Submitting a stool sample to determine if you’re able to lose weight on a newly characterized diet doesn’t seem like the greatest way to start the day, yet new evidence from investigators at the University of Copenhagen shows that the bacteria we all have in our gut may play a decisive role in personalized nutrition and the development of obesity. Findings from the new study—published recently in the International Journal of Obesity in an article entitled “Pre-Treatment Microbial Prevotella-to-Bacteroides Ratio, Determines Body Fat Loss Success during a 6-Month Randomized Controlled Diet Intervention”— reveal that the ratio of two gut bacterial species can determine whether an individual can lose weight by following dietary recommendations characterized by a high content of fruit, vegetables, fibers, and whole grains.

“Human intestinal bacteria have been linked to the increasing prevalence of overweight and obesity, and scientists have started to investigate whether the intestinal bacteria can play a role in the treatment of overweight,” explained senior study investigator Arne Astrup, Ph.D., professor and head of the department of nutrition, exercise, and sports at the University of Copenhagen. “But it is only now that we have a breakthrough demonstrating that certain bacterial species play a decisive role in weight regulation and weight loss.”

The research team found that relationship between two groups of intestinal bacteria was decisive for whether overweight people lose weight on a diet that follows the Danish national dietary recommendations and contains a lot of fruit, vegetables, fiber, and whole grains. In the study, 31 subjects ate the New Nordic Diet for 26 weeks and lost an average of 3.5 kg (7.6 pounds), whereas the 23 subjects eating an Average Danish Diet lost an average of 1.7 kg (3.7 pounds). Thus, weight loss was on average 1.8 kg (3.9 pounds) greater in the subjects on the New Nordic Diet.

Interestingly, when the study participants were segmented by their level of intestinal bacteria, it was found that people with a high proportion of Prevotella bacteria in relation to Bacteroides bacteria lost 3.5 kg more in 26 weeks when they ate a diet composed by the New Nordic Diet principles compared to those consuming the Average Danish Diet. Moreover, subjects with a low proportion of Prevotella bacteria in relation to Bacteroides did not lose any additional weight on the New Nordic Diet. Overall, approximately 50% of the population has a high proportion of Prevotella bacteria in relation to Bacteroides bacteria.

“The study shows that only about half of the population will lose weight if they eat in accordance with the Danish national dietary recommendations and eat more fruit, vegetables, fibers, and whole grains,” noted lead study investigator Mads Fiil Hjorth, Ph.D., assistant professor in the department of nutrition, exercise, and sports at the University of Copenhagen. “The other half of the population doesn’t seem to gain any benefit in weight from this change of diet. These people should focus on other diet and physical activity recommendations, until a strategy that works especially well for them is identified.”

The results from the current study show that biomarkers, e.g., fecal samples, blood samples, or other samples from our body, which says something about our state of health, should play a far greater role in nutritional guidance simply because biomarkers allow us to adapt the guidance to the individual.

“This is a major step forward in personalized nutritional guidance. Guidance based on this knowledge of intestinal bacteria will most likely be more effective than the ‘one size fits all’ approach that often characterizes dietary recommendations and dietary guidance,” concluded Dr. Hjorth.