Metcalf and colleagues from the Peninsula Medical School and University of Plymouth suggest that in children it may be excess bodyweight causing inactivity rather than sedentary behaviour causing weight gain.
Increased prevalence of childhood obesity in the UK has been widely documented and concern is growing about the implications this could have for future risk of cardiovascular and metabolic disease. Although obesity may have many causes, it is commonly reported that prolonged episodes of sedentary behaviour lead to excess weight gain. Evidence from cross-sectional studies revealing significant inverse associations between objectively measured physical activity and body weight/percentage body fat (%BF) are used to support this argument. It should, however, be acknowledged that the alternative view of increased fatness causing inactivity is equally viable as these studies are not able to determine the true direction of causality. In an attempt to clarify the relationship between bodyweight and physical activity, Metcalf and colleagues applied the rule of temporality to determine the dominant direction of causality between objective measures of physical activity and %BF in children. In total, data from 202 children (107 boys and 95 girls) living in Plymouth and participating in the EarlyBird study were collated. Physical activity and %BF were measured using Actigraph accelerometers and dual energy x-ray absorptiometry, respectively, at four annual time points from the age of 7 to 10 years. Data analysis revealed a trend for girls to be less active than boys and for both girls and boys to become less active as they got older. When the associations between changes in %BF and physical activity were investigated no significant relationships between physical activity and subsequent change in %BF were found, although significant relationships were reported between %BF and changes in physical activity. Percentage body fat at 7 years predicted changes in physical activity from 7-10 years (r = -0.17, p = 0.02) whilst physical activity at 7 years did not predict changes in %BF from 7-10 years (r = -0.01, p = 0.8). Analysis revealed that a 10% increase in %BF at age 7 equated to a four minute reduction in physical activity from 7-10 years. These data from Metcalf and colleagues suggest for the first time that in children it is an increase in %BF that may lead to physical inactivity and not the more traditional belief that sedentary behaviour underpins weight gain and obesity. Although further research is warranted to clarify these findings and investigate their reproducibility in wider populations, these data indicate that it may be more appropriate for interventions aimed at tackling and preventing childhood obesity to focus upon dietary intake rather than increasing physical activity per se.
Full reference for paper:
Metcalf, B.S., Hosking, J., Jeffery, A.N., Voss, L.D., Henley, W. and Wilkin, T.J. (2010) Fatness leads to inactivity, but inactivity does not lead to fatness: a longitudinal study in children (EarlyBird 45). Archives of Disease in Childhood, doi: 10.1136/adc.2009.175927.
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