Low-fat and low-carb diets produced the same weight loss

Researchers at Stanford tested the idea in the DIETFITS trial, randomizing 609 adults to a healthy low-fat or healthy low-carbohydrate diet for 12 months, then checking whether genotype or insulin secretion predicted who lost more weight on which plan.

Both groups lost similar amounts of weight over the year, several kilograms on average, with no meaningful difference between diets overall. Neither the three-gene diet-responsiveness score nor baseline insulin secretion predicted which individuals did better on low-fat versus low-carb, despite the trial being large enough to detect such an interaction if it existed.

The result undercut the idea, popular at the time, that personalized nutrition based on genotype could reliably steer people toward the diet suited to their biology. Both diets worked about equally well when they emphasized whole foods and cut added sugar and refined grains, suggesting food quality mattered more than macronutrient ratio or genetic matching.

The gene test did not pick a winner.