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Body and mind

Only half lose weight on a high-fibre diet

Diets sometimes promise more than they can deliver. Sometimes they even contradict each other. Confusion about contradictory and ineffective diets can sometimes make people give up trying to improve their health. Now Danish researchers may have revealed some of the reasons. Their study found that the ratio between two types of gut bacteria was associated with whether overweight people lost weight. The new results explain why one-size-fits-all dietary guidelines may not always be helpful.

How can carbohydrate, protein and fat be both healthy and unhealthy? How can different studies even confirm that various types of food and diets have positive and negative effects? New research in which half the participants ate a standard Danish diet and half a fibre-rich new Nordic diet reveals that our intestinal bacteria partly explain why research can draw such vastly different conclusions.

“The studies show that only about half the population can lose weight if they follow the official dietary guidelines and eat more fruit, vegetables, fibre and whole grains. The other half does not seem to lose weight from these dietary changes,” says Mads Fiil Hjorth, Assistant Professor, Department of Nutrition, Exercise and Sports, University of Copenhagen.

Variation from person to person

The 26-week trial included 62 participants. The ones following the new Nordic diet shed an average 1.8 kg more than those who ate a standard Danish diet. The researchers observed substantial variation in the results and therefore decided to examine more closely the differences between the participants. They found the reason for the difference in the stool samples they had taken from the participants before the study began.

“We categorized the participants by their gut microbiota, and those with a high ratio of Prevotella bacteria to Bacteroides bacteria lost 3.5 kg more over 26 weeks by following the new Nordic diet than those eating a standard Danish diet.”

Conversely, participants who had a low Prevotella-to-Bacteroides ratio lost the same amount of weight regardless of which diet they ate.

“Since 2011, we have known that the Prevotella-to-Bacteroides ratio varies geographically and from person to person, and in recent years there have been attempts to link this to various disease risks. However, this is the first time that anyone has shown that the proportions of bacteria are important in determining whether a person succeeds in losing weight or not.”

One size does not fit all

This study is one of many confirming that gut bacteria play a major role in health. The trials also show that gut microbiota potentially play a decisive role in a person’s individual metabolism and accumulation of fat and that changes in the proportions of gut bacteria can strongly affect health.

“Nevertheless, the Prevotella-to-Bacteroides ratio remains fairly constant for each person – even if we try to alter it through dietary changes. Naturally, we hope to find another strategy that works well for the people with fewer Prevotella bacteria who do not lose weight eating a fibre-rich diet. However, until then, they should instead focus on following other recommendations about diet and physical activity.”

It is too early to conclude why the Prevotella-to-Bacteroides ratio results in this difference. Previous studies showed that the Prevotella bacteria produce other and more short-chain fatty acids than the Bacteroides bacteria when breaking down the same fibre, and these short-chain fatty acids influence the secretion of the hormones that control appetite. The ratio between bacteria may therefore indicate a person’s health status and can help in targeting appropriate guidelines.

“This is a major step forward in the efforts to tailor diet for each individual. Current dietary guidelines are based on a one-size-fits-all approach that does not always work, whereas individual information obtained through stool samples and blood tests can play an important supplementary role to dietary guidelines because it provides opportunities to tailor the guidelines and thereby treatment to each person.”

Pre-treatment microbial Prevotella-to-Bacteroides ratio determines body fat loss success during a 6-month randomized controlled diet intervention” has been published in the International Journal of Obesity. The study was carried out in a collaboration between researchers from the Department of Nutrition, Exercise and Sports, University of Copenhagen; National Food Institute, Technical University of Denmark; Gelesis Inc., Boston; and Steno Diabetes Center.

Mads Fiil Hjorth
Associate Professor
My research has primarily focused on movement behaviors (physical activity and sleep) and the link to nutritional behavior, obesity and health (mental and metabolic). Recently, we made a dramatic breakthrough in the area of personalized nutrition showing that diets should be tailored to different groups of individuals. This is something scientists have searched for in decades and might be one of the major reasons why one superior diet will never be identified. In the future I will follow this lead and design RCT’s that answers these specific questions.