Ever wonder if you're genetically prone to losing weight? The answer might be hiding in your poop.

Recent research has shown that a stool sample can teach doctors a lot more about your health than just whether you’ve been eating enough roughage and fiber. That's because gut bacteria expelled in your waste can shed light on what’s going on with your immune system, gastrointestinal status, and even brain health. (Yes, we're talking about gut bacteria again.)

Researchers at the University of Copenhagen have tied the type of bacteria found in your feces to whether or not you can easily drop pounds from your waist. In a new study, the scientists gave 31 participants the New Nordic Diet—root veggies, leafy dark greens, fruit and berries, salmon, wild game, and foraged food—for 26 weeks; participants lost almost 8lbs. The other 23 people in the study ate an average Danish, or Western, diet and lost around 4lbs.

The study participants were then divided into groups based on the gut bacteria populations in their—you guessed it—stool samples. The curious result? Only about half of the people on the "healthier" New Nordic Diet lost weight if they had certain bacteria levels. “The other half of the population doesn't seem to gain any benefit in weight from this change of diet," said assistant professor Mads Fiil Hjorth, Ph.D., at the department of nutrition, exercise, and sports at the University of Copenhagen. "These people should focus on other diet and physical activity recommendations until a strategy that works especially well for them is identified.

"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."