Don't waste money on a diet plan that claims to match meals to your DNA - it won't make a difference, a new study has found.
A recent trend in healthy eating circles has been a 'genotype' diet. Proponents claim different people need to eat different foods based on their DNA - for example, some people are supposedly better at burning fat, while others should eat carbs.
To test this, Stanford University researchers put more than 600 overweight but otherwise healthy people on either a low-fat or low-carb diet for a year, after testing their DNA for genes linked to high fat or carb metabolism.
They found both groups lost weight equally well, averaging around 6kg. But there was no link whatsoever between what diet they were on and the tested genes.
"There was no significant difference in weight change among participants matched vs mismatched to their diet assignment," the researchers wrote.
"I had this whole rationale for why these three [DNA variants] would have an effect," said study co-author Christopher Gardner, citing a 2010 study he conducted that suggested genotypes could be the key to weight loss.
"But let's cut to the chase: We didn't replicate that study, we didn't even come close. This didn't work."
He says his previous study appears to have been blighted by outliers - women who failed to lose weight turned out to have gone through significant emotional upheavals, such as a divorce, whilst women who lost weight were fanatic calorie counters.
The latest study made sure participants stuck to their diets so the results would be more accurate.
He called it a "reminder that so many emotional, economic, metabolic, social, and other forces affect someone's chance of losing weight that the effect of genes gets lost in the noise".
"We were so excited and thought this would work. It's humbling."
Companies that develop gene-based diets told science news site STAT the genes Dr Gardner tested for aren't the only ones that can influence weight loss, and other personal info - such as blood types - are also important. But blood type diets have also been shown to be of dubious use.
The latest study was published in the Journal of the American Medical Association.