate were progressively replaced by lipids as source of energy. Exogenous glucose contributed markedly to total carbohydrate oxidation and decreased the percentage of energy derived from lipids. In addition, ingestion of exogenous glucose resulted in a significant economy of endogenous carbohydrates and permitted to prolong the duration of exercise. Key words: Exercise -- Respiratory quotient - Fuels -- Stable isotopes. Introduction
During muscular exercise, the respiratory quotient (RQ) significantly exceeds its value at rest, a finding suggesting that carbohydrates serve as a major energy substrate in that condition. However, more detailed analysis of the respiratory quotient data suggested that the muscle not only uses carbohydrates for its aerobic metabolism, but also oxidizes lipids (Dill et al., 1932; Christensen and Hansen, 1939). New techniques, using radio-active isotopes (Friedberg et al., 1960; Issekutz et al., 1965, 1966; Paul, 1970, 1975) or vascular catheterizations (Carlson and Pernow, 1959; Wahren et al., 1971) have given formal proof, with regard to carbohydrates as well as to lipids, of increased mobilization and oxidation during muscular contractions. The respective contributions of the various substrates utilized is a controversial matter. It depends mainly on intensity and duration of the work (Keul et al., 1972;
248
F. Pirnay et al.
W a h r e n et al., 1975). It is know that the level of body stores, as well as the nature of the food ingested during the preceeding days, clearly modify the energy sources (Bergstrrm et al., 1967). O n the other hand, few investigations have been made on the immediate effect of glucose intake during the actual exercise. Dill et al. (1932) observed that the running time of a dog on a treadmill was much lengthened when it was given glucose. Christensen and H a n s e n (1939) also noted that an exhausted subject can prolong his effort on a bicycle, if he ingests 200 g of glucose. Prolongation of the effort was attributed to the recovery of a normal blood glucose level. The problem of the immediate utilization of ingested glucose is still debated. Our aim was to study the influence of an oral load of 100 g of glucose on the distribution of substrates utilized during prolonged exercise, and to evaluate the possible contribution of the exogenous glucose to the energy supply.
Material and Methods
Principles The energy provided by protein metabolism was calculated by multiplying the quantity of urinary nitrogen by its caloric coefficient. The bladder was emptied before exercise; all the urinary output produced during and immediately after exercise was collected. The amounts of carbohydrates and of lipids oxidized were calculated from the non-proteic RQ values using classical tables (Lusk, 1928). Exogenous glucose consumption was measured using as metabolic tracer a "naturally enriched laC-glucose". Due to isotopic effects during photosynthesis, some plants such as maize and sugar-cane contain slightlyhigher quantities of 13C than other plants (Smith and Epstein, 1971). As a consequence, the animals who feed on these substances, partially reproduce the ~3C enrichment in their expired CO2. Thus, naturally enriched ~3C-glucosecan be used as tracer for metabolic studies in animals and in man (Duchesne et al., 1973; Lacroix et al., 1973; Shreeve, 1973; Lacroix and Mosora, 1975). The ratio of isotopes 13 and 12 of carbon, measured in the expired CO2, enabled us to calculate the proportion of CO2 provided by the ingested glucose. The total amount metabolizedin the body requires, in addition, the determination of the total volume of expired CO2. The details of the method have been recently published (Mosora et al., 1976).
Techniques The expired CO2 and 02 consumption were continuously measured in open circuit. The subject breathed atmospheric air from two Tissot gasometers which determined the volume of ventilated air. He was alternately connected to one gasometer or the other by means of magnetic valves controlled by a time clock. The expired air was then conveyed by a weak resistance expiratory valve, through a mixer, before flowingout. After coldtrapping of the water vapour, a sample of expired air was analyzed for its O2 content, using a paramagnetic analyser (Servomex). COz concentration was measured with an infrared analyzer (Godart Capnograph). A manometer, set in parallel, constantly controlled any variation of pressure, and a flowmeter, fitted to each analyzer, ensured constant flows in the analysis circuits. Both analyzers were calibrated, after measurement, with three gases of known concentration. The respiratory quotient was obtained by the ratio between the expired CO2 and 02 consumption determined continuously and simultaneously.The exogenous glucose utilizedwas measured as from the total volume of expired CO2 and from the 13C/12C ratio, by the technique devised by Mosora et al. (1976).
Glucose Ingestion and Muscular Exercise
249
To assess the 13C/12Cisotopic ratio, a sample of expired air was collected in a rubber bag of about 1 1. The isotopic ratio was measured by means of a high sensitivity mass spectrograph with a double collector (Varian/Mat CH5) which compares peak height ratios of masses 45 and 44 of the CO2 extracted from the tested air with a standard sample of CO2. The heart rate was measured from a thoracic lead of the electrocardiogram. Every 15 rain, blood was withdrawn through a soft catheter set in an antecubital vein for blood glucose determination using the method of Hoffman (1932) adapted to the Technicon Auto Analyzer.
Muscular Exercise The subjects, fasted overnight, walked on a 10% uphill treadmill. Speed was constant and set between 4 and 5 km/h. In these conditions, oxygen consumption averaged about 50% of the individual maximum. The exercise required an 02 consumption of 1.8-2.1 l/min, while the heart rate varied between 144 and 162 beats/min. After a 15 rain exercise adaptation period and without stopping walking, the subjects ingested, in I - 2 min, 100 g of "naturally enriched 13C-glucose"diluted in 400 ml of water. We have used anhydrous glucose prepared from maize (Glucopur, Glucosuries Reunies, Alost, Belgium). Melting point determinations showed that this glucose was partially hydrated and calculations indicated that the amount of pure glucose in the load given was in fact 95 _+ 1 g (mean + SD; 4 determinations).
Subjects Seven volunteer male students, accustomed to physical exercise but without particular training were investigated. They were between 21 and 25 years old, and had a mean body weight close to their ideal body weight: 103 + 2.4% [Metropolitan Life Insurance Company (1959)]. A preliminary oral load test of I00 g of glucose showed, for all subjects, normal glucose tolerance (Wilkerson, 1964) and no glycosuria.
Results The distribution of energy substrates utilized during exercise is shown in Figure 1. Protein oxidation, as evaluated from urinary nitrogen excretion, was very low and provided between 1 and 2% of the total energy. A t the beginning o f the exercise, the contributions of lipids and o f c a r b o h y d r a t e s in the metabolism were almost equal. A s the test was on without glucose ingestion, c a r b o h y d r a t e s were gradually less utilized. Their contribution decreased from 43.9 to 18.7% after 4 h o f exercise. The energy was then mainly supplied by lipid oxidation. The same Figure 1 illustrates, throughout the exercise, the influence exerted by the ingestion o f 100 g o f glucose. Distribution o f energetic substrates utilized for aerobic metabolism was clearly modified. The ingested glucose rapidly participated in oxidations. Fifteen minutes after its ingestion, the expired air was already enriched in 13C. Utilization of exogenous glucose increased during the first hour, reached a m a x i m u m 6 0 - 9 0 min after its ingestion, then progressively decreased. The contribution o f exogenous glucose to energy supply is quantitatively important, since it m a y provide as much as 27.2 + 7.4% of the total energy. The ingestion of glucose simultaneously increased the total energy made available from carbohydrates. After glu-
%
I
pr~
[---] ['pids M end~176 ~ ex~176 carbohydrates
100.
gtucose
-r~
i +
80
2 el_ cl_
~ 60 cn
~ 40c LI
"
I
20-
EXERCISE BEFORE GLUCOSE
0
30
60
90
120
150
180
210
I
I
I
I
I
I
I
I
30
60
90
120
150
180
210
240
TIME {min)
Fig. 1. Respective contributions of the various energy fuels during prolonged muscular exercise. For each period, the left column corresponds to the control test effected without any glucose ingestion while the right column corresponds to the test effected after the oral ingestion of glucose at time 0. The two columns to the far left represent data obtained during the last 5 min of the exercise-adaptation period. Results are exposed as mean + SD (standard deviation). The same three subjects were investigated under both experimental conditions. During the last period (210-240 rain), two out of the three subjects were exhausted when exercising without glucose ingestion. For that period, the left column corresponds to the value recorded in the only subject and is not figured. When given glucose, all three subjects completed the program
o,o
I
pr~
[ipids
end~167176
exogenous
carbohydrates~
gtucose
100-
80(D_ CL
-
60-
+I ++ +++ ~
cI.U
20"
T
T
......... i .......... ,i~!)i!ii! /!:iSS:. ~,:i~i!!iii +~++ X'X-X'3:
......... EXERCISE BEFORE GLUCOSE
i:!:i:!:!:i!:!i.;:;:;:;:;:;:;:;:.: iiiiiiiii
0
30
60
90
I
I
I
I
30
60
90
120
.'2"2"X.X'2. 9...,.........+
120 i 150
X'X-X'X!-X' r.,;.....+... " :-2"X-X! ! ii!iiii : 150 i 180
180 I 210
210
TIME
240
(rain)
Fig. 2. Respective contributions of the various sources of energy during muscular exercise after 100 g glucose intake. Means and standard deviations of seven experiments are shown. The first column to the left represents data obtained during the last 5 min of the exercise-adaptation period
Glucose Ingestion and Muscular Exercise
251
cose ingestion, the contribution of carbohydrates to aerobic metabolism always remained superior to that observed under control conditions. Total carbohydrate oxidation increased temporarily during the first 2 h after glucose intake, before gradually decreasing. The part played by exogenous glucose in energy supply was confirmed in a greater number of subjects whose results are shown in Figure 2. Slight individual variations in the evolution of the phenomena were observed, but the dispersion of the results was rather weak. In all subjects, the ingested glucose participated in cellular oxidations, and permitted total carbohydrate oxidation to provide between 50% (first hour) and 30% (last hour) of the total energy requirements.
Discussion
The respective contributions of the various substrates utilized during long-term muscular exercise, correspond to the results reported in the literature (Keul et al., 1972; Wahren et al., 1975; Paul, 1975). In fasting individuals, the proportion of the energy supplied by lipid oxidation gradually increased in prolonged exercise. In our investigations, it reached more than 80% during the fourth hour of the exercise. This value is in good agreement with the 62% measured by Ahlborg et al. (1974) in the active leg, as well with Paul's results (1975), who reported that lipid contribution may reach 80-90% of the energy requirement. In addition to the duration of the exercise, its intensity modifies the respective contributions of the substrates utilized. Light exercises mobilize greater quantities of lipids, while intense efforts preferentially use carbohydrates (Keul et al., 1972). Lipid participation increased in well trained individuals (Paul, 1975) or after a low carbohydrate diet (Christensen and Hansen, 1939; Bergstr6m et al., 1967). Glucose ingested during exercise modifies the distribution of oxidized substrates. Using "naturally labeled 13C-glucose" we have defined the early an important contribution of ingested glucose in muscular metabolism (Pirnay et al., 1977). The large utilization of exogenous glucose is consistent with the results of Benade et al., (1973) who gave a 14C-sucrose load. These authors reported that the participation can reach 44.2% of the carbohydrate metabolism, and started 10 min after the ingestion of the labeled sucrose. Under other experimental conditions however, Costill et al. (1973) reported that the contribution of exogenous glucose to total carbohydrate oxidation appeared smaller, but these authors restricted their analysis to a 1 h period after the ingestion of 32 g of glucose. Glucose intake induced a parallel increase in total carbohydrate metabolism and a corresponding decrease in lipid oxidation. This modification in fuel utilization is maximal during the second hour, when it reached 10%. The increase in total carbohydrate metabolism, however, remained lower than the amount of energy made available from exogenous glucose, thus sparing endogenous carbohydrates. As shown in Table 1, the quantities of endogenous carbohydrates oxidized during exercise were lower when the subjects were given glucose during the test. This sparing effect was already obvious 30 min after glucose intake and remained significant until the 150th rain. It represented 0.3-0.5 g of glucose per minute. In the first 2 h, cumulative endogenous glucose consumption was evaluated at 118.6 and 79.3 g respectively in control and glucose-given subjects. When the test was prolonged, the
F. Pirnay et al.
252 Table 1. Carbohydrate oxidation Period min
Without glucose ingestion
With glucose ingestion (100 g)
Endogenous glucose
Total
g/30 rain
Exogenous
Cumul (g) g/30 rain
Cumul (g) g/30 min
Endogenous Cumul (g) g/30 rain
Cumul (g)
0 30
M SD
31.9 4.3
31.9 4.3
33.3 5.3
33.3 5.3
4.7 1.9
4.7 1.9
28.6 5.3
28.6 5.3
30 60
M SD
31.2 3.0
63.1 8.4
36.7 5.6
69.9 10.8
16.1 5.0
20.8 6.8
20.5 3.5
49.1 7.4
60 90
M SD
30.4 1.6
93.6 14.2
35.5 6.1
105.4 16.1
19.4 5.2
40.2 11.2
16.1 2.6
65.2 11.4
90 120
M SD
25.0 5.1
118.6 21.1
30.8 7.3
136.2 22.9
16.6 1.9
56.9 12.8
14.2 5.8
79.3 16.2
120 150
M SD
20.9 7.2
139.6 28.7
28.4 5.3
173.6 34.6
12.9 4.2
74.9 9.1
15.5 9.5
98.6 18.4
150 180
M SD
17.7 6.6
157.3 31.4
26.1 2.0
199.8 36.6
9.3 5.8
84.3 5.1
16.8 7.8
115.6 24.6
180 210
M SD
14.3 3.5
171.6 38.2
24.2 3.8
223.8 40.3
6.5 3.6
90.7 4.7
17.6 7.2
133.1 26.2
210 240
M
(13.6)
(185.2)
18.6 6.5
242.4 46.6
4.1 2.0
94.8 4.2
14.5 8.5
147.6 33.6
Results are given as mean (M) + standard-deviation (SD)
m
/100 mi
BLOOD
GLUCOSE
120-
IO0
- .....
w i t h glucose without
glucose
80
60
(I
30
6'0
90
120 150 I~0 2'10 2'/.0 TIME {min)
Fig. 3. Mean blood glucose concentrations of three subjects during exercise with and without glucose intake
influence o f glucose ingestion g r a d u a l l y decreased, but persisted for 3 h. It should be p o i n t e d out t h a t these calculations o f the e n d o g e n o u s c o n t r i b u t i o n o f glucose in e n e r g y supply are a p p r o x i m a t e . I n fact, t h e y neglect other p a t h w a y s o f glucose f o r m a t i o n and b r e a k d o w n , w h i c h m a y be i m p o r t a n t in m u s c u l a r exercise ( Y o u n g et
Glucose Ingestion and Muscular Exercise
253
al., 1967; Wahren et al., 1975). Despite these restrictions, it appears as if the contracting muscles used a lesser amount of their carbohydrate stores. Normally, the muscular glycogen level strongly decreases during long-term effort, and its depletion m a y be the main limiting factor (Bergstrrm et al., 1967; Hultmann et al., 1967, 1971; Costill et al., 1971). Ingestion of glucose seems to delay this limit. In our investigations, it enabled to prolong their exercise two subjects out of three, thereby confirming the results of others (Dill et al., 1932; Christensen and Hansen, 1939; Bergstrrm et al., 1967). Simultaneously, blood glucose level (Fig. 3) gradually decreased in fasting exercising subjects and showed a mean value of 65 mg/100 ml at the end of the test. It constantly remained at a higher level throughout exercise when exogenous glucose was given. Both mechanisms, sparing of endogenous carbohydrates and maintenance of a higher blood glucose level, m a y contribute to enhance tolerance to prolonged effort. Early and high oxidation of exogenous glucose as shown in this study, warrants carbohydrate ingestion during long-term exercise.
Acknowledgements. This work was supported by the Fonds National de la Recherche Scientifique (F.N.R.S.), the Fonds de la Recherche Fondamentale Collective (F.R.F.C.) and the Fonds de la Recherche Scientifique Mgdicale (F.R.S.M.) of Belgium and by a grant-in-aid of Chemic Griinenthal GmbH (Federal Republic of Germany).
References
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