Growth and Concentrations of Amino Acids in Plasma of Rats Fed Four Levels of Amino Acids ,

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Abstract
Amino acids in plasma and liver were measured to determine whether they would reflect dietary intakes of weanling rats fed 4 levels of amino acids in the same proportions when blood was sampled at an interval of fasting at which it was least affected by either absorption or starvation. To determine the interval of fasting to be used, animals fed casein were fasted 3, 6, 9 or 12 hours in the first experiment. Plasma obtained after the 6-hour interval appeared to be least affected by absorption or starvation since it contained neither the highest nor lowest levels of most ninhydrin-reacting materials and contained few materials formed during metabolism. In the second experiment diets were formulated to contain 115, 100, 85 or 70% of proposed minimal requirements. Although the nitrogen intake of rats fed the lowest level of amino acids was only 56% of the highest level, only the concentration of lysine in plasma was related directly to intake at all levels, and that of threonine was related at the three lower levels. Plasma concentrations of other amino acids and ninhydrin-reacting materials are discussed in relation to intake and to levels in the liver. Carcass nitrogen was the only parameter of growth that was related directly to intake of all levels of amino acids.