Increasing leisure-time activity and reducing sedentary behaviors such as watching television will both independently attenuate genetic predisposition to obesity, a new study reveals [1].

"We cannot change genes, but we can do something to change the influence of genes, by doing some exercise and by reducing our sedentary behavior, primarily by watching television less," lead author Dr Qibin Qi (Harvard School of Public Health, Boston, MA) told heartwire. Qi presented the results of the research today at EPI|NPAM 2012, the Epidemiology and Prevention/Nutrition, Physical Activity and Metabolism Scientific Sessions 2012.

Qi says he and his team's work is the first to study the modifying effect of TV watching, "which is the most common sedentary behavior," on genetic predisposition to obesity based on a risk score. They also examined the effects of physical exercise on this score.

Because they found independent effects of the two behaviors, Qi says he "wants to emphasize the importance of both." And he says those who increased physical activity didn't necessarily reduce TV viewing time. So an additional goal is to try to get people to exercise while watching TV, "at least stand up and move around," he suggests.

Activity attenuates effect size for BMI; TV viewing increases it

Qi said statistics show that people in Europe and the US watch, on average, four to five hours of TV per day. It is already known that the greater the daily duration of television viewing, the higher the risk of obesity and related disease, independent of physical activity, he noted.

He and his colleagues set out to examine whether leisure-time TV watching and physical activity modify the genetic predisposition to elevated adiposity in women and men, using data from the Nurses' Health Study (7740 women with genotyping data) and the Health Professionals Follow-up Study (4564 men with genotyping data).

They assessed body-mass index (BMI) and average weekly time spent watching TV and performing physical activity, both self-reported and validated (with an r=0.97 between reported BMI and measured BMI).

Qi explained that most previous studies on gene-lifestyle interaction and obesity have largely focused on a single locus, the fat mass and obesity-associated gene (FTO). Instead, he and his colleagues calculated a genetic predisposition score. "The novel thing we did was include 32 genes, including FTO, and we created a score that estimates the overall genetic predisposition to obesity," where each point score corresponded to each BMI-increasing allele, he noted.

The results from women and men were pooled by meta-analysis.

Overall, each BMI-increasing allele was associated with an increase of 0.13 kg/m2 in BMI. The effect size for BMI in those in the highest physical-activity quintile was attenuated compared with that in individuals in the lowest physical-activity quintile (0.08 kg/m2 vs 0.15 kg/m2; p for interaction <0.001).

In contrast, the genetic effect on BMI was more pronounced in people who spent >40 hours watching TV than in those who spent 0 to one hour per week (0.34 kg/m2 vs 0.08 kg/m2; p for interaction=0.001).

One-hour-a-day walk cuts difference between max and min score by 50%

Qi and colleagues found that each 4-METs/day increment in physical activity-equivalent to one hour per day of brisk walking-was associated with a 0.06-kg/m2 reduction in BMI (approximately 46% of the main effect of each additional BMI-increasing allele), while each two-hour/day increment in TV watching was associated with a 0.03-kg/m2 increase in BMI (23% of the main effect).

"We estimated that the difference in BMI (4 kg/m2, equivalent to 11.6 kg in body weight for a person 1.7 m tall) between individuals with a genetic predisposition score of 13 (minimum) and those with a score of 43 (maximum) could be reduced by half (2.1 kg/m2, 6.1 kg in weight) by one hour per day of brisk walking or increased by 25% by two hours per day of TV watching," they say.

"Our data suggest that both increasing exercise levels and reducing sedentary behaviors, especially TV watching, independently may mitigate genetic disposition to increased BMI."

Qi acknowledges, however, that genetic testing is not yet widespread, so in the meantime, doctors and the public should use family history of obesity to guide them. "Maybe ask about relatives who are obese, to see if there is a high genetic risk," he advises.