In the era of personalized nutrition, there might be value in getting your stool tested and your gut bacteria counted before starting on a new diet. The results can be used to predict whether a particular diet will work for you. This follows a study in the International Journal of Obesity, published by Springer Nature, which shows that the increasingly popular fibre-rich "New Nordic Diet" might not work for everyone. Its success depends on the particular combination of bacteria in the intestines of the dieter. The study was led by Mads Hjorth and Arne Astrup of the Department of Nutrition, Exercise and Sports at the University of Copenhagen in Denmark.

"These results are a breakthrough demonstrating that certain bacterial species play a decisive role in weight regulation and weight loss," explains Astrup. "Now we can explain why a high fibre diet does not always lead to weight loss. Human intestinal bacteria is an important part of the answer and will from now on play a role in the treatment of the overweight."

A group of 62 overweight participants were randomly assigned to follow either the "New Nordic Diet" or the "Average Danish Diet." These eating plans vary greatly in the volume of dietary fibre and wholegrain being consumed. The former is the more fibre-rich option and places greater emphasis on wholefoods such as vegetables and fruits. The participants' weight and body measurements were taken before and after they started their 26-week diets.

The results of their stool samples were used to divide participants into two different enterotype or gut bacteria groups. This was done based on the abundance of Prevotella bacteria types found in their intestines compared to Bacteroides species. About half of the group fell in the high volume Prevotella-to-Bacteroides group, whereas the other half were placed in the low ratio group. After the initial 26-week study period, all 62 participants followed the New Nordic Diet for another year.

On average, the 31 subjects who ate the New Nordic Diet for 26 weeks lost 3.5 kilograms, whereas the 23 subjects following the Average Danish Diet lost 1.7 kilograms. The New Nordic Diet worked best for participants in the high volume Prevotella group. They lost 3.15 kilograms more body fat when they followed the New Nordic Diet compared to the Average Danish Diet. Their waistlines also decreased more significantly, and their weight loss was maintained after following the diet for one year. The type of diet followed had no influence on how much weight participants in the low ratio group lost.

"People with a high Prevotella/Bacteroides ratio were more susceptible to body fat loss on a diet rich in fibre and wholegrain compared to an average Danish diet," Hjorth explains.

"The health promoting aspects of the New Nordic Diet in terms of body weight regulation seem mainly to apply to a subset of the population," he adds. "This could apply to as much as half of the population."

Hjorth says that research into the human gut microbiota is increasingly playing a role in personalizing nutrition. He believes that the two relatively stable groups or enterotypes of bacteria species into which people can be grouped could be valuable markers to predict whether specific diets will work for them or not.