Some dieters firmly believe in avoiding fat, others avoiding carbs. Does it matter? New research says yes and no.

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On Nutrition

Most people who come to see me for nutrition counseling want to lose weight, even if their main goal is to improve some aspect of their health. For example, managing their blood sugar and cholesterol to reduce the risk of developing type 2 diabetes or heart disease. But for every patient convinced that low-carbohydrate is the way to go for weight or health, there’s another who is firmly in the low-fat camp. Does it matter? New research says yes and no.

Results of the DIETFITS Study (Diet Intervention Examining the Factors Interacting With Treatment Success) were published Feb. 20 in the Journal of the American Medical Association. I’ve been following the progress of this research since interviewing the lead author — Christopher Gardner, Ph.D., professor of medicine at Stanford University — a few years ago and hearing him present his results more recently at a conference.

Gardner’s team randomly assigned 609 healthy men and women ages 18 to 50 (average age 40) to either a low-fat or a low-carb diet. They spent eight weeks going as low as possible with fat or carbs — 20 grams was the goal — then stayed there for a few more weeks. After that, they gradually increased fat or carbs until they reached a point where they were still “low,” but felt they could be satisfied eating that way long after the 12-month study was over. Gardner’s idea was that they would arrive at lifelong eating plan, rather than just following a restrictive diet that would end, as diets do.

By the end of the study, the low-fat diet group was averaging around 57 grams of fat per day, while the low-carb group averaged around 132 grams of carbs daily. This was significantly lower than what they were eating before the study — and it was high quality.
• The low-carb group was encouraged to eat foods like avocados, nuts and seeds, pasture-raised eggs, sustainable fish and grass-fed beef.
• The low-fat group leaned toward whole grains, beans and lentils, seasonal fruit, lean meat and low-fat dairy.
• A common denominator for all was abundant vegetables and fresh salads. Participants also received instruction on preparing meals at home from whole and minimally processed foods.

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The researchers hypothesized that who did well on each diet would depend on insulin levels and genotype — did they have a gene pattern that could affect how they metabolize fat or carbs? But when the big reveal came, neither one mattered. Within each group, there were huge variations in weight loss — some individuals lost more than 60 pounds, while others gained 15-20 pounds. The average weight loss for each group was the same — 13 pounds — and there was no significant interaction between assigned diet and genotype or insulin levels.

The moral of this scientific story? There’s no one perfect way to eat for everyone. My takeaways:
• Go for quality. The study participants were encouraged to choose the highest quality foods that they could reasonably find, afford and enjoy. “On both sides, we heard from people who had lost the most weight that we had helped them change their relationship to food, and now they were more thoughtful about how they ate,” Gardner said.
• If you already like eating lower carb, or lower fat, and it works for you, stay the course. (Not sure? Consider a more moderate Mediterranean-style diet, which has well-supported health benefits.)
• Remember that nutrition is only one tool in your toolbox when it comes to improving health. Physical activity, adequate sleep, social connection and stress management matter, too.