Br. vet.

J.

(1975),

131 ,

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FOOD INTAKE REGULATION IN PIGS. IV. THE INFL UENCE OF DIETARY THREONINE IMBALANCE ON FOOD INTAKE, DIETARY CHOICE AND PLASMA ACID PATTERNS By D. W.

ROBINSON

Department of Animal Science, University oj California, Davis, California 956 16 SUMMARY

The response of pigs to threonine imbalance diets was a rapid and marked depression in food intake. Plasma threonine values showed rapid depression due to the imbalance. Both food intake and plasma threonine values were restored by dietary supplementation of threonine. When given a choice between threonine imbalance and protein-free diets pigs chose to eat the protein-free diet. INTRODUCTION

It has been well established that amino acid imbalances and deficiencies will depress growth in pigs (Robinson & Lewis, 1963). Few studies have attempted, however, to separate the growth depression due to impaired metabolism from the primary, and probably major, effect which is a food intake depression in ad libitum fed pigs. In laboratory animals attention has been drawn to the food intake depressing effects of amino acid imbalance (Waddell, 1958) and to the speed with which this depression is manifest (Harper & Kumta, 1959). Animals force fed amino acid imbalanced diets grow well (Leung, Rogers & Harper, 1968) and it is therefore clear that the primary effect of an imbalance is upon food intake and not growth. Several factors have been shown to influence the response of imbalanced diets, notably the level of protein in the diet preceding the imbalance. Both high protein and protein-free diets cause a delayed response relative to a low protein diet (Peng, Benevenga & Harper, 1969). The present study was designed to investigate the food intake response of pig to diets imbalanced with respect to threonine, the plasma amino acid patterns resulting from the feeding response, and finally the ability of pigs to detect and select ag:.1inst a threonine imbalance. MATERIALS AND METHODS

Fifteen Duroc gilts were allocated to three experimental diets (five animals per treatment) shown in Table I. These consisted of a "Basal" (B) synthetic diet of 4 per cent protein, an "Imbalanced" (I) amino acid diet consisting of the

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BRITISH VETERINARY JOURNAL, 13 1, 5 TABLE I PERCENTAGE COMPOSITION OF BASAL IMBALANCED AND CORRECTED DIETS

Ingredients Sucrose Corn starch Casein Cellulose Vegetable oil Dicalcium phosphate Sodium chloride Min. + vito mix* L-threonine DL-methionine L-tryptophan L-leucine L-isoleucine L-valine L-histidine-HCl L-phenylalanine L-lysine-HCl L-arginine-HCl Sodium bicarbonate

Basal

Imbalanced

48.80 30.00 4.00 8·00 3.00 3.70 0.50 1·80

45" 25 26·00 4.00 8·00 3.00 3.70 0.50 1·80

0·20

0·55 0·24 0·84 1·20 1.30 0.36 0·54 0.90 0.30 1·02 100·00

100·0

Corrected 44.80 26·00 4.00 8·00 3.00 3.70 0.50 1·80 0·45 0·55 0·24 0·84 1·20 1.30 0.36 0·54 0.90 0.30 1·02 100·00

* Contained per kilogram of feed: Minerals: K 4.08 g; S 2·24 g; Mg 0·4 mg; Mn 0·12 g; Zn O·II g; Fe 0.31 g; Cu 0·006 g; I 0.0024 g; Co 0·0004 g; Mo 0·0001 g; Se 0·0001 g; Fl 0.064 g. Vitamins: A 6750 U.S.P.; D22250 U.S.P. Menandione 5"6 mg; Vitamin E 2·5 i.u.; Riboflavin 9 mg; Thiamin HC14·5 mg; Panthothenic acid 16·7 mg; Niacin 45.1 mg; Choline Chloride 1225 mg. Antibiotic: Bacitracin 22 mg (from bacitracin methylene disalicylate). basal diet plus all the essential amino acids added, except threonine to the level of the N.R.C. (1968) requirements, and a "Corrected" (C) diet consisting of the basal diet plus all the essential amino acids including threonine. All animals were fed the basal diet (B) during a preliminary experimental period to accustom them to the synthetic diet. Eight days after food intake has stabilized, the experiment was started by placing five pigs on the imbalanced diet (I), five on the corrected diet (C), and five on the basal control diet (B). Food intake was recorded daily during the experiment. Blood samples were collected at 48 hand 7 days after exposure to experimental diets for the assay of plasma amino acid patterns. Following an initial test period of 8 days on the three diets during which a clear-cut food intake response was observed, a further period of study involved observing the food intake response, and dietary choice when pigs were offered a choice of protein-free (PF) and imbalanced (I) diets; corrected (C) and imbalanced (I ), and a choice between protein-free (PF) and corrected (C). Blood samples were drawn from the carotid sinus after 48 hand 7 days of exposure to the experimental diets. Samples from five pigs in each group were pooled for amino analysis, and consequently statistical differences between the treatments were not computed. Analytical methods were by methods similar to those of Stein & Moore (1954).

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RESULTS AND DISCUSSION

Within 24 h a clear-cut food intake response to the experimental diets was seen (Fig. I) in which the imbalanced diet produced a highly significant (P < 0·01) fall in food intake and the corrected diet a significant (P < 0·01) rise in food intake, in response to the additional essential amino acids which effectively raised the dietary protein from 4 to almost 12 per cent. The slight depression in food intake on the basal diet was due to the start of the bleeding schedule and would presumably have been similar in all groups. Values for all the essential amino acids rose in the plasma of pigs fed the corrected diet (C) by comparison with those fed the basal diet (B) (Table II). In the plasma of pigs fed the imbalanced diet (I) all the essential amino acid values rose with the exception of threonine which fell to very low levels of 4.8 fLmol/IOo ml compared with 11·8 (B) and 45.6 (C). After 7 weeks on the diets the plasma amino acid patterns remained essentially unchanged. Following 7 days on the imbalanced diet some pigs were placed for 5 days on the corrected diet and as can be seen from the last column (I -> C) plasma threonine values rapidly returned to normal.

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I

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Fig. I. The voluntary food intake and diet preference of pigs offered a protein-free threonine, threonine imbalanced diet, threonine corrected and basal diet combinations. • Threonine corrected, 0 protein-free diet, 0 threonine-imbalance. 4-

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BRITISH VETERINARY JOURNAL, 13 1, 5 TABLE II

PLASMA-FREE AMINO ACIDS FROM PIGS FED A BASAL 4 PER CEN T CASEIN DI ET (B), AN IMBALANCED DIET WITH RE SPECT TO THREONINE (I) , AND THE IMBALANCED DIET WITH THREONINE ADDED, I.E, A CORRECTED DIET (C), ALL VALUES IN MICROMOLES PER 100 ML OF BLOOD

Time of sampling diet 48 hours

Thr Ser Gly Ala eys Val Met Iso Leu Tyr Phe Lys His NH3 Arg

7 days

B

I

C

B

I

C

I--+C

11,8 4 2'0 73'2 80'0 3 2'8 3 2'6 98 '6 5,8 16,8 8,8 10'4 32 '2 16,8 37'0 5,6

4,8 43'0 39'4 57'0

45'6 4 1,6 44,8 68,8 115'0 146 '0 66'0 36 '4 45'6 23'2 67'6 12 ,8 26'4 8,6

6'2 4 2 '0 44,8 53'2 25'2 76 '0 124'0 40 '0 29'2 32 '8 33'2 60'0 30 '0 29'0 15,6

45'6 44,8 55'6 81,6

76 '4 126,6 44'4 33'0 34'6 23'8 63'2 13'4 50 '4 13'8

13'4 44,8 72 '2 78 '0 10,8 26'0 122'0 8,6 13,8 11,8 9,8

63'0 44,6 45'4 72 '8 16,8 13 1'0 158'0 65'8 3 1'2 42 '4 19'4 62 '4 14,6 27'4 8'7

93'0 146'0 5 2 '0 28,6 37'8 24'0 55'4 18'4 25'0 6,6

In the second part of the experiment an immediate voluntary rejection of the imbalanced diet was made and preference given to a protein-free diet (Fig, 2), When the protein-free diet was replaced by the corrected diet a clear preference for the corrected diet was made and food intake increased rapidly in response to the effectively higher protein level of the corrected diet, When the corrected diet was replaced with the protein-free diet there was an initial but short-lived preference for the imbalanced diet and a rapid switch to the protein-free diet. Offering the corrected diet in place of the imbalanced diet resulted in a gradual change to the corrected, On day 23 the protein-free diet was removed so that pigs were forced to eat the corrected diet, Where the protein-free diet was then returned it appeared that a clearer choice for the corrected diet was being made as a result of the one day of "training", These results demonstrate that the pig responds in an identical way to the laboratory rat, which has been shown to respond not only to threonine imbalance (Leung, Rogers & Harper, 1969) but also to histidine and tryptophan and isoleucine (Leung & Rogers, 1969), with marked reduction in food intake, Also there is almost complete preference for a protein-free diet which would not support life, over an imbalanced, diet, which if eaten (as when force fed) would support life, Harper, Benevenga & Wohleuter (1970) has suggested that the dramatic response to amino acid imbalances has two components, first, a component which associates ingestion of an imbalanced diet with an undesirable effect in the animal, which is unlikely to be taste or smell because the omitted amino

FOOD INTAKE REGULATION IN PIGS. IV

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25

c

.2

Ci. E

§

15

u

'8 ~

10

1

I

I

I

I

I

2

4

6

8

10 12 Days

I

I

I

I

I

14

16

18

20

Fig. 2 . The voluntary food intake of pigs fed either a basal, threonine imbalanced, or threonine corrected diet . • Basal diet, 0 threonine imbalance, • threonine corrected.

acid constitutes such a small component of the large number added to the corrected diet. Second, a component which is a learned response in which selection of the protein-free diet is made possible by recognition of some differences between the two diets offered. This learned response might easily use taste, smell, or the position of the feed trough as a cue once the initial choice has been made. Precisely what the mechanism is which leads to such a rapid rejection of an imbalanced diet is not clear at the present time. The elegant work of Leung & Rogers (1969) has clearly demonstrated however, that centres in the central response system are specifically sensitive to the plasma amino acid patterns since infusions of small amounts of the missing amino acid from an imbalanced dietary mixture will prevent the food intake depression when infusions are made via the carotid artery but not when infused through the jugular vein. ACKNOWLEDGEMENTS

This study was financed by Grant No. 427 from the Nutrition Foundation, New York, and by generous donations from the Tanabe-Seiyaku Co. Ltd, of Osaka, Japan. The expert technical assistance of Karen Hazelwood, Mike Broderick and Jim Robb is gratefully acknowledged.

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REFERENCES HARPER, A. E., BENEVENGA,j. & WOHLEUTER, R. M. (1970). Physiol. Rev. 50,428. HARPER, A. E. & KUMTA, U. S. (1959). Federation Proc., 18, 1136. LEUNG, P. M. B., ROGERS, Q. R. & HARPER, A. E. (1968). J. Nutr. 95,474. LEUNG, P. M. B., ROGERS, Q. R . & HARPER, A. E. (1969). J . Nutr. 95,483. LEUNG, P. M. B. & ROGERS, Q. R. (1969). Life Sci. 8, Part II, p. I. NATIONAL RESEARCH COUNCIL. (1968). Nutrient requirements of domestic animals. No.2, Nutrient Requirements of Swine. Washington, D .C. National Academy of Sciences. PENG, Y. N., BENEVENGA,j. & HARPER, A . E. (1969). Am. J. Physiol. 216,1020. ROBINSON, D. W. & LEWIS, D . (1963). J. Sci. Fd. Agric. 14,806. STEIN, W. H. & MOORE, S. (1954). J. Biol. Chem. 211,915. WADDELL, j. (1958). In Processed Plant Protein Foodstuffs, Chap. 13, p. 307, ed. A. S. Altschul. " New York: Academic Press. (Accepted for publication 7 June 1973)

Le controle de I'absorption alilnentaire chez Ie pore. IV. L'influence du desequilibre alilnentaire de la threonine sur I'absorption, Ie choix alilnentaire et les groupes acidiques du plasma (Robinson) ResUDle. La n!ponse de pores envers des regimes avec desequilibre en "threonine se traduisit par une baisse rapide de I'absorption alimentaire. Les valeurs de la threonine du plasma montn!rent une baisse rapide due an desequilibre. Et I'absorption alimentaire et les valuers d e la threonine du plasma furent retablies apres un apport de threonine. Devant Ie choix entre un regime avec desequilibre en threonine et un regime sans proteines, les pores choisirent Ie regime sans prottines. Regulierung der Nahrungsaufnahme bei Schweinen. IV. Einftuss von Threoninbilanzstorungen auf NahrungsaufnahDle, Diatwahl und Saureverhaltnisse iIn PlasIDa (Robinson) ZusaDlDlenfassung. Die Reaktion von Sehweinen auf eine Diiit mit nicht ausbalanziertem Threoningehalt bestand in einem rapiden Zurtickgehen der Nahrungsaufnahme. Threoninwerte im Plasma zeigten ein rapides Absinken infolge der Bilanzstiirung. Nahrungsaufnahme und Threoninwerte wurden durch Zusatz von Threonin zur Diiit wieder ins Gleichgewieht gebracht. Wenn die Tiere zwischen Nahrung, die zu Threonin-mangel ftihren wtirde, und proteinfreier Nahrung wiihlen konnten, frassen sie das proteinfreie Futter. Regulacion de la ingestion de alilnentos en el cerdo. IV. La infIuencia del desquilibrio de la treonina dietaria sobre la ingestion de alilnentos, eleccion de dietas y patron de los licidos plasDlliticos (Robinson) ResUDlen. La respuesta de los cerdos a las dietas con desequilibrio de treonina fue una nipida depresion de la ingestion de alimentos. Los valores de treonina plasmatiea mostraron una rapida depresion debido al desequilibrio. Se restauraron tanto la ingestion de alimentos como los valores de treonina plasmatiea mediante el suplemento dietario de treonina. Cuando se presento una eleccion entre las dietas con desquilibrio de treonina y libres de protein as, los cerdos eseojieron la dieta libre de proteinas.