Abstract
Objective. To investigate whether birth weight acts as a biological determinant of later aerobic fitness, and whether fat-free mass may mediate this association. Methods. The European Youth Heart Study (EYHS) is a population-based cohort of two age groups (9 and 15 years) from Denmark, Portugal, Estonia and Norway. Children with parentally reported birth weight >1.5 kg were included (n = 2 749). Data were collected on weight, height, and skinfold measures to estimate fat mass and fat-free mass. Aerobic fitness (peak power, watts) was assessed using a maximal, progressive cycle ergometer test. Physical activity was collected in a subset (n = 1 505) using a hip-worn accelerometer and defined as total activity counts/wear time, all children with >600 minutes/day for ≥3 days of wear were included. Results. Lower birth weight was associated with lower aerobic fitness, after adjusting for sex, age group, country, sexual maturity and socio-economic status (ß = 5.4; 95% CI: 3.5, 7.3 W per 1 kg increase in birth weight, p < 0.001). When fat-free mass was introduced as a covariate in the model, the association between birth weight and aerobic fitness was almost completely attenuated (p = 0.7). Birth weight was also significantly associated with fat-free mass (ß = 1.4; 95% CI: 1.1, 1.8, p < 0.001) and fat-free mass was significantly associated with aerobic fitness (ß = 3.6; 95% CI: 3.4, 3.7, p < 0.001). Further adjustment for physical activity did not alter the findings. Conclusion. Birth weight may have long-term influences on fat-free mass and differences in fat-free mass mediate the observed association between birth weight and aerobic fitness.
Acknowledgements
The authors would like to thank all the staff and volunteers involved with the European Youth Heart Study.
Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.