Physical activity impact on BMI

March 2, 2013

John DiTraglia

Contributing Columnist

One of the most mythical of myths about obesity is that exercise will help you lose weight and/or keep it off. The fact that this myth is indeed a myth is well established by many studies now. We should probably stop doing studies that try to show that exercise will help you lose weight. However, maybe because doctors don’t want to believe that this is a myth, more studies keep being done.

In the beating a very dead horse category is a project, presented in abstract in February’s Pediatrics from a group in Australia that studied the correlation between physical activity and BMI over a three-year period in 182 overweight/obese 5 to 10-year-olds. (1) They measured physical activity by a seven-day accelerometer monitor. Baseline activity did not predict BMI change. However for every 100cpm increase in change in activity over three years, BMI z score fell by 0.11. That was statistically significant.

They conclude, “Sustained increase in moderate-vigorous physical activity predicts reducing BMI z score over three years in overweight/obese children… However, the small BMI change associated with even the largest activity changes may explain disappointing BMI outcomes of brief primary care interventions targeting physical activity.”

So the impact of increased physical activity of this project is disappointing too and furthermore it takes a long time. But more importantly, this was a correlation, not a treatment result. In other words, kids who didn’t get this much fatter moved around more. That doesn’t necessarily mean that moving around more on purpose would make you slightly less fat.

1.Trinh A et al. Physical activity and 3-year BMI change in overweight and obese children. Pediatrics 2013;131:e470-7.