Supplemental essential amino acids augment the somatomedin-C/insulin-like growth factor I response to refeeding after fasting

Plasma somatomedin-C (Sm-C)/insulin-like growth factor I (IGF-I) concentrations have been shown to reflect changes in nitrogen balance induced by manipulation of nutrient intake. To assess the Sm-CIGF-I response to refeeding a protein restricted diet in which the nitrogen source was supplemented with essential amino acids, six normal adults were fasted for five days, then refed for nine days diets consisting of 35 kcals/kg and 0.48 g protein/kg body weight. In one diet, 80% of the nitrogen was supplied as essential amino acids, and in the other, 80% was supplied as nonessential amino acids. Following the first fast/refeed cycle, a control diet was eaten for two weeks before the fast was repeated and the other test diet was ingested. The refeeding diets were given a random order. Plasma Sm-CIGF-I fell from a pre-fast mean of 1.64 ± 0.24 U/mL (mean ± 1 SEM) to 0.67 ± 0.18 (P < 0.001) following fasting, and rose to 1.41 ± 0.19 U/mL (P < 0.001) following ingestion of the diet with supplemental essential amino acids. In contrast, when the same subjects were refed a diet in which 80% of the nitrogen was in the form of nonessential amino acids, the plasma Sm-CIGF-I concentrations rose from 0.74 ± 0.17 to 1.15 ± 0.15 U/mL (P < 0.01). This increase was significantly less than that observed after ingestion of the essential amino acid supplemented diet (0.74 ± 0.10 U/mL v 0.40 ± 0.11 U/mL; P < 0.02). Changes in estimated nitrogen balance correlated with changes in Sm-CIGF-I (r = 0.81). The mean daily estimated nitrogen balance on the diet with supplemental essential amino acids was −1.62 ± 0.20 g/d, whereas subjects fed the diet high in nonessential amino acids had significantly more negative estimated nitrogen balance (−3.4 ± 0.18 g/d, P < 0.001). Serum prealbumin concentrations also declined with fasting and rose with refeeding. As with Sm-CIGF-I the increase in prealbumin concentrations was significantly greater when subjects were refed the diet supplemented with essential amino acids. These results support the concept that plasma Sm-CIGF-I measurements may be a useful method for monitoring the response to nutritient intake, and may be helpful in constructing diets which provide more favorable nitrogen balance during protein restriction.