Negative energy balance plays a major role in the IGF-I response to exercise training

Circulating IGF- I is correlated with fitness, but results of prospective exercise training studies have been inconsistent, showing both increases and decreases in IGF- I. We hypothesized that energy balance, often not accounted for, is a regulating variable such that training plus an energy intake deficit would cause a reduction in IGF- I, whereas training plus energy intake excess would lead to an increased IGF- I. To test this, 19 young, healthy men completed a 7- day strenuous exercise program in which they were randomly assigned to either a positive energy balance [ overfed ( OF), n = 10] or negative energy balance [ underfed ( UF), n = 9] group. IGF- I ( free and total), insulin, and IGF- binding protein- 1 were measured before, during, and 1 wk after the training. Weight decreased in the UF subjects and increased in the OF subjects. Free and total IGF- I decreased substantially in the UF group ( P < 0.0005 for both), but, in the OF group, IGF- I remained unchanged. The UF group also demonstrated an increase in IGF- binding protein- 1 ( P < 0.027), whereas glucose levels decreased ( P < 0.0005). In contrast, insulin was reduced in both the OF and UF exercise- training groups ( P < 0.044). Finally, within 7 days of the cessation of the diet and training regimen, IGF- I and IGF- binding protein- 1 in the UF group returned to preintervention levels. We conclude that energy balance during periods of exercise training influences circulating IGF- I and related growth mediators. Exercise- associated mechanisms may inhibit increases in IGF- I early in the course of a training protocol, even in overfed subjects.