Serum levels of total and free IGF-I and IGFBP-3 are increased and maintained in long-term training

The goals of this study were to determine whether the long-term training regimens experienced by competitive collegiate swimmers would result in altered levels of total and free serum insulin-like growth factor I (IGF-I) as well as IGF-binding proteins (BP) IGFBP-1 and -3. Two male (Teams 1M and 2M) and one female (Team 2F) teams were studied at the start of training, after 2 mo of training, after 4 mo (2-4 mo had the highest volume of training), after 5 mo (near the end of tapering; only for Team 1M), and several days after training was over. For Team 1M, total IGF-I concentrations were increased by 76% after 4 mo and were subsequently maintained at this level. Total IGF-I responses were more variable for Teams 2F and 2M. Free IGF-I levels were increased nearly twofold for all teams at 2 mo and were maintained or increased further with subsequent training. Only the levels of free IGF-I for Team 1M returned to pretraining values after training had ended. Training had little effect on IGFBP-1 levels. For all teams, serum IGFBP-3 was elevated by 4 mo of training (for Team 2F it was increased at 2 mo) by 30-97% and remained at these higher levels thereafter. The ratio of total IGF-I to IGFBP-3 was not increased by training in any group. These data indicate that serum levels of total and free IGF-I and total IGFBP-3 can be increased with intense training and maintained with reduced training (tapering). The findings show that changes in free IGF-I levels are not accounted for by alterations in the total IGF-I/IGFBP-3 complex or in IGFBP-3 levels and indicate that there are other important determinants of free IGF-I.