Protein utilization in growth: effect of lysine deficiency on serum growth hormone, somatomedins, insulin, total thyroxine (T4) and triiodothyronine, free T4 index, and total corticosterone.

We have studied the effect of a lysine-deficient diet on the growth of young rats and on serum levels of GH, somatomedins [insulin-like growth factors (IGFs) I and II], insulin, total T4 and T3, free T4 index, and total corticosterone. Rats eating a wheat gluten diet consumed about one third as much lysine as controls eating an isocaloric and isonitrogenous casein diet and grew at approximately 56% of the control rate. The mean (+/- SEM) GH level in the experimental group (68 +/- 9 ng/ml) was significantly lower (P less than 0.01) than that in the controls (106 +/- 17 ng/ml), but was not correlated with age or body weight and was only weakly correlated with total IGF. In contrast, total IGF and IGF-I were significantly correlated with age and body weight (r = 0.86 and r = 0.84, respectively; P less than 0.01). The levels of these somatomedins in the wheat gluten-fed animals were consistently and significantly lower than those in their age-matched controls, but not significantly different from those in their weight-matched controls, throughout the study. Serum total T4 and T3 (but not the free T4 index) and corticosterone were significantly elevated in the experimental rats, perhaps representing a serum binding globulin adaptation to lysine deficiency that is not clearly understood. In this study, we have compromised the ability of growing rats to use dietary protein anabolically to examine the nutritional effects of qualitative protein deficiency on growth and the growth-promoting endocrine system.