Low-Carb vs. Low-Fat Diet: What the DIETFITS Trial Found

Last updated October 30, 2026

Short answer: In the DIETFITS trial, 609 adults followed either a healthy low-fat or a healthy low-carb diet for a year. The low-fat group lost 5.3 kg and the low-carb group 6.0 kg, a 0.7 kg difference that was not statistically significant. Neither a person's genetic pattern nor how much insulin they released predicted which diet would work better for them.

What DIETFITS tested

The trial enrolled 609 adults aged 18 to 50 without diabetes and with a BMI of 28 to 40. They were randomly assigned to a healthy low-fat diet (305 people) or a healthy low-carbohydrate diet (304 people) for 12 months. In total 481 people (79%) completed the trial.

The question the trial also asked

Many people believe the right diet depends on your body. The study checked whether a particular genotype pattern or the amount of insulin a person released after sugar predicted who would lose more on each diet. Neither did. The authors concluded there was no significant difference in weight change between the diets, and that neither factor helped identify which diet works better for which person.

What this does not tell you

• Keto. The published summary calls the second diet a healthy low-carbohydrate diet and does not describe it as ketogenic, so these results should not be read as evidence about keto diets.
• Older adults or people with diabetes. The trial enrolled adults 18 to 50 without diabetes.
• Years beyond the first. The trial followed people for 12 months.
• Your individual response. These are group averages, and individuals will differ.

How it fits with other diets

A 2016 systematic review of trials lasting at least a year found the Mediterranean diet produced weight loss similar to low-carbohydrate diets. Across these studies, no single diet clearly wins on weight, and the plan you can keep up appears to matter more. See our Mediterranean diet page and intermittent fasting page.

Practical takeaways

• Choose the version you can keep doing, since the trial found no meaningful winner on average.
• Both diets in the trial were healthy versions, per its title, so the results may not apply to a diet built mostly on processed foods.
• If you take medicine for diabetes or another condition, check with your prescriber before a big dietary change.

Related: lifestyle vs. GLP-1 weight loss and weight loss percentage calculator.