Long-Term Impact of Preterm Birth on Exercise Capacity in Healthy Young Men: A National Population-Based Cohort Study

Background: Increasing numbers of survivors of preterm birth are growing into adulthood today. Long-term health-effects of prematurity are still poorly understood, but include increased risk for diabetes, obesity and cardiovascular diseases in adult life. To test if reduced physical fitness may be a link in the causal chain of preterm birth and diseases in later life, the association of preterm birth and adult exercise capacity was investigated. The hypothesis was that preterm birth contributes independently of other risk factors to lower physical fitness in adulthood. Methods and Findings: Population-based national cohort study of all males conscripting for military service in 1993-2001 and born in Sweden 1973-1983, n = 218,820. Data were retrieved from the Swedish Conscript Register, the Medical Birth Register and the Population and Housing Census 1990. Primary outcome was the results from maximal exercise test (W-max in Watt) performed at conscription. Association to perinatal and socioeconomic risk factors, other co-variates and confounders were analysed. General linear modelling showed that preterm birth predicted low W-max in a dose-response related pattern, with 25 Watt reduction in W-max for the lowest gestational ages, those born <= 27 weeks. Low birth weight for gestational age also independently predicted low W-max compared to normal and high birth weight (32 Watt reduction for those with a birth weight Standard Deviation Score <2). Low parental education was significantly associated with reduced W-max (range 17 Watt), as well as both low and high current BMI, with severe obesity resulting in a 16 Watt deficit compared to W-max top performance. Conclusion: Being born preterm as well as being born small for gestational age predicts low exercise capacity in otherwise healthy young men. The effect size of being born preterm equal or exceed that of other known risk factors for unfitness in adults, such as low parental education and overweight.