Serum insulin-like growth factor I and physical performance in prepubertal Bolivian girls of a high and low socio-economic status

The aim of the study was to determine if a decrease in serum insulin-like growth factor I (Igf-I) levels under marginal malnutrition is responsible for the lower physical performance of girls of a low socio-economic status (LSES). Girls were selected after physical examination (Tanner's stage 1) and anthropometric measurements (height, body mass or mb, body mass index or BMI = mb height2). Lean body mass mb,l was measured after skinfold thickness determination; serum IGF-I, by radioimmunoassay; maximal O2 consumption, (V˙O2max), directly during incremental exercise up to exhaustion; and maximal aerobic power (W˙max), using the force-velocity test. LSES girls (n = 31) had been malnourished in the past and, currently, were suffering from marginal malnutrition: they were smaller (135.2 ± 5.5 vs 146.1 ± 4.3 cm), lighter (31.7 ± 3.9 vs 37.6 ± 5.0 kg), exhibited a lower mb,l (24.2 ± 2.5 vs 27.5 ± 3.0 kg) but same BMI compared with HSES (high socio-economic status) girls (n = 32). Igf-I levels (27.7 ± 7.9 vs 34.1 ± 6.5 nmol · l−1), V˙O2max (45.26 ± 4.72 vs 50.74 ± 6.02 ml · min−1 · kg−1 LBM) and W˙max (6.00 ± 1.15 vs 8.70 ± 1.53 W · kg−1mb,l were lower in LSES girls. Moreover, the differences in every parameter were not the consequence of the younger age (10.8 ± 0.9 vs 11.2 ± 0.6 years) of the LSES girls. Our results provide evidence that the lower W˙max of undernourished prepubertal girls was partly the consequence of alterations in muscle function at the qualitative level, as a result of a decrease in Igf-I levels. Conversely, under normal nutritional conditions, anthropometric characteristics only are explicatory factors for physical performances.