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.