Fat-free mass mediates the association between birth weight and aerobic fitness in youth.
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.


CONCLUSIONS
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.
MRC Epidemiology Unit, Institute of Metabolic Science, Cambridge, UK.
C L
CL
Ridgway
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S
Brage
S
S
Anderssen
L B
LB
Sardinha
L B
LB
Andersen
U
U
Ekelund
D056044:Actigraphy / Q000295:instrumentation; D000293:Adolescent; D000367:Age Factors; D000704:Analysis of Variance; D001724:Birth Weight; D001823:Body Composition; D001827:Body Height; D015992:Body Mass Index; D001835:Body Weight; D002648:Child; D005060:Europe; D005080:Exercise Test; D005260:Female; D006801:Humans; D016014:Linear Models; D008297:Male; D009043:Motor Activity; D010809:Physical Fitness; D012737:Sex Factors; D012885:Skinfold Thickness