Objective To establish in children whether inactivity is the cause of fatness or fatness the cause of inactivity.
Design A non-intervention prospective cohort study examining children annually from 7 to 10 years. Baseline versus change to follow-up associations were used to examine the direction of causality.
Setting Plymouth, England.
Participants 202 children (53% boys, 25% overweight/obese) recruited from 40 Plymouth primary schools as part of the EarlyBird study.
Main outcome measures Physical activity (PA) was measured using Actigraph accelerometers. The children wore the accelerometers for 7 consecutive days at each annual time point. Two components of PA were analysed: the total volume of PA and the time spent at moderate and vigorous intensities. Body fat per cent (BF%) was measured annually by dual energy x ray absorptiometry.
Results BF% was predictive of changes in PA over the following 3 years, but PA levels were not predictive of subsequent changes in BF% over the same follow-up period. Accordingly, a 10% higher BF% at age 7 years predicted a relative decrease in daily moderate and vigorous intensities of 4 min from age 7 to 10 years (r=−0.17, p=0.02), yet more PA at 7 years did not predict a relative decrease in BF% between 7 and 10 years (r=−0.01, p=0.8).
Conclusions Physical inactivity appears to be the result of fatness rather than its cause. This reverse causality may explain why attempts to tackle childhood obesity by promoting PA have been largely unsuccessful.
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Funding This report from the EarlyBird Diabetes Study was funded by the Bright Futures Trust, Diabetes UK, Smith's Charity, the Child Growth Foundation, the Diabetes Foundation, the Beatrice Laing Trust, Abbott, Astra-Zeneca, GSK, Ipsen and Roche. None of the sources funding this study had any involvement in the study design, collection or analysis of data, interpretation of findings or writing of the manuscript.
Competing interests None.
Ethical approval Ethical approval was obtained from the Plymouth local research ethics committee (South and West Devon Health Authority) in 1999. Informed consent and assent were obtained from parents and children, respectively.
Provenance and peer review Not commissioned; externally peer reviewed.