People's success in losing weight while dieting may vary widely because of the different way they metabolise their food, say scientists.
No one shares exactly the same response to eating the same food because their gut bacteria are different, researchers at the Weizman Institute in Israel suggested.
Eight hundred people were given standardised breakfasts containing 50 grams of carbohydrates for a week to look at the impact on their blood sugar levels.
Different people had different blood sugar level responses to the same kind of food, overturning the idea that certain foods are intrinsically healthy for everyone.
Researchers said the discovery could explain why weight-loss diets that work for some individuals fail so badly for others.
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Dr Eran Elinav, lead scientist on the study from the Weizmann Institute, said:
"Measuring such a large cohort without any prejudice really enlightened us on how inaccurate we all were about one of the most basic concepts of our existenc - which is what we eat and how we integrate nutrition into our daily life," he said.
The finding also means the glycaemic index (GI) of a food, which scientists use to rank its blood sugar impact, cannot be measured objectively but should be tailored to each individual person.
One obese middle-aged woman who had failed to succeed with a range of diets learned her blood sugar levels spiked after eating tomatoes.
"For this person, an individualised tailored diet would not have included tomatoes but may have included other ingredients that many of us would not consider healthy, but are in fact healthy for her," said Dr Elinav.
"Before this study was conducted, there is no way that anyone could have provided her with such personalised recommendations."
Gut bacteria appear to be the main factor in causing different blood sugar level responses to food, tests showed.
The scientists were able to reshape gut bacteria colonies by changing the diets of individual participants, thereby reducing post-meal blood sugar in participants.Reuse content