Why is it so hard to lose weight? According to a new Weizmann Institute study by Professor Eran Segal and Dr. Eran Elinav, it may be because different people’s bodies respond differently to the same meal, depending on their gut bacteria.

The study, published in the November 19 issue of the journal Cell, followed a group of 800 people for a week and continuously monitored their blood sugar levels. It turns out that the foods most likely to make people’s blood sugar spike diverge widely. For instance, many people’s blood sugar rose sharply after consuming a sugary dessert, but others experienced a blood sugar surge after white bread but not glucose. One participant even saw a sharp rise after eating tomatoes, which she had been consuming under the mistaken impression that they were good for her.

Segal told The Times of Israel that his team decided to focus on blood sugar “because it’s a very relevant parameter for weight management and fat storage, and of course diabetes, cardiovascular diseases, even cancer.”

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Not all calories are created equal, says Segal. Frequent blood sugar surges are directly responsible for weight gain.

“When your blood sugar levels spike, your body secretes insulin, an anabolic hormone that signals the cells to uptake the added sugar and convert it to fat in your cells.”

Many of us have felt tired after a particularly rich meal, and perhaps noticed our waistline expanding the next day, but the truth is, says Segal, most people can’t tell when their blood sugar goes too high.

“That’s why diabetes is known as the silent disease. Even when they become diabetic, some people won’t notice it necessarily. When it’s really bad symptoms start to emerge. But 40 percent of adults worldwide are pre-diabetic, which is a treatable condition, and many don’t even know it.”

A twist on conventional wisdom

The conventional wisdom in nutrition circles is that blood sugar levels are connected to a food’s glycemic index and glycemic load. For instance, glucose has a glycemic index of 100 while a baked potato has a glycemic index of 76. Diets with an abundance of potatoes, white bread, white rice and sweet beverages are more likely to lead to insulin resistance, followed by diabetes.

Conversely, low-carb diets like Atkins and South Beach tend to be successful, explains Segal, because they limit those carbs that send people’s blood sugar soaring.

“Those diets work in the short term,” says Segal, “yet they’ve been proven to fail because eventually people go back to their old habits of eating carbohydrates. Carbs are tasty and they’re all around us.”

The breakthrough of Segal and Elinav’s study is that “if a food has a low glycemic load for the population average, it might not be low for you personally. We had striking findings as to how individualized the responses are.”

For instance, when Segal monitored his own blood sugar, he discovered that dark chocolate and ice cream did not cause much of a spike, but sushi sent his blood sugar through the roof. One participant’s blood sugar responded to bananas but not to cookies with the same number of calories, while another person’s blood sugar did the opposite.

Segal and Elinav collected data on a total of 50,000 meals. In addition, they collected information about each participant’s age, lifestyle, and blood parameters and also took a swab of their gut bacteria. Using this information, they developed an algorithm that learned which gut bacteria and other factors correlate with which glucose responses. A new person who did not participate in the study can use this algorithm to determine which carbohydrates are safe and which he or she should shy away from.

“We also proved that the algorithm works. We did dietary interventions on people and showed that we can give the same person what we call a ‘good diet’ and ‘bad diet,’ equal in calories but one will give them high sugar levels and the other low — and this is predicted by the algorithm.”

The study, called the Personalized Nutrition Project, is looking for volunteers in Israel for a second phase. Volunteers must have been diagnosed as pre-diabetic to qualify.

Why are gut bacteria such a big deal?

If you read health-related articles on the Internet, it’s hard to escape news about the impact of the microbiome (gut bacteria) on weight, health, mood and even conditions like autism. Some gut health enthusiasts go so far as to endorse fecal transplants, while others advocate drinking human breast milk.

But how can we separate the science from the pseudoscience? Dr. Omry Koren, a researcher into the human microbiome at Bar Ilan University’s medical school, says there is currently no scientific research into the microbial effects of adults, as opposed to babies, drinking breast milk. Fecal transplants are done in hospitals to treat Clostridium difficile infections, but other uses are still in early research stages.

Koren does say that there are observable differences in the microbiome of obese and thin people. The microbiome of thin people tends to be more diverse. When researchers transferred gut bacteria from obese humans into thin mice, those mice grew fatter than a control group eating the same diet.

Koren and his colleagues published a study in Nature this year showing that dietary emulsifiers — which are found in many processed foods — change gut bacteria in mice in a way that is linked to metabolic syndrome, obesity and colitis.

How can humans change their microbiome?

“Changing your diet has the quickest impact,” Koren tells The Times of Israel, “but it won’t cause a permanent change, because the bacteria in your intestines are also influenced by factors like genetics and your immune system.”

For instance, if a person stops eating meat and becomes a vegan, their microbiome will change in response.

But Koren won’t say whether the change is for better or worse because scientists don’t yet know which groups of bacteria are better or worse.

“For instance there are groups of bacteria associated with metabolic syndrome that are very important and beneficial in pregnancy.”

Koren says that his colleagues Eran Segal and Eran Eliav at the Weizmann Institute are among the first to offer actionable advice related to the microbiome.

“Because each person responds to food differently, there is no one-size-fits-all dietary recommendation, so we’ll have to wait for apps with individualized food plans,” says Koren.

Segal says the plan is to bring their algorithm to the public at large “in less than a year.”

They will do this first by reducing the set of inputs needed to use the algorithm, and second, licensing their technology to an external organization. Eventually, you will be able to send a swab of your gut bacteria to this organization by mail, and then download an app with personalized diet recommendations. The recommendations will be based on how other people with similar age, health, lifestyle and gut bacteria profiles reacted to meals in the studies.

Asked whether the app will be suited to people who don’t have a lot of money or time to invest but simply want to be healthier, Segal says, “Yes, that is the idea.”

In the meantime, Koren, the Bar-Ilan researcher, offers health advice that he tries to follow in his own life.

“I exercise a lot, try to eat well, and try not to overdo it in anything, not go to extremes.”