Eating plans that focus on matching your diet to your DNA or to the rate at which your body pumps out insulin (a hormone that has an impact on fat storage) have been popular recently. But a new study published today in the Journal of the American Medical Association found that neither genetic makeup nor the amount of insulin the body secretes determined how much weight people lost on either a low-fat or a low-carbohydrate diet.

Still, there are a few takeaways from the new study that may make losing weight a little easier, says the lead researcher, Christopher Gardner, Ph.D., a professor of medicine in the Stanford Prevention Research Center at Stanford University. 

What the Researchers Did

It’s no secret that an eating plan that works for one person may not be right for another. “We wanted to find out which diet is better for who,” Gardner says.

At the start of the study, Gardner and his colleagues hypothesized that certain genetic makeups would determine whether someone would do better on a low-fat or a low-carb diet.

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They also wanted to test the theory that individuals who are insulin-resistant—which means they must produce excess insulin to process carbohydrates—would respond better to a low-carb diet.

The 609 overweight or obese men and women in the study had a blood test to determine their gene type and a glucose tolerance test to see whether they were insulin-resistant.

But without knowing the results of those tests, the researchers randomly assigned the participants to either a low-fat or low-carb diet group, so both groups contained people with different gene types. 

At the start of the study, the researchers instructed the participants to stick to around 20 grams of fat or carbohydrates a day, depending on which diet group they were in.

After eight weeks, the study participants got to gradually increase the amount of fat or carbohydrates in their diet until they reached a level they could maintain over the long run. “They had to look us in the eye and say, ‘I could eat like this forever,’ ” says Gardner.

The weight changes in both the low-fat and low-carb groups were similar over 12 months, with some people losing as much as 60 pounds. The average weight loss in both groups was around 11 pounds.

Yet neither genetic type nor insulin resistance seemed to explain why different diets worked for different people, says Gardner. 

Lessons Learned

For the most part, the study subjects were highly successful at sticking with their assigned plans. That’s probably because they were given the freedom to eat a “low-fat” or “low-carb” diet on their terms, rather than being given a specific allotment of nutrients or calories, Gardner says. 

“Among the people who were successful," he says, "what we really heard from them was that we changed their relationship with food.” All study participants, regardless of which diet they were following, were instructed to focus on three basic tenets: minimizing refined grains and added sugars, eating more vegetables, and choosing minimally processed, nutrient-dense, home-cooked foods whenever possible.

Determining a healthier way of eating that you can comfortably stick with over the long run can be a key to weight-loss success. “Although personalized medicine may have some insights, the issue is still adherence after you lose weight,” says William Dietz, M.D., Ph.D., chair at the Redstone Global Center for Prevention and Wellness at George Washington University.

The focus on diet quality throughout the study, however, may have had an impact on some of the findings. For example, had the low-fat group been eating refined carbohydrates in the form of white flour and added sugars instead of healthy types of carbohydrates, people with insulin resistance may have been less likely to lose weight, says Gardner. Bottom line: The quality of your food choices counts, regardless of your diet’s overall makeup.  

So does satiety. Throughout the course of the study, subjects were instructed to eat foods that helped them feel fuller for longer. “There isn’t one way to do that,” says Gardner, who points out that personal preference, social norms, culture, and other factors play roles in helping you eat less without feeling deprived.

Researchers are looking to uncover more about how your biology might predict slim down success. Until then, using trial and error to find the approach you can stick with is the personalized strategy that can help you reach and stay at your healthiest weight.