There's a new reason not to super-size those french fries. Researchers have found that people with genetic risk factors for obesity are more likely to gain weight when eating fried foods than people without these genetic risks.

The study was simple: It took nearly 10,000 women and 6,500 men from the Nurses' Health Study and a replication cohort of more than 20,000 women from the Women's Genome Health Study. Researchers then observed the effects of diet on body mass index (BMI), a rough measurement of body fat.

Researchers from the Harvard School of Public Health found that people in the top third of genetic risk for adiposity, or the propensity for cells to store fat, who ate fried food more than four times a week had a higher BMI than those who ate fried food one time or less a week.

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This was one of the largest studies to date showing a correlation between fried food consumption and increased BMI, based on 32 genetic variants with a known link to BMI. 

"It is not surprising there are some dietary factors that may interplay with our genome in determining obesity risk," said study co-author Dr. Lu Qi, an assistant professor in the department of nutrition at the Harvard School of Public Health. "But before our study, we knew little about which foods play a role. In this regard, it is a bit surprising that fried food is the one." 

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Blame Your Genes

The 32 genetic variants were identified through previous studies, according to Qi. This study is one of the first to suggest that people with a greater genetic risk of obesity are more susceptible to weight gain from eating fried foods.

Study participants answered questionnaires about their dietary habits and changes to BMI. In the food frequency questionnaire, participants were asked whether they consumed fried foods at home or away from home, and while researchers did not ask how the foods were fried, most of these foods in the U.S. are deep fried.

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Weighing the Impact of Fried Foods

Even on the low-consumption end of the study spectrum, genetic disposition to obesity resulted in a higher BMI. Also, women tended to gain more weight than men did.

Eating fried foods more than four times a week may not seem like a lot when you're getting three square meals a day, but the impact of each additional meal of fried food is huge. Researchers found that the combined genetic effect on BMI among people who ate fried foods more than four times a week was almost double the effect on BMI among people who ate fried foods less than once a week.

These findings help explain why for some people, unhealthy eating habits have a greater impact than for others. Ultimately, diet modification could combat or lessen the effect of a higher genetic risk of adiposity. 

"[It's] more calories in than out. Frying makes foods more delicious [but] results in more intake of energy," Qi says. More energy in but less energy expended leads to packed on pounds.

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