INDIVIDUAL AMINO-ACID BALANCES IN YOUNG LEAN AND OBESE ZUCKER RATS FED A CAFETERIA DIET

The amino acid composition of the diet ingested by reference and cafet eria diet-fed lean and obese Zucker rats has been analyzed from day 30 to 60 after birth. Their body protein amino acid composition was meas ured, as well as the urinary and faecal losses incurred during the per iod studied. The protein actually selected by the rats fed the cafeter ia diet had essentially the same amino acid composition as the referen ce diet. The mean protein amino acid composition of the rat showed onl y small changes with breed, age or diet. Cafeteria-fed rats had a high er dietary protein digestion/absorption efficiency than reference diet -fed rats. Obese rats wasted a high proportion of dietary amino acids when given the reference diet, but not on the cafeteria diet. In all c ases, the amino acids lost as such in the urine were a minimal portion of available amino acids. In addition to breed, the rates of protein accretion are deeply influenced by diet, but even more by the age - or size - of the animals: cafeteria-fed rats grew faster, to higher body protein settings, but later protein accrual decreased considerably; t his is probably due to a limitation in the 'blueprint for growth' whic h restricts net protein deposition when a certain body size is attaine d. Obese rats, however, kept accruing protein with high rates througho ut. Diet composition - and not protein availability or quality - induc ed deep changes in amino acid metabolism. Since the differences in the absolute levels of dietary protein or carbohydrate energy ingested by rats fed the reference or cafeteria diets were small, it can be assum ed that high (lipid) energy elicits the changes observed in amino acid metabolism by the cafeteria diet. The effects induced in the fate of the nitrogen ingested were more related to the fractional protein ener gy proportion than to its absolute values. Cafeteria-fed rats tended t o absorb more amino acids and preserve them more efficiently; these ef fects were shown even under conditions of genetic obesity. There were deep differences in handling of dietary amino acids by dietary or gene tically obese rats. The former manage to extract and accrue larger pro portions of their dietary amino acids than the latter. The effects of both 'models' of amino acid management were largely additive, suggesti ng that the mechanisms underlying the development of obesity did not r un in parallel to those affecting the control of amino acid utilizatio n. Obesity may be developed in both cases despite a completely differe nt strategy of amino acid assimilation, accrual and utilization.