Losing weight is hard, and sometimes it seems like no matter what you do, you can’t shed those unwanted pounds. But as it turns out, the solution for your weight loss woes may lie in your poop, according to a fascinating new study published in the International Journal of Obesity.

Intestines are filled with bacteria, a community that is known as the microbiome. And the type of bacteria in your microbiome could be affecting how much weight you lose depending on what type of diet you’re on. Researchers examined poop from 62 people who were overweight, and randomly assigned them to spend 26 weeks on the New Nordic Diet, which usese fiber in whole grains, or the Average Danish Diet.

Researchers found that those with a high count of Prevotella or Bacteroides bacteria who were on the high fiber NND diet lost an average of about 7 pounds more body fat than those on the ADD. But the two diets didn’t result in any signficant difference in those with low ratios of those bacteria. Which means that if you’re not successful with one diet, it may be because it doesn’t match up with your gut bacteria, and you should keep trying different diets until you find one that works.

“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. 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” says Professor Arne Astrup, Head of the Department of Nutrition, Exercise and Sports at the University of Copenhagen, Denmark.