Why Are Some People Fat? One Gene, Not Diet, Might Be the Problem

High fat diets could be detrimental for people with a gene mutation, even if they're eating fewer calories. Joe Raedle/Getty Images

It often seems that some possess superhuman eating powers allowing them to down an entire pizza while remaining rail-thin. Others only need to think of a slice and gain five pounds. Now one doctor says there's evidence genetics could be behind some of these differences. Regardless of how much you eat, your weight may be out of your hands.

Related: Low-calorie diet could help reverse Type 2 diabetes

"Sure, you need to watch your food consumption, but sometimes people can just look at food and get fat," said Vann Bennett, a biochemist at Duke University.

Bennett led a new investigation that targeted variants of the ankyrin-B gene, found in millions of people, that could leave some people predisposed to being overweight. Prior studies have shown that mice with alterations of the gene gained more weight, even when receiving the same amount of food and exercise as their counterparts. This time, Bennett's team discovered why this happens.

They engineered mice to have several common modifications of the gene found in humans. They observed that mice who had mutations of ankyrin-B took more glucose into their fat cells, which in turn made more fat. Typically, the plasma membrane acts as a barrier to prevent glucose from entering these cells; the alteration kept the gate open.

The change may serve a useful purpose. "Probably this is not always a bad thing," Bennett told Newsweek. "It could help people survive famines in the past. But today we have so much food, that it probably is a bad thing." Our modern diets of fast food drive-thru windows and aisles of packaged snacks make the alteration work against us.

Dieters have long been told to watch their calories and exercise more, but this new finding suggests that a blanket approach doesn't work for everyone. And the study illustrates a common problem for people: increased weight gain as a function of age.

Bennett explains that the mice packed on the pounds gradually, beginning at around 10 months old, which is the equivalent of a 45-year-old person. Our metabolism naturally slows with age, making it harder to maintain the weight of our 30-year-old selves when we're 50. Now add an unruly ankyrin-B gene, and it may seem impossible to stay slim.

While old dieting advice is typically centered around counting calories, there's mounting evidence that a calorie just isn't a calorie, which Bennett affirms. Research has shown that energy consumed from protein impacts our bodies differently than when it comes from sugar, though buzzy plans like the Twinkie Diet have been modeled off the calorie counting approach.

A nutrition professor popularized his Twinkie Diet after losing 27 pounds in two months from eating snack cakes. Scott Olson/Getty Images

The mice in the study gained more weight when on high-fat diets. Bennett believes this is because once the fat cells received the glucose and start making more fat, they become sensitive to other fats. This means that popular high-fat diets like Atkins might work for some, but could actually backfire for others. "That kind of diet long-term likely would be not good for people with this mutation," he said. "They would tend to put on more fat even though the number of calories is the same."

Despite being studied in mice, the researcher believes further research on this gene, and possibly others, could potentially create a field of customized diets and health plans based on genetics. But although DNA sequencing is becoming cheaper, it's definitely not mainstream yet. Still, Bennett envisions such assessments being performed at birth one day.

For now, frustrated dieters can take comfort with one mantra: It's not you, it's your genes.