Abstract
Amino acid imbalances refer to the deleterious effects that occur when a second-limiting amino acid or mixture of amino acid lacking a particular limiting amino acid is supplemented in diets marginal in one or more indispensable amino acids. In spite of variation in the conditions that have been used to induce amino acid imbalances, such as protein level in the diet, the extent of difference in total nitrogen content between basal and imbalanced diets, and kinds of amino acids used as imbalancing agents, the conspicuous common features of amino acid imbalances have been a decreased concentration of the limiting amino acid in blood, depression of feed intake and weight gain, and increased dietary content of the limiting amino acid needed to correct the imbalances. There is strong evidence that a decrease in the concentration of a limiting amino acid detected in the anterior prepyriform cortex of the brain is followed by behavioral effects, especially a decrease in feed intake. This might be due to the competition between the limiting amino acid and the amino acids in the imbalancing mixture for transport from blood into brain. One of the biochemical responses of animals fed amino acid imbalanced diets is a rapid decrease in the concentration of the limiting amino acid, which are due in part to an increase in catabolism of the limiting amino acid by the increased activities of enzymes involved in the catabolism of the amino acid. Practically, specific amino acid imbalances could be induced in swine and poultry diets that have been supplemented with lysine, methionine, tryptophan when threonine, isoleucine, valine, etc. are potentially third- or fourth-limiting in diets. In these cases supplementation of the limiting amino acid could be beneficial in preventing the decrease of feed intake that could otherwise occur as a result of amino acid imbalance.
Highlights
The concept of amino acid balance had been introduced by two pioneering researchers, Osborne and Mendel (1914)
Leung and Rogers (1969) were the first to examine the role of the brain in recognizing the imbalances. They observed that the depressed feed intake of rats fed an imbalanced diet could be restored by the infusion of a small amount of the most limiting amino acid into the carotid artery a few hours before feeding the imbalanced diet, whereas a similar amount of the same amino acid infused into the jugular vein had no effect on feed intake
This study suggests that the concentration of limiting amino acid in blood supplying the brain determines the feed intake changes in animals fed amino acid imbalanced diets
Summary
The concept of amino acid balance had been introduced by two pioneering researchers, Osborne and Mendel (1914). They understood that the nutritional value of a protein was dependent upon the proportions of the various indispensable amino acids it contained. Harper (1964) categorized the adverse interaction of disproportionate balance of amino acids as imbalances, antagonisms, and toxicities. According to Harper’s definitions, imbalances refer to the deleterious effects that occur when a second-limiting amino acids or a mixture of amino acids lacking the limiting amino acid is supplemented to diets marginal in one or more indispensable amino acids. The adverse effects of the imbalanced diet may be alleviated by the addition of the most limiting amino acid to the diet
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