Which diet is best for fat loss? What the research found

Pick any diet, keto, Mediterranean, IIFYM, low-fat, intermittent fasting, and someone will tell you it’s the best one for fat loss. A 2009 randomised controlled trial published in the New England Journal of Medicine tested this directly. Sacks et al. put 811 overweight adults on calorie-restricted diets for two years, varying the macronutrient composition across four groups: high-fat versus low-fat, and high-protein versus average protein, with carbohydrate targets shifting accordingly. At six months, all groups had lost approximately 6 kg, roughly 7% of starting body weight. At two years, the average maintained loss was 3-4 kg, with no statistically significant difference in weight loss between any of the diet types. High-fat, low-fat, high-protein, high-carb, the macro split did not determine the result. What did? The participants who attended the most nutrition counselling sessions lost the most weight, regardless of which diet they were following. Adherence mattered far more than the composition.

How the study was designed

Four groups, one shared goal: find out whether macronutrient composition drives fat loss once calorie intake is controlled.

Each group was assigned a different macro target:

• High-fat, average protein
• High-fat, high-protein
• Low-fat, average protein
• Low-fat, high-protein

Carbohydrate intake shifted to accommodate the fat and protein targets in each group. All participants received structured individual and group nutrition counselling over the two years. This was important, the researchers weren’t leaving people to manage the diet alone. Everyone had support. The only thing that varied systematically was the macro split.

That design allowed one clean question to be answered: once you control for the calorie deficit and give everyone the same level of support, does it matter whether the diet is high-fat or low-fat, high-protein or average?

What the results actually showed

At six months, the answer was unambiguous. All four groups had lost roughly the same amount, approximately 6 kg, or about 7% of starting body weight. No group had a statistically significant edge.

Four different macronutrient splits, the same fat loss result at six months

Sacks et al. (2009) found no significant difference in weight loss between high-fat, low-fat, high-protein, and average-protein diets across 811 overweight adults at six months or two years.

By two years, average maintained loss had settled at 3-4 kg across all groups. Some regain is expected as structured adherence drifts over time, but the pattern held. No diet type pulled ahead. The macro ratio did not change the outcome.

What actually separated people who lost more

The most instructive finding wasn’t in the diet comparisons at all.

Across all four groups, participants who attended more nutrition counselling sessions lost more weight. Consistently. Regardless of whether they were on a high-fat or low-fat plan. Showing up, getting guidance, and sticking to the plan had a bigger influence on results than anything on the plate, which is exactly the structure regular nutrition coaching is built to provide rather than leaving someone to interpret a diet on their own.

This points to something straightforward that tends to get buried under macro debates: the limiting factor for most people isn’t which diet they’re on. It’s maintaining a consistent calorie deficit over weeks and months, and understanding what actually drives fat loss versus fat burning helps that stay consistent. Accountability and structure support that in ways macronutrient tweaks simply don’t, which is also why knowing who to actually ask for nutrition guidance matters as much as the plan itself.

Reference: Sacks FM, Bray GA, Carey VJ, et al. Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates. N Engl J Med. 2009;360(9):859-73.