Why Losing Weight Is So Hard for Some People

A new study shows people’s bodies react to the same foods in very different ways, adding to a growing body of research that suggests people may be better able to achieve weight loss if their diet was tailored to them, rather than following general advice about foods to eat and avoid.

In the new study, published this month in the journal Cell, a team of Israeli researchers looked at a variety of biomarkers in 800 people between the ages of 18 and 70. For one week, the men and women wore devices that measured their blood sugar levels every five minutes. They also used a mobile app to closely record their food intake, sleep and exercise. In addition, they filled out questionnaires about their health, and provided blood and stool samples for testing.

The researchers found that blood sugar levels varied widely among people after they ate, and these levels were highly variable even when the researchers had the people eat the exact same meal. Sometimes a food that would result in low blood sugar for one person, would cause high blood sugar for another. This information, they argue, suggests umbrella recommendations for how to eat don’t do the trick.

“For many years, our thinking has been that people develop obesity, diabetes and other diet-related diseases because they are not compliant with our dietary advice,” lead researchers Eran Segal and Eran Elinav of the Weizmann Institute of Science in Israel told TIME in a joint email about their findings. “However, based on our study, another possibility is that people are in fact compliant but that the dietary advice that we are giving them is inappropriate.”

“We believe a take-home message for people from our work is that if a diet did not work for you, it may be the diet’s fault and not your fault,” they add.

What the researchers believe could be responsible for these differences is the microbiome—trillions of bacteria that live in the gut and differ wildly from person to person. Another recent study published in the journal Obesity Research & Clinical Practice found that even if they exercised and ate the same amount, an adult in 2006 is heavier than one in 1988. The study authors also suggested that changes in the microbiome could be at play, amid other possibilities.

“We are only just beginning to explore how the complex microbiome influences our physiology and health,” says study author Jennifer Kuk, a professor of kinesiology and health science at York University. “This [new study] is another that shows promising evidence that the microbiome may play an important role in how we regulate body weight and could be a novel target for future weight loss interventions.”

Segal and Elinav say they’re already moving that science forward. In their study, they also took all the data they collected and created algorithms that were able to predict how a person’s blood sugar would respond to the food they ate. They say that down the line, they believe their algorithm could be used to create personalized diets for people.

“We showed that the comprehensive profile that we measured can be used to achieve and design personally tailored diets,” they told TIME. “Our vision is to be able to derive predictions and personalized diets using a small set of inputs that people could fill out in questionnaires and a single microbiome sample, and we believe that this is both achievable in the near future and that that would be cost effective.”