Low-Fat or Low-Carb Diets: Does One Work Better?

Few debates in nutrition run as long as low-fat versus low-carb. Each has devoted fans and plausible reasons to work. In 2018, researchers at Stanford published a large trial designed to put both to a fair test, and also to see whether genes or insulin levels could predict who would do better on which. This guide walks through what it did and why its answer is practical. It is general education, not medical advice.

The study: DIETFITS

The trial, called DIETFITS, was published by Christopher Gardner and colleagues in the Journal of the American Medical Association. It enrolled 609 adults aged 18 to 50 without diabetes, with a BMI between 28 and 40. They were randomly assigned to a healthy low-fat or a healthy low-carb diet for 12 months.

• Both groups got the same kind of support. They attended group sessions with dietitians, and the emphasis was on quality: vegetables, whole foods, minimal added sugar and refined flour, and cooking at home.
• Neither group was given calorie targets. The goal was to eat well and stay with it.
• Researchers also tested two ideas that had been popular: that a person’s genetic pattern, or their insulin response, could tell who would do better on which diet.

What they found

• Weight loss was similar. After 12 months, the low-fat group had lost an average of about 5.3 kg (around 12 lb) and the low-carb group about 6.0 kg (around 13 lb). The difference was not statistically significant.
• Wide variation within each group. Some people lost 20 kg or more on either diet, while others gained a little.
• Genes and insulin did not predict the winner. Neither the genetic pattern nor insulin response identified who would do better on which diet.
• Some health markers moved differently. Both groups improved blood pressure, insulin and glucose, while LDL cholesterol favored the low-fat group and triglycerides and HDL favored low-carb.

Why the answer is useful

• Quality seemed to matter more than the label. Both groups cut added sugar and refined grains and ate more vegetables and whole foods.
• Both can work. There is no need to force yourself into the style you dislike.
• Staying with it is the bigger question. The big spread within each group points to adherence and fit.

Things to keep in mind

• Some people dropped out. About 80 percent completed the 12 months.
• The participants were adults without diabetes. Results may not apply to people with diabetes or other conditions.
• “Healthy” mattered. The trial used whole-food versions of each diet, not processed low-fat products or low-carb snack foods.

How to use this

• Start with what you enjoy. Pick the way of eating you can see yourself doing for a long time.
• Focus on the shared basics. More vegetables, fewer sugary drinks and refined snacks, and food you cook yourself. Our ultra-processed foods guide goes into that.
• If you lean low-carb, see our guides on keto. If you prefer a middle path, the Mediterranean diet is one of the best-studied.
• Check the label. Our guide to reading a nutrition label helps with either approach.
• Talk to a clinician or dietitian if you take medication, have diabetes or heart disease, or have a history of disordered eating.

The takeaway

If you were hoping the research would crown a single winner, it did not, and that is helpful news. A well-built version of either diet worked about equally on average. The best choice is the one you can enjoy, afford and keep doing.

Sources

• Gardner CD, Trepanowski JF, Del Gobbo LC, et al. Effect of low-fat vs low-carbohydrate diet on 12-month weight loss in overweight adults and the association with genotype pattern or insulin secretion: the DIETFITS randomized clinical trial. JAMA, 2018;319(7):667-679.
• Stanford Medicine news release on the DIETFITS trial, 2018.