Fat DNA Is Not Destiny

It was enough to make people who believe they have “fat genes” give up. Among 438 adolescents who carry a form of the gene called FTO (fat mass- and obesity-associated) linked to obesity, there was no effect of physical activity on body mass index, found a 2009 study that followed them from infancy to age 15: teens who carry the obesity form of the FTO gene and exercised were no less obese than teens who carry the gene and were couch potatoes.

Let this be a lesson in the truth of the cliché that no single study should be taken as Truth, especially on something as complicated as human health. A new study—and yes, this one should also be taken provisionally—suggests that exercise can indeed mitigate the effect of genes for obesity. Importantly, this research has the virtue of doing something that no previous study on the FTO gene and exercise has done: actually measure how active kids were, rather than taking their word (through a questionnaire) for it.

When adults have two copies (one each from mom and dad) of a particular form of FTO, they weigh an average of 6.6 pounds more than adults who have a different version of the gene; they also have a 67 percent higher risk of type 2 diabetes. Adults who carry a single copy of the obese form of FTO (inherited from one parent) weigh, on average, just over three pounds more than people with no copies, found a 2007 study. An estimated 40 percent of whites carry at least one copy.

The new study, in Archives of Pediatrics & Adolescent Medicine, finds that in adolescents who spent less than 60 minutes per day in moderate to vigorous physical activity, fat DNA was indeed destiny. The kids’ body-mass index was an average of .65 higher if they carried one copy of the gene and 1.3 if they had two copies. (For someone who stands 5-foot-5, that’s the difference between weighing 132 pounds and 140.) The inactive kids also had an average of 1.7 percent more body fat per gene, and an extra half inch around their waist.

But kids who exercised for at least the recommended 60 minutes per day did not—again, on average—have a higher BMI, more body fat, or a larger waist than kids who carried zero copies of the fat gene.

Following exercise recommendations, conclude the authors, “may offset the genetic predisposition to obesity associated with the FTO” gene. Indeed, three earlier studies (including this one from 2009 and this one from 2008) also suggested that exercise can negate the effect of the “fat” gene. Like the studies that reached the opposite conclusion—exercise all you want, but carrying fat DNA is destiny—they, too, used questionnaires rather than actually measuring how much kids exercised. By doing the latter, this new research study takes a big step toward showing that our fate—or at least our BMI—lies as much in our gym membership as in our DNA.