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Tryptophan utilization in a threonine-induced amino acid imbalance in weanling rats: gain in weight and carcas nitrogen with two feeding methods



Tryptophan utilization in a threonine-induced amino acid imbalance in weanling rats: gain in weight and carcas nitrogen with two feeding methods



J. Nutrition. 89: 149-157



Diets containing 8% vitamin-free casein sup plemented with DL-methionine (control) or with DL methionine, DL-threonine, DL-phenylalanine, L-lysine and L-histidine (imbalanced) were given to weanling rats in experiments lasting for 2 weeks; the latter diet was known to depress growth in rats because of a threonine imbalance. In one series of experiments diets were given freely and the effects of tryptophan and nicotinic acid supplements on growth and tryptophan utilization were studied. In a second series of experiments the protein portion of the two diets was given twice a day so that the casein intake from each source was the same, the remainder of the diet being given freely; the effects of protein intake and of tryptophan and nicotinic acid supplements on growth, tryptophan utilization and carcase N were studied. In the first series, supplementing the imbalanced diet with 0.02% tryptophan increased weight gain from 10 g to 28 g, a value almost identical with that in the control group, 27 g. Weight gains were greater at higher levels of tryptophan supplementation, food intake also increased progressively. Weight gains on the imbalanced diet increased progressively as dietary protein was raised from 9 to 13.5%; at the higher level gains were similar to those of the control rats. The addition of 2.5 mg nicotinic acid per 100 g to the imbalanced diet increased growth at each protein intake. Better weight gains were made with the imbalanced than with the control diet when the protein was given separately from the rest of the diet, particularly at low protein intake and with a nicotinic acid supplement. Carcase analyses showed that protein was somewhat better utilised from the imbalanced than from the control diet; with the former diet, but not the latter, a nicotinic acid supplement caused an increase in body N. It is concluded that the threonine imbalance can be alleviated by supplements of tryptophan or nicotinic acid or by giving the protein separately from the rest of the diet; the alleviation resulting from the last method may be caused by differences in the metabolic pathways of tryptophan.

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Accession: 014790443

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PMID: 5912948


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