What is "healthy" food for some may not be for others, researchers found. (Barbara Damrosch)

If you've ever tried out the latest diet fad only to find yourself gaining weight and feeling awful and wondered what you were doing wrong, scientists now have an explanation for you.

Israeli researchers, writing in the journal Cell this week, have found that different people's bodies respond to eating the same meal very differently — which means that a diet that may work wonders for your best friend may not have the same impact on you.

Lead authors Eran Segal and Eran Elinav of the Weizmann Institute of Science focused on one key component used in creating balanced diet plans like Atkins, Zone or South Beach. Known as the glycemic index or GI for short, it was developed decades ago as a measure of how certain foods impact blood sugar level and has been assumed to be a fixed number.

But it's not. It turns out that it varies widely depending on the individual.

The researchers recruited 800 healthy and pre-diabetic volunteers ages 18 to 70 and collected data through health questionnaires, body measurements, blood tests, glucose monitoring and stool samples. They also had the participants input lifestyle and food intake information into a mobile app that ended up collecting information on a total of 46,898 meals they had.

Each person was asked to eat a standardized breakfast that included things like bread each morning.

They found that age and body mass index, as expected, appeared to impact blood glucose level after meals, but so did something else. Different individuals showed vastly different responses to the same food, even though their own responses remained the same day to day.


(iStockphoto)

"There are profound differences between individuals — in some cases, individuals have opposite responses to one another," Segal explained.

The researchers said the findings show that tailoring meal plans to individuals' biology may be the future of dieting and the study yielded many surprises for individuals. One example involves a middle-aged woman who tried and failed with many diets. Tests showed that her blood sugar levels spiked after eating tomatoes — which the researchers said appeared to be a poor choice for her since high blood sugar has been associated with heart problems, obesity and diabetes — but since she didn't know this, she was eating them as part of her healthy diet plans several times a week.

Elinav said the work "really enlightened us on how inaccurate we all were about one of the most basic concepts of our existence, which is how we eat and how we integrate nutrition into our daily life."


( Melina Mara/The Washington Post)

To drill down even deeper into the question of why such vast differences exist, the researchers designed another experiment that involved personalized dietary interventions on 26 new volunteers. The goal was to reduce post-meal blood sugar levels. The clinicians designed two sets of specialized meals — breakfast, lunch, dinner and up to two intermediate meals — for each person that were theorized to be a "good" diet or a "bad" diet. Every participant followed the diets for a full week. The good diets worked, and not only did they see their blood sugar levels going down, they found alterations in their gut microbiota. One interesting finding was that even though the diets were very personalized, several of the changes in the microbiota were similar for participants.

This appears to imply, the researcher said, that we're "really conceptually wrong" in our thinking about the obesity and diabetes epidemic.

We think "we know how to treat these conditions, and it's just that people are not listening and are eating out of control," Segal said, "but maybe people are actually compliant and in many cases we were giving them the wrong advice."

By using the information from the study, the researchers were able to come up with the holy grail of dieting: an algorithm that takes hundreds of factors about a person and turns them into a tailor-made meal plan. The results were pretty surprising to both the doctors and participants. “It wasn't just salad every day,” Segal told The Atlantic. “Some people got alcohol, chocolate, and ice-cream, in moderation."

Lua Wilkinson, a doctoral candidate in nutritional sciences at Cornell, said the study is interesting because it shows "the way people control their blood sugar involves more than just carb intake or glycemic index. However, she cautioned, "it said nothing about health effects or weight loss."

Nonetheless, many of the participants of the study were so excited that they spread word of the experiments to their friends and family and now the researchers have more than 4,000 on their waiting list for their next study.

This post has been updated.

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