Changes in Plasma Somatomedin‐C in Response to Ingestion of Diets with Variable Protein and Energy Content

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Abstract
We have attempted to determine the relative importance of dietary intake of protein and energy in restoring plasma immunoreactive somatomedin‐C (Sm‐C) concentrations after fasting. Ten healthy human volunteers were fasted for 5 days, then divided into two refeeding groups. One group (variable energy) was refed 1.0 g protein/kg ideal body weight, and in 9‐day sequences, 11, 18 and 25 kcal of energy/kg. The other group (variable protein) was given 35 kcal energy/kg, and in 9‐day sequences, 0.2, 0.4, and 1.0 g protein/kg. When subjects were refed the variable energy diets there was no significant increase in Sm‐C at 11 kcal/kg (0.47 ± 0.13 to 0.45 ± 0.12 U/ ml), suggesting that there is a threshold energy requirement below which optimal protein intake is not sufficient to raise the Sm‐C. When subjects were refed 18 and 25 kcal/kg, it became apparent that the more energy added to the diet, the greater the absolute concentration of Sm‐C attained (0.66 U/ ml on 18 kcal/kg and 0.97 U/ml on 25 kcal/kg). Sm‐C correlated with nitrogen balance (r = 0.58) during refeeding with the variable energy diets and was a good indicator of acute directional change in nitrogen balance. However, Sm‐C was not a reliable indicator of nitrogen repletion, since it rose almost to control levels on the 25 kcal/kg diet while nitrogen balance remained slightly negative. During refeeding of the variable protein‐adequate energy diets, there was a significant increase in Sm‐C at the lowest protein intake (at 0.2 g protein/kg, Sm‐C increased from 0.49 ± 0.22 to 0.80 ± 0.34 U/ml; p < 0.01), suggesting that the ingested energy facilitates the efficient utilization of small quantities of dietary protein. As more protein was ingested, the Sm‐C exceeded basal, prefast values in some subjects. Nitrogen balance correlated well with plasma Sm‐C (r = 0.71) on the variable protein diets and appeared to be a good indicator of acute changes in nitrogen accretion. These studies suggest that both energy and protein are important for restoration of Sm‐C after fasting. At extremely low levels of intake, however, it appears that energy restriction causes more impairment of the postfast recovery in plasma Sm‐C than does an equivalent restriction in protein intake. (Journal of Parenteral and Enteral Nutrition 8:407–411, 1984)