IGF-I LEVELS IN 188 ADOLESCENTS DURING TWO YEARS OF TRAINING AT A COMPETITIVE LEVEL

Abstract

It is known that physical activity cause an acute increase in growth hormone (GH) and it has lead to the hypothesis that GH and insulin-like growth factor I (IGF-I) levels are increased in people active in sports. In contrast, endurance training of adolescents have shown a decrease in IGF-I suggesting that intensive training might reduce growth and compromise final height. The objective was therefore to evaluate the long-term changes of IGF-I in children training at a competitive level. Age, height and pubertal development were recorded and blood samples drawn from 188 adolescents (90 males and 98 females) in swimming (Sw), tennis (Te), team handball (TH) and gymnastics (Gy) four times with half a year apart during two years of investigation. IGF-I was measured by radioimmunoassay. Levels of IGF-I in both sexes were significantly different between the sports at the first test (P = 0.04, Kruskal-Wallis) being higher in TH than in gymnasts (P = 0.009, Mann-Whitney). No differences were found in the following tests apart from female TH showing higher levels of IGF-I than Te and Gy at test 2 (P = 0.03, Mann-Whitney). All sports but female TH showed a significant increase in IGF-I levels by the end of the investigation period. All measured IGF-I levels were within existing normative data for Danish children. Z-scores for height did not decrease throughout the investigation period in any of the sports. A linear regression model showed that height, age and pubertal development were significantly associated with IGF-I levels. Importantly, training hours per week (median 7 hours, range 2–18) were not associated with IGF-I levels. Thus IGF-I seemed to be independent of the overall training stimuli though acute reduced levels might have occurred. Our results confirm that IGF-I in adolescents in competitive sports is well adapted to the training stimuli and that IGF-I levels were exclusively dependent on height, age and pubertal development. Supported by The Danish Research Council (grant no. 9600260)