Diet dilemma: why weight loss depends on gut bacteria

Some people lose weight more easily than others and this could be due to the type of bacteria in the dieter’s intestines, according to a study published in the International Journal of Obesity.

The study randomly assigned 62 overweight participants to follow either the New Nordic Diet or the Average Danish Diet. The Nordic diet is common in countries such as Iceland, Norway and Denmark and contains a higher volume of dietary fibre and wholegrain. This includes fibre-rich, plant-based foods such as berries, cruciferous vegetables, root vegetables, apples and pears, fish and lean meats. The participants’ weight and body measurements were taken before and after they started their 26-week diets.

To establish the bacteria in their intestines, stool samples were taken and participants were divided into two groups depending on the abundance of Prevotella bacteria types compared to Bacteroides species. About half of the group fell into 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.

Led by Mads Hjorth and Arne Astrup of the Department of Nutrition, Exercise and Sports at the University of Copenhagen, the study found that the success of the diet and participants’ ability to lose and maintain weight depended on this particular combination of bacteria.

“These results are a breakthrough demonstrating that certain bacterial species play a decisive role in weight regulation and weight loss,” stated 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.”

Of the 31 people following the New Nordic Diet for 26 weeks, participants lost an average of 3.5 kg, whereas the 23 subjects following the Average Danish Diet only lost 1.7 kg. The study found that participants in the high-volume Prevotella group were more susceptible to weight loss with the New Nordic Diet compared to the Average Danish Diet, losing about 3.15 kg more body fat. However, the type of diet had no impact on how much weight participants in the low ratio group lost.

“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,” said Hjorth. “This could apply to as much as half of the population.”

Hjorth suggested that personalised nutrition could be aided by research into gut bacteria in the future, as grouping people into one of the two types of enterotypes or bacteria could indicate whether specific diets will work for them or not.