There’s long been debate over whether low-fat or low-carb diets are better for weight loss. Some dieters swear by plans that eliminate grains, fruits and other carbohydrates, while others defend diets that cut down on red meat, dairy and other fatty foods. So who’s right?
Both — and neither, according to new research published in JAMA.
In a 600-person, year-long study, the two eating styles helped dieters drop almost exactly the same number of pounds — and there didn’t seem to be much rhyme or reason as to who succeeded on which plan, explains study author Christopher Gardner, director of nutrition studies at the Stanford Prevention Research Center.
“We really did get a lot of people on both diets to be successful,” Gardner says. “You can be successful on either one. There isn’t any one diet that anyone has to follow.”
The people in the study were all between 18 and 50 years old, and were all overweight or obese but otherwise healthy. The whole group attended nutrition classes, and each person was directed to minimize their intake of sugars, refined flours and trans fats while eating plenty of vegetables and nutrient-dense foods. There were no explicit restrictions on calories, and everybody was encouraged to adopt healthy habits like cooking at home and sitting down for structured meals with family members.
Half the group was randomly assigned to significantly decrease how many carbohydrates they ate, while the other half cut way back on fat.
Individual results after a year were quite varied — one person lost 60 pounds, while another gained 20 — but the average weight loss in each group was almost identical: 11 pounds in the low-fat group, compared to 13 pounds in the low-carb group.
What’s more, the researchers had hypothesized that certain genetic and metabolic markers identified in past studies would predict which people would succeed on which diet — but that didn’t turn out to be the case, Gardner says. About 30% of people in the study group had a genetic signature that, in theory, should have pointed to success on the low-fat diet, while 40% had a low-carb “profile”. But the data didn’t show any strong correlation between these signatures and weight loss on the corresponding plan, Gardner says. Neither did measures of insulin resistance, which the team also thought would be related to success.
“Wouldn’t that be helpful, if we just had this cool, simple genetic test, and you’ll see which one you are? But it failed,” Gardner says. “We would love to personalize it, and we failed to personalize it.”
While that may be frustrating news for dieters, Gardner says there is a silver lining to the finding. The most successful dieters, regardless of which group they were assigned to, credited their achievement to a reframed relationship with food — that is, they began eating more mindfully, cooking at home more often and focusing on whole foods.
“That was more powerful than differentiating between low-carb or low-fat: just getting them to be a lot more mindful about what they were eating,” Gardner says. “It’s almost more accessible to everybody. It’s not so much about that food — it’s really about [changing] this crazy way that Americans eat.”
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