Differential regulation of IGF-1 and IGF-binding protein-1 by dietary composition in humans

Although it is known that circulating levels of the insulin-like growth factors (IGFs) and the IGF-binding proteins (IGFBPs) fluctuate in response to changes in nutritional status, there is little information regarding either relative contributions from different dietary components or regulation by insulin in nondiabetic subjects. To define dietary contributions to IGF regulation, the authors examined the effects of fasting and hypocaloric diets of differing nutritional composition on serum IGF-1 and a IGFBP-1 in 16 healthy, obese adult women. Subjects received an isocaloric diet for 6 days, followed by 14 days of calorie restriction (fasting or a hypocaloric diet enriched in either protein, fat, or carbohydrate), and by 4 days refeeding. All diets produced 6-8% weight loss over 14 days with little difference between groups. The "protein-sparing" diet sustained nitrogen balance (+1.2 g/d, versus -4.5 g/d for the other three groups; p < 0.05). Serum IGF-1 levels decreased during calorie restriction with fasting or with diets high in fat or carbohydrates (CHO; combined mean 40 +/- 7%) but showed little change with the high protein regimen (3 +/- 16%; p < 0.05 compared to the other diets). In contrast, IGFBP-1 increased during calorie restriction in all four groups but significantly less with the high CHO diet (43 +/- 17% above baseline) than with the other diets (168 +/- 31%; p < 0.05). Levels of IGF-1 were correlated with nitrogen balance (r = 0.51; p < 0.05) but levels of IGFBP-1 were not. Although IGFBP-1 levels inversely correlated with measures of insulin secretion, IGF-1 levels did not.(ABSTRACT TRUNCATED AT 250 WORDS)