Walking briskly for an hour a day can halve the tendency to obesity in people genetically predisposed to the condition, while
a sedentary way of life that includes 4 hours or more of TV viewing a day can increase it by 50%, said researchers at an
American Heart Association meeting in San Diego this week.
Lead author Dr Qibin Qi, a post-doctoral research fellow in the Department of Nutrition at Harvard School of Public Health in Boston, Massachusetts, and colleagues, examined data on 7,740 women and 4,564 men taking part in two prospective studies: the Nurses' Health Study and Health Professionals Follow-up Study.
Qi told the press:
"In our study, a brisk one-hour daily walk reduced the genetic influence towards obesity, measured by differences in BMI, by half. On the other hand, a sedentary lifestyle marked by watching television four hours a day increased the genetic influence by 50 percent."
BMI or body mass index is the ratio of a person's weight in kilograms to the square of their height in meters (expressed as kilograms/meter squared or kg/m2). A BMI score of 30 kg/m2 or more rates as obese.
Qi and colleagues note that previous studies on the links between genes, lifestyle and obesity have largely focused on the FTO gene, and overall physical activity, and we know little about links with sedentary behavior, such as TV watching, and the influence of other gene variants.
So they designed their study to look at the links between leisure-time physical activity, TV watching and the effect of several genes on obesity.
The physical activity and TV watching data they used had been collected two years before measuring BMI. Genetic data on predisposition to diseases came from biological samples.
For the effect of genes on obesity, Qi and colleagues based their calculations on 32 genes known to increase BMI. They calculated differences in BMI per point of a genetic predisposition score, corresponding to each BMI-increasing gene.
When they analyzed the data, they found:
- Each BMI-increasing gene was linked to a 0.13 kg/m2 increase in BMI.
- The effect was lessened in people with the highest level of physical activity (0.08 versus 0.15 kg/m2 in those with the lowest physical activity levels).
- The genetic influence on BMI was stronger in people who spent 40 hours a week watching TV (0.34 compared with 0.08 kg/m2, for those who watched TV an hour or less per week).
- The equivalent of brisk walking for one hour per day was linked to a 0.06 kg/m2 reduction in the genetic influence on BMI.
- Every additional 2 hours a day of TV watching was linked to a 0.03 kg/m2 increase in the genetic effect on BMI.
"Greater leisure-time physical activity attenuates the genetic predisposition to increased BMI, whereas sedentary lifestyle indicated by prolonged TV watching accentuates the genetic effects on BMI."
They note their findings suggest that "both increasing exercise levels and reducing sedentary behaviors, especially TV watching, independently may mitigate the genetic predisposition to increased BMI".
Qi said the average American watches TV for between 4 and 6 hours a day.
At present, members of the public can't have gene tests for obesity, so in the meantime, Qi said doctors should ask about family history as the next best way to assess genetic predisposition.
Qi said we don't know exactly how the 32 genes they assessed impact BMI.
"These genes were just identified in the past five years and the exact functions of the genetic variants are still unknown. Future studies will be needed to uncover the underlying mechanisms," he explained.
Written by Catharine Paddock PhD