Genetics help determine whether a frequent diet of fried food will make you fat, according to a new Harvard study.
Eating fried food more than four times a week had twice as big an effect on body size for people at high genetic risk of obesity compared with people at low risk, researchers found after analyzing data from three U.S. trials.
Moreover, the more pro-obesity genes you carry, the bigger you'll get chowing down on fried chicken and such, the researcher said.
Such findings help explain why Americans' overall bad health habits don't affect everyone equally, said Claude Bouchard, chairman of genetics and nutrition at the Human Genomics Laboratory of Pennington Biomedical Research Center, in Baton Rouge, La.
"Our dietary habits and our lack of physical activity is driving the obesity epidemic, but the force of the behavioral driver is not the same in everyone," Bouchard said.
"We also have a biology driver, and that driver for some people is very minor," he said. "For others, it's a powerful agent enhancing the risk of behavior."
Other studies have noted similar interactions between these genetic risks and other environmental risk factors for obesity, including physical activity and total calorie intake, Bouchard said. People who are more sedentary or eat more are more likely to gain weight if they carry these genetic risks.
In the future, genetic testing could help reveal who is at higher risk of obesity, so they can take preventive measures, Bouchard said.
"It's not a sentence for obesity, but rather an increased susceptibility to obesity," he said. "Your peers can afford to have an extra serving or be sedentary and they will be OK, but for you it won't happen. That's important to know."
The findings, published March 19 in the journal BMJ, are based on an analysis of more than 37,000 men and women who participated in three U.S. health trials.
For each person, the researchers looked at a panel of 32 known genetic variants associated with obesity. They tracked fried food consumption using questionnaires and looked at each person's body-mass index (BMI), a measurement of body fat based on height and weight.
People with genetic risk scores in the top third tended to have a BMI that was 1 unit higher in women and 0.7 units higher in men if they ate fried food four or more times a week, compared with people at the same risk who ate such food less than once a week.
But for participants with the lowest genetic risk, little differences was seen between those who ate the most fried food versus very little -- just half a unit in women and 0.4 units in men.
A BMI of 25 or more is considered overweight, while a BMI of 30 or more is considered obese.
The research team also found that a person's overall risk of obesity from fried foods increases exponentially with every 10 genetic variants, or alleles, that predispose them to obesity.
Those who eat less than one serving a week have a 61 percent increased risk of obesity for every 10 risk genes, but that person's risk rises to 112 percent with one to three servings of fried food a week.
"If you have people who have 30, 35, 40 risk alleles, their total risk on average would be much, much larger," Bouchard said.
The researchers don't have the biological evidence to say why these genes inflate the body's reaction to fried foods, said senior author Lu Qi, an assistant professor of nutrition at the Harvard School of Public Health. He said, however, that they believe it may stem from the way some genes are tied to the body's energy balance.
"It's likely through the interplay between the genetic factors and the dietary factors in managing energy balance," he said.
For more information on genes and obesity, visit the Harvard School of Public Health.