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Lysine Requirement of the Growing Rat: Plasma-free Lysine as a Response Criterion^1,2,

Department of Animal Science, University of Minnesota, St. Paul, Minnesota 55101

Five experiments were conducted to reevaluate the lysine requirement of the growing rat and to determine the effect of different feeding methods on the concentrations of plasma free lysine when diets containing graded levels of lysine were fed to growing rats. The lysine requirement for maximum weight gain/feed was 0.60% of the diet containing equivalent of 10% dietary protein (N x 6.25). If rats were fed for 1 hour every 12 hours for 17 days, and if blood was collected 6 hours after the last feeding, plasma free lysine remained at a low and constant level until dietary lysine was increased to the level that supported maximum gain and gain/feed, and it increased rapidly and linearly as dietary lysine was increased beyond this level. If rats were fed ad libitum, and blood was collected 6 hours post feeding, a similar pattern was evident for plasma free lysine, but considerable variation was evident in the increases. If rats were fed ad libitum, trained to eat twice daily for 2 days or fasted 12 hours and then fed for a 1-hour period, and blood was collected 6 hours post feeding, plasma free lysine started to increase before dietary lysine was adequate to support maximum gain and gain/feed, especially just prior to or at the level that was adequate. Following the large increase in plasma free lysine when dietary lysine was adequate, relatively smaller increases resulted as dietary lysine was increased beyond this level.

² A preliminary report of these results was presented at the annual meeting of the American Society of Animal Science, Stillwater, Oklahoma, July 29, 1968 (J. Anim. Sci. 27: 1156 (abstr.)).

³ NIH Trainee in Nutrition; supported by NIH Training Grant GMO 1041 from the National Institute of General Medical Science.

Manuscript received 28 January 1970.