To review systematically all publications of the effects of exercise on endogenous insulin-like growth factor (IGF) to clarify the nature of this association.
We reviewed 115 research studies in humans by subgroup of population (age; sex; athletic training status), physical activity exposure (resistance vs. aerobic activity; duration of activity) and study design.
Fifty percent of studies reviewed found no difference in total circulating IGF-1 as a result of exercise; 37% showed an increase, and 13% observed decreases in IGF-1 levels with exercise. Age influenced the effects of exercise on IGF levels. Exercise appeared to decrease IGF-1 levels in children, but to increase levels in young adults. Similar results were found for IGFBP-3.
It is not yet possible to determine if exercise affects IGF levels. Important methodologic differences among studies, as well as concerns about study quality, limit the ability to draw firm conclusions.