Lysine deficiency alters diet selection without depressing food intake in rats

Under states of protein deficiency, the dietary limiting amino acid, rather than protein content, can act as the dietary stimulus to control diet sele ction. If fact, threonine-deficient rats will alter their diet selection pa tterns solely on the basis of very small changes (0.009 g/100 g) in the die tary threonine concentration. In these studies, we assessed whether lysine- deficient rats will also alter their diet selection patterns on the basis o f small changes in dietary Lys concentration. In all experiments, growing r ats were adapted to diets in which the protein fraction (purified amino aci ds or wheat gluten) was limiting in Lys. They were then given a choice betw een the adaptation diet (AD) diet and a slightly more deficient diet. Rats that were adapted to a Lys-deficient diet (0.25 g Lys/100 g) selected their AD over diets containing as little as 0.01% less Lys (P < 0.01) within 5 d . To determine how deficient rats must be before they alter their selection patterns, rats were adapted to diets containing various levels of Lys, i.e ., 2 levels below the requirement for growth and 2 levels above the require ment for growth, but below the requirement for maximal nitrogen retention. Only rats adapted to diets containing Lys below their requirement for growt h selected their AD over a diet containing 0.05% less Lys (P < 0.005). Fina lly, to determine whether rats will alter their selection to whole protein- based diets, rats were adapted to 25% wheat gluten diets supplemented with 0.03-0.21% Lys. Rats selected the AD over a diet containing as little as 0. 09% less supplemental Lys by d 4 of the trial (P < 0.05). We conclude that rats are sensitive to changes as small as 0.01% in dietary Lys concentratio n, but that sensitivity requires prior adaptation to Lys-deficient diets.