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.

The effect of coprophagy on apparent neutral detergent fiber (NDF) digestibility, and calcium absorption was evaluated by housing rats in four types of cages: regular, metabolic, and two anticoprophagy cages: a short linear tube cage which allowed the rat forward and backward movement of about one-half a body length and a long tube cage assembled to form a square which allowed the rat to move in one direction through the tube but not turn around. Three-day weight gains of animals in the regular or metabolism cages were greater than those of rats housed in either anticoprophagy cage. In contrast, average food intake did not differ among the four housing conditions. Wet, but not dry, fecal weights were greater in the two groups of rats in the anticoprophagy cages. Neither fecal NDF (% dry wt) nor apparent NDF digestibility was affected by housing conditions. Apparent calcium absorption was decreased by the anticoprophagy housing. The differences in body weight and apparent calcium absorption suggest that prevention of coprophagy in the rats produces significant changes in the efficiency of food and nutrient utilization. The failure to detect differences in NDF digestibility indicates that coprophagy has little impact on the study of fiber digestibility.

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We have studied the effect of a calorie-restricted diet on the growth of young rats and on serum levels of GH, somatomedins (insulin-like growth factors I and II), total T4 and T3, free T4 index, and total corticosterone. Experimental rats consumed the same quantities of protein and carbohydrate as control animals, but less fat, so that their calorie intake was approximately 76% that of controls. The mean (+/- SEM) GH level in the experimental group (78 +/- 21 ng/ml) was not significantly different from that in the control group (89 +/- 31 ng/ml). In contrast, serum total insulin-like growth factor and insulin-like growth factor IGF-I, while not correlated with serum GH, were significantly correlated with age and body weight (r = 0.93 and r = 0.69, respectively; P less than 0.01). The levels of these somatomedins in the calorie-restricted rats were significantly lower than those in their age-matched, but not weight-matched, controls after approximately 2 weeks of study. Serum T4, T3, and free T4 index were all significantly reduced in the experimental animals and may represent an adaptive response to calorie restriction. Serum corticosterone levels in the experimental and control rats were essentially identical. In this study, by restricting calorie intake we have compromised the ability of growing rats to use dietary protein anabolically, creating a useful model to examine in some detail nutritional influences on growth and the growth-promoting endocrine system.

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