The most famous argument in the diet world, and the largest trial designed specifically to settle it — with a result neither camp expected.
The trial
The DIETFITS study (Gardner et al., JAMA 2018) randomly assigned 609 adults (aged 18–50, BMI 28–40, without diabetes) to a healthy low-fat or healthy low-carbohydrate diet for 12 months[1].
Participants attended 22 group sessions with health educators focused on reaching the lowest fat or carbohydrate intake they could sustain long term, with emphasis on food quality[1].
loss between the diets
The result
A draw. The trial found no significant difference in 12-month weight loss between the healthy low-fat and healthy low-carbohydrate diets[2],[3]. It also found that genotype pattern and insulin secretion did not predict which diet suited a given person[1] — the very hypothesis the study was built on.
So what did determine success?
A secondary analysis published in the American Journal of Clinical Nutrition linked weight-loss success to dietary adherence and diet quality — across both diets[4]. In other words, the right question isn't "which diet is better?" but "which diet can you stick to?"
What this means for you
- Choose what fits your palate and your life: if you love rice and bread, keto is a poor choice for you no matter how "optimal" it looks in theory.
- Quality was shared by both arms: both groups focused on real food — not keto-with-sausages, nor low-fat-with-sweets.
- A calorie deficit is the common denominator — whichever road takes you there (calculate it).
- Beware "genetic diet" sellers: the trial tested that exact idea and found no support for it[1].