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On the Futility of Screening for Genes That Make You Fat

  • J. Lennert Veerman mail

    Affiliation: School of Population Health, The University of Queensland, Brisbane, Queensland, Australia

  • Published: November 01, 2011
  • DOI: 10.1371/journal.pmed.1001114

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Original article did NOT say exercise produces a normal weight

Posted by JSShapiro on 02 Nov 2011 at 18:57 GMT

genes may predispose to weight gain, but this weight can be lost by extra physical activity

This commentary says, "genes may predispose to weight gain, but this weight can be lost by extra physical activity." The original PLoS Medicine article said nothing of the sort. It said that active adults had a BMI 0.79 units lower than inactive adults, which is nowhere near enough to move an obese person to the normal range. (That takes a minimum of 5 BMI units.) For an adult of average height, 0.79 BMI is about five pounds. In children, the effect of exercise on BMI was not significant at all.

My personal opinion is that exercise has a huge positive effect on health, but only a very modest effect on weight. The original PLoS Medicine article actually confirms that exercise has only a small effect on weight in adults--and showed no significant effect at all in children.

No competing interests declared.

RE: Original article did NOT say exercise produces a normal weight

lennertveerman replied to JSShapiro on 03 Nov 2011 at 12:42 GMT

"Original article did NOT say exercise produces a normal weight." Well, the commentary did not exactly claim that, either, as it did not state that all weight gain was due to genetic predisposition. FTO doesn't seem to contribute more than 1-2 kg, on average and physical activity (or even just exercise) can certainly reduce weight by that much.

But I admit that in my thinking I was extrapolating a bit from the evidence in the original paper by Loos and colleagues. I would argue that there are two reasons to think the study underestimates the attenuating effect of PA on the fattening effect of FTO.

First, the study suffers from inevitable limitations in the measurement of physical activity (PA). This would lead to a degree of misclassification in exposure variable PA, which would bias the results towards ‘no effect’ – i.e., reduce the attenuating effect PA on BMI that was found (27%).

Furthermore, the study used a dichotomous classification of physical activity, comparing the least active 20% or so with the rest. On average, the 'active' persons probably weren't very active at all. If the degree of attenuation of the weight-enhancing effect of FTO increases with increasing PA in a continuous manner, the gene might not have any measurable effect on BMI at all in the most active 1%.

(To complicate matters, since no adjustment was made for diet, and diet quality correlates with PA, we may actually be talking about the combined effect of the two. I hope for more clarity when actual mechanisms for these associations are found.)

No competing interests declared.