Plasma free lysine levels of growing female rats given a lysine-deficient diet supplemented with graded amounts of lysine were higher than those of males. In both sexes, plasma free lysine values rose rapidly in response to added dietary lysine, and reached a maximum at a dietary lysine concentration of approximately 1.0%. This amount of dietary lysine was about 0.2% greater than that found necessary for maximum growth. Male rats given diets containing rapeseed oil and graded amounts of lysine showed reduced growth and lower plasma lysine and threonine levels as compared to animals given similar diets containing corn oil, but the amount of dietary lysine required for maximum plasma lysine levels was not influenced by the growth rate. Plasma free threonine levels showed a reciprocal relationship with those for lysine. It was concluded that measurement of the ratio between plasma free lysine and threonine levels may provide a sensitive indication of the adequacy of the dietary lysine content.
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Morrison et al. (1961) studied this question.
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