Tryptophan Utilization in a Threonine-induced Amino Acid Imbalance in Weanling Rats: Plasma amino acid and liver pyridine nucleotide concentrations

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Abstract
Growth studies with niacin-free, low casein diets, to which threonine had been added, indicated that this amino acid-imbalanced mixture could be well-utilized when the protein was fed separately but not when the entire diet was fed ad libitum. In the present experiment 4 treatments were used involving both ad libitum and separate feeding of a balanced protein (casein) and an imbalanced protein mixture (casein with added threonine). Poor growth occurred whenever the imbalanced protein was fed ad libitum, regardless of pre-treatment of the animal. Since a growth depression resulted when animals were fed the imbalanced diet ad libitum following a week of good growth with the same protein mixture fed separately, there appeared to be no adaptation to the imbalanced mixture per se. The differences in plasma amino acid patterns did not explain the differences in the growth and acceptability of the imbalanced protein by the 2 feeding methods. Plasma threonine was elevated in animals fed the imbalanced diet regardless of the growth of the animals. Plasma tryptophan did not appear to be depressed more than other amino acids, and the lowest plasma amino acids were observed with the group fed the imbalanced mixture separately in which good growth had occurred. No differences in liver pyridine nucleotide concentrations were observed between treatments at either of the 2 time-intervals tested to suggest an alteration in the tryptophan-niacin pathway. It appeared that use of a feeding method which ensured ingestion of larger quantities of protein per feeding may have improved the utilization of amino acids from an imbalanced mixture.