Plasma GH And IGF-I Concentrations Are Affected By Increased Consumption Of Dietary Protein In Healthy Males

Abstract

Dietary protein intake has been shown to impact levels of circulating Insulin-like Growth Factor I (IGF-I). However, the effects of habitual consumption of diets containing varied levels of dietary protein on circulating IGF are limited to observational studies. Furthermore, intervention studies focused on the relationship between diet and IGF-I were often short term, lacked dietary control or associated with an appreciable energy deficit. PURPOSE To examine the response of circulating IGF-I levels to habitual consumption of increased amounts of dietary protein during aeucaloric diet intervention in healthy, exercising males. METHODS Male runners (n=5, mean ± SEM, 20 ± 0.6yrs, 0 ± 1.2kg, 9 ± 0.5% body fat) participated in a randomized, crossover eucalroic diet intervention design, where they consumed either a low (0.8 g/kg; LP), moderate (1.8 g/kg; MP), or high (3.6 g/kg; HP)protein diet for 4 weeks. At week 4, Growth Hormone (GH), total IGF-I and free IGF-I were assessed following an overnight fast. RESULTS GH was lower for MP and HP vs LP (0.3 ± 0.2 and 0.6 ± 0.3 vs 2.4 ± 0.5 ng/mL, p<.05), while total IGF-I was increased on MP vs LP (139 ± 19 vs 93 ± 10 ng/mL, p<.05) and tended to be higher for HP vs LP (125 ± 23 vs 93 ± 10 ng/mL, p<.15). CONCLUSION These data demonstrate that, despite a diminution in GH, habitual consumption of dietary protein in excess of the RDA (i.e. 0.8 g/kg/d), is associated with increased circulating levels of total IGF-I when energy balance exists. However, since no changes were observed in free IGF-I, the biological implications of these findings are limited and warrant further investigation focused on the relationship between dietary protein intake and IGF-I. Supported in part by the National Cattleman's Beef Association and the University of Connecticut Research Foundation