People who primarily eat unprocessed foods may experience similar weight loss on a low-carbohydrate or low-fat diet—suggesting that there's little merit to genetic tests that claim to determine the best diet for a particular person, according to a study published Tuesday in JAMA.
Previous research has suggested an individual's insulin levels or genes could be a good indicator of their response to certain diets. According to Los Angeles Times' "Science Now," that research has led to a surge in products and services offering genetic tests to help people find the diet that is right for them.
However, for the new study, the researchers from Stanford University's Prevention Research Center sought to determine whether a person's insulin levels or genetic makeup actually influence their response to a low-fat versus a low-carb diet.
The researchers randomly assigned 609 adults ages 18 to 50 who were overweight or obese to follow a low-fat diet or a low-carb diet for a 12-month period. At the start of the study, the participants received glucose and DNA tests intended to determine whether they might be predisposed to benefit from one type of diet over another.
Throughout the year, participants attended 22 small-group meetings, where they were encouraged to avoid processed food and follow a healthy diet, consisting of home-cooked meals of whole foods. They were encouraged to be physically active and told to consume the lowest level of carbs or fat that they could sustain "indefinitely." Participants recorded their exercise levels and types of food consumed, but they were not told to count or restrict their calorie intake.
After 12 months, the researchers found that the average weight loss among both groups was similar: The 305 individuals in the low-fat diet group lost an average of about 11.5 pounds, compared with an average of about 13 pounds for the 304 individuals in the low-carb group. Overall results varied, however, with some participants losing up to 60 pounds and others gaining 15 pounds.
The researchers found that the insulin and genomic tests were not good indicators of a subject's success on a diet. They wrote, "Neither of the two hypothesized predisposing factors was helpful in identifying which diet was better for whom."
Instead, the researchers found adherence to a healthy diet appeared to make the biggest difference in weight loss success.
Lead study author Christopher Gardner said the findings suggest that "precision medicine is not as important as eating mindfully, getting rid of packaged, processed food," and avoiding unhealthy eating habits, such as eating while watching television.
However, some experts say the study's methodology was not rigorous enough to rule out the possibility that genes and insulin levels play some role in predicting an individual's diet success.
For instance, Susan Roberts of the USDA Nutrition Center on Aging at Tufts University, said the study relied on self-reported food choices and did not give participants a strict diet to adhere to. She said, "I would bet that if they had implemented their intervention in a way that got the participants actually adhering to the diet recommendations, they would have shown what past studies show, which is that there are individual differences in response to different diets."
Frank Hu, nutrition chief at Harvard's School of Public Health, noted that the study did not test for all gene variations that might affect individual responses to weight loss diets. But Hu said, "In any weight loss diets, adherence to the diet and the overall quality of the diet are probably more important than any other factors."
Reshmi Srinath, director of the Weight and Metabolism Management Program at Mount Sinai Hospital and Icahn School of Medicine at Mount Sinai, said, "The study population is also unique in being very healthy, without comorbidity, and with access to high-quality food. This may not reflect the typical overweight or obese patient" (O'Connor, "Well," New York Times, 2/20; Tanner, AP/Sacramento Bee, 2/20; Monaco, MedPage Today, 2/20; Rapaport, Reuters, 2/20; Healy, "Science Now," Los Angeles Times, 2/20).
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