The effect of exercise on plasma somatomedin-C/insulinlike growth factor I concentrations.

Because inadequate intake of energy or protein causes reduction in the plasma concentrations of somatomedin-C/insulinlike growth factor I (Sm-C/IGF-I), we have completed studies to determine whether the energy deficit induced by vigorous exercise sustained for 1 week would also be associated with a reduction in this peptide. When six healthy, exercise-conditioned males were fed a diet consisting of 35 kcal/kg ideal body weight (IBW) and 1.0 g protein/kg and exercised so that 14.1 to 16.3 kcal/kg of energy was expended each day, their plasma Sm-C/IGF-I concentrations declined from a mean basal value of 1.15 +/- 0.78 U/mL (1 +/- SD) to 0.62 +/- 0.41 U/mL during the last two days of a seven-day exercise period (P less than .05). This decrease in Sm-C/IFG-I occurred almost entirely in the first four days of the study period. After 3 weeks for reacclimation, the calorie intake of each volunteer was reduced by the same number of calories as had been expended in the form of exercise during each day of the exercise week (mean of 15.1 kcal/kg IBW/d). On this regimen the mean plasma Sm-C/IGF-I declined from 1.12 +/- 0.57 U/mL, to 0.69 +/- 0.37 U/mL on the last two days of dietary restriction. The magnitude of fall with dietary restriction was not significantly different from that observed during exercise. During the last three days of the exercise period the mean daily nitrogen balance was -1.60 +/- 1.70 g/d, while this value was -3.50 +/- 1.73 g/d during the last three days of dietary restriction (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)