What's considered 'healthy' could be different for everyone. 

By Grace Elkus
Updated November 20, 2015
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We've probably all been there—a friend swears by her new diet, but when you give it a try, you don't see the same results. What gives?

According to a new study conducted by the Weizmann Institute of Science in Israel, individual people can have extremely different responses to the same foods, and universal diets and dietary recommendations might not be as helpful as previously thought. The results are published in the journal Cell.

Eight-hundred non-diabetic people participated in the week-long study, during which they received standardized meals for breakfast. The participants used a mobile app to report their lifestyle habits and food intake, and additional data was collected through health questionnaires, blood tests, body measurements, glucose monitoring, and stool samples. A total of 46,898 meals were measured.

Although the glycemic index (which is often used to develop healthy diet plans) ranks foods based on how they affect the blood sugar levels of groups of people, the researchers saw that how the food was metabolized varied greatly from person to person—and that the glycemic index of any given food depends largely on the individual. The age and body-mass index (BMI) of the participants were found to be strongly linked with differences in metabolism, as were gut microbes.

The individualized feedback was particularly compelling, and provided support for the importance of personalized nutrition plans. One woman's blood sugar spiked when she ate tomatoes—a food that any universal diet would have advertised as a healthy choice.

"For this person, an individualized 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," Eran Elinav, one of the researchers, said in a statement.

Because the increase of blood glucose levels can lead to pre-diabetes and impaired glucose tolerance, and has been linked to obesity, hypertension, non-alcoholic fatty liver disease, and cardiovascular disease, these results are crucial to understanding how to best help people identify which foods are best for their health.

"After seeing this data, I think about the possibility that maybe we're really conceptually wrong in our thinking about the obesity and diabetes epidemic," said Eran Segal, the other lead researcher. "The intuition of people is that we know how to treat these conditions, and it's just that people are not listening and are eating out of control—but maybe people are actually compliant but in many cases we were giving them wrong advice."