Low-fat or low-carb: which diet leads to more weight-loss?
Common dieting advice urges people to either eat fewer carbohydrates or less fat in order to shed weight.
But a new study found neither approach is better than the other.
Nor is a person's genetics or insulin metabolism a key factor in whether a diet works for them or not, said the report in the Journal of the American Medical Association (JAMA).
The findings could have implications for the weight loss industry, and particularly the latest trend of DNA dieting, which claims to point people to the best diet for their genes.
"We've all heard stories of a friend who went on one diet - it worked great - and then another friend tried the same diet, and it didn't work at all," said lead author Christopher Gardner, professor of medicine at Stanford University.
"It's because we're all very different, and we're just starting to understand the reasons for this diversity. Maybe we shouldn't be asking what's the best diet, but what's the best diet for whom?"
The study enrolled 609 people - 57% women - aged 18 to 50 and randomly assigned them to either a low-fat or low-carb diet for a year.
At the end, the average weight loss was 5.9kg (13 pounds) in both groups.
Some individuals lost far more - up to 27kg (60 pounds), while some gained 9kg (20 pounds). But researchers were unable to find any link between dietary approach and superior weight loss.
After a year, "there was no significant difference in weight change between a healthy low-fat diet vs a healthy low-carbohydrate diet," said the report.
At the beginning, "participants got part of their genome sequenced, allowing scientists to look for specific gene patterns associated with producing proteins that modify carbohydrate or fat metabolism," said the report.
They also drank a shot of glucose on an empty stomach so researchers could measure their bodies' insulin outputs.
"Neither genotype pattern nor baseline insulin secretion was associated with the dietary effects on weight loss," it said.
What seemed to help people lose weight was following a single strategy: eat less sugar, less refined flour, and as many vegetables and whole foods as possible.
"On both sides, we heard from people who had lost the most weight that we had helped them change their relationship to food, and that now they were more thoughtful about how they ate," said Gardner.