Voluntary food intake variation in chickens on lysine-free diet is attributed to the plasma lysine concentration.

1. Growing chickens decrease their voluntary food intake when they receive a diet deficient in a single essential amino acid. Our previous studies suggest that the decreased food intake was associated with some metabolic changes. 2. In order to reveal the involvement of plasma lysine fluctuations in the reduction of food intake, we examined whether maintaining the plasma lysine concentration in chickens on a lysine-free diet (the purified diet contained no lysine) restored the food intake to that of the control (lysine hydrochloride 11.9g/kg) group. 3. Male egg-type chickens at 21d of age were injected with lysine at doses of 0.1g/ml one hour after presenting the lysine-free diet. This injection increased the plasma lysine concentration one hour later and kept it similar to that of the control group for the following 2h. Chickens ate the lysine-free diet as much as the control diet when their plasma lysine concentration was kept at a similar level to the control group. Injection of saline or alanine (0.12g, isonitrogenous to lysine 0.1g) into the crop of chickens on the lysine-free diet did not bring about the variations of food intake and plasma lysine concentrations as observed in those with lysine. 4. These findings show that the food intake variation was attributed to the plasma lysine concentration in the chickens on the lysine-free diet.