The New Nordic Diet is based on large amounts of fiber, especially root vegetables, dark greens, and foraged plants. For some people, it works well to shed the pounds and shrink the waistline. But for others, it’s just another diet that doesn’t work.

The outcome of the diet may not depend on willpower, however. The kinds of bacteria in the intestines may be more responsible for whether specialized diets such as the New Nordic work at all, according to a new study in the International Journal of Obesity.

“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. This could apply to as much as half of the population,” said Mads Hjorth, one of the authors, of the University of Copenhagen.

The researchers looked at 62 people who underwent long-term dieting. Their waists and weight were measured, and a sample of their stool was categorized by quantitative PCR analysis between two enterotypes: a biome dominated by species from the Prevotella bacterial genus, or the Bacteroides genus.

The participants were then randomly assigned to either the New Nordic Diet, based on the planets and fiber and fish intake, or the Average Danish Diet, based on traditional nutritional recommendations and limitations.

The groups stayed on their respective diets for 26 weeks. But then all 62 participants maintained the New Nordic Diet for another whole year, according to the study.

The New Nordic Diet generally worked better for weight loss, with an average of 3.5 kilograms compared to 1.7 for the group following the Average Danish.

But the dieters that benefited most were those with more Prevotella bacteria. Those subjects lost 3.15 kilograms more body fat, and their waistlines decreased more significantly than their peers who had more Bacteroides in their intestines, the study reports.

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

The cause-and-effect is not entirely clear. One theory is that Prevotella species have more short chain fatty acid production, they write.

“From our analysis we cannot determined specific bacterial species responsible for the dietary effects that we observe but only highlight the relative abundance of Prevotella species as important in the classification of microbiota profiles,” they write.

But the overall bacterial profiles could explain why the New Nordic Diet – or its cousin the “Mediterranean Diet” – works so well for some, but not for others, the authors conclude.

“These results are a breakthrough demonstrating that certain bacterial species play a decisive role in weight regulation and weight loss,” said Arne Astrup, another of the authors. “Now we can explain why a high-fiber 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.”

Astrup has previously been part of a comparison between the New Nordic Diet and the Average Danish Diet, last year in the Journal of Proteome Research. The team assessed blood samples of 145 individuals using gas chromatography – mass spectrometry, and found that the New Nordic improved insulin sensitivity through increasing ketosis and gluconeogenesis.