Differential Regulation of IGF-1 and IGF-Binding Protein-1 by Dietary Composition in Humans

ABSTRACT 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  μ U/ml but fell to a plateau when insulin levels exceeded 30 μ U/ml. In conclusion, IGF-1 is more sensitive to dietary protein, whereas IGFBP-1 appears to be regulated largely via secretion of insulin. These findings suggest that levels of IGF-1 may constitute a particularly useful marker of nitrogen balance, while levels of IGFBP-1 may serve as an index of insulin action on the liver.