The effect isn't huge. But for some people, the extra pounds could be the difference between being "normal" weight and overweight.

Eating fried foods four or more times each week is associated with two extra points on the body mass index scale when you carry 10 obesity risk genes, the study found.

But here's the surprising part: For people who don't have any of the obesity genes, one of those extra BMI points melt away. In essence, those mozzarella sticks are more fattening for those of us predisposed to a bigger waistline.

To find this interaction between genes and fried foods, epidemiologist Lu Qi and his team analyzed the dietary habits of nearly 30,000 adults in the U.S. They also calculated each person's genetic risk for obesity by analyzing 32 genes known to be linked to the disease. Having these genes doesn't mean you're obese. Instead, they increase your risk for having a higher BMI at some point in your life.

As expected, people who ate more fried foods gained more weight than those who indulged less frequently. And people who carried more of the obesity genes also had higher BMIs.

But the volunteers who had both risk factors — eating lots of fried food and a high number of fat genes — had the largest BMIs of all groups and the biggest risk for obesity.

In other words, a love of fried food may magnify your genetic disposition for adding on pounds. Qi and his colleagues think the same may be true for sugary drinks.

A few years ago, they performed a similar experiment — instead looking at fried foods, they analyzed people's weekly consumption of sodas, fruit juices and lemonade.

Drinking at least one sugary beverage each day was associated with a bump in BMI by about 1.8 points, when people carried 10 of the obesity genes. The soda penalty shrink by nearly half for people without the risk genes, the team reported in The New England Journal of Medicine.

"In Westernized societies, we are all exposed to calorie-dense food, sedentary lives, stress and sleep deficit," geneticists Alexandra Blakemore and Jessica Buxton, from Imperial College London, wrote in an comment about the current study. "Some people seem relatively insensitive to these environmental pressures, while others are severely affected and become obese."

The Harvard studies offer an explanation for this observation. But they come with limitations.

For starters, Qi and his team couldn't tell whether the fried foods or sugary drinks caused the increase in BMI. That's because a love of fried foods could be an indicator of other unhealthy habits. In the study, people who indulged more often also tended to watch more TV and get less exercise.

The findings are also averages over a large number of people. "They have poor predictive power for any given individual," Blackmore and Buxton write.

Nevertheless, the synergy between fried food and genes offers one more reason to take pause before ordering that aromatic fried chicken — even if you don't have obesity genes.

And it's pretty easy to figure out whether you do, Christpher Ochner tells USA Today. You don't need a fancy DNA test. "Just take a look at your parents," he says.

Copyright 2014 NPR.

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French-Fry Conspiracy: Genes Can Make Fried Foods More Fattening