When two people follow the same weight-loss diet to the letter, but one fails to lose weight, the problem might be their bodies' different responses to the same foods, a recent Israeli study suggests.

That's because when two people eat the same meal, one may experience a spike in blood sugar levels when the other person doesn't, the study found. Over time, elevated blood sugar can lead to health problems like obesity and diabetes.

Many popular diets such as Atkins, The Zone and South Beach center on a component known as the glycemic index (GI), a fixed ranking of foods based on how fast and how high they raise blood sugar after meals. The index was originally developed to help people with diabetes choose foods that wouldn't cause blood sugar to rise too high.

The logic behind the weight-loss diets is that consuming foods with a lower GI like fish, lean meat and vegetables can help keep blood sugar low and promote weight loss.

"The idea behind the low-GI diet is that the glycemic response to a certain food is an intrinsic property of the food which means that we should be able to predict how a certain individual would respond to some food by looking at the average response of a small group of other people to that food," senior study authors Eran Segal and Eran Elinav of the Weizmann Institute of Science in Rehovot said by email. "Our study demonstrates that this cannot be done."

To see how foods are digested, Segal, Elinav and colleagues recruited 800 adult volunteers and collected data through health questionnaires, body measurements, blood tests, glucose monitoring, stool samples and a mobile app used to report lifestyle and food intake for a total of 46,898 meals.

In addition, they asked participants to eat similar meals for breakfast each day.

As expected, age and body weight appeared to impact blood sugar levels after meals.

But the study also found that different people show vastly different responses to the same food, even though their individual responses remained the same from day to day.

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For example, one middle-aged woman in the study had tried and failed a number of diets over the years. Tests found that tomatoes - a food considered healthy on the standardized glycemic index because it's believed to have little impact on blood sugar - were linked to blood sugar spikes after meals for this particular woman.

Overall, the participants showed wide variation in their blood sugar responses to a single food type, such as bread, the study team found. They also varied significantly in the type of meal that provoked the highest blood sugar rise. Researchers also found that two standardized meals designed to be equivalent on the glycemic index could produce opposite blood sugar responses in an individual.

Researchers also did a second experiment with 26 of the participants to see if they could provide personalized nutrition recommendations that would lower blood sugar. When they recommended meals with foods they knew wouldn't raise blood sugar for each person, the people did indeed have lower blood sugar levels after meals.

The study team also noted changes in the gut microbe population after people followed their customized diet.

One shortcoming of the study is its reliance on participants to accurately report on their own food and drink consumption, the authors acknowledge in the journal Cell.

The results, while intriguing, may also be difficult for doctors to use in real life to guide patients to the best nutrition plan, said Allen Taylor, a nutrition researcher at Tufts University in Boston who wasn't involved in the study.

"This is a very complicated paper because of the many techniques used and the many levels of interpretation of data that are required," Taylor said by email.

Many things can influence how individuals respond to certain foods, including genetics, environment, exercise, overall nutrition, the order foods are consumed and the makeup of bacteria in the gut, Taylor added.

"Diet should always be considered as a whole," Taylor noted. "The effect of one component of a diet may differ depending on the other dietary components."