According to Gram Research analysis, a randomized controlled trial of 10 professional handball players found that a regular balanced diet caused specific changes to gut bacteria composition and function over 8 weeks, while a Paleo diet produced no significant microbiome alterations. Both diets maintained stable bacterial diversity, though individual athletes showed highly variable responses. The Paleo group consumed 40% less carbohydrate and more fat, but both groups had insufficient vitamin D intake.
A new study compared how two different eating plans affected the gut bacteria of professional handball players. Ten athletes followed either a Paleo diet (high fat, low carbs) or a regular balanced diet for 8 weeks. Researchers found that the regular diet caused some changes to gut bacteria, while the Paleo diet didn’t create major shifts. Both diets had enough fiber, but both groups weren’t getting enough vitamin D. The study suggests that for athletes, neither diet dramatically changed their gut health over this time period, though individual athletes responded differently to each eating plan.
Key Statistics
A 2026 randomized controlled trial of 10 professional handball players found that a regular balanced diet induced specific taxonomic and functional changes to gut bacteria, while a Paleo diet produced no significant microbiome alterations over 8 weeks.
According to research reviewed by Gram, handball players following a Paleo diet consumed approximately 40% less carbohydrate and a higher proportion of unsaturated fatty acids compared to those eating a rational diet, though both groups had insufficient vitamin D intake.
A pilot study of 10 athletes found substantial inter-individual variability in how gut bacteria responded to dietary intervention, with overall microbiome richness remaining stable at the group level despite individual differences.
Research from Frontiers in Microbiology showed that both the Paleo and rational diets maintained high dietary fiber intake in professional athletes, yet both groups failed to meet vitamin D requirements over the 8-week intervention period.
The Quick Take
- What they studied: How eating a Paleo diet (lots of meat, vegetables, and fat; no grains) compared to a regular balanced diet affected the helpful bacteria living in athletes’ stomachs
- Who participated: Ten professional male handball players, split into two groups of five. One group ate Paleo-style for 8 weeks, the other ate a regular balanced diet with the same number of calories
- Key finding: The regular diet caused some measurable changes to gut bacteria types and their functions, but the Paleo diet didn’t create significant changes. Both groups’ overall bacterial diversity stayed stable, though individual athletes showed different responses
- What it means for you: For athletes, switching to a Paleo diet probably won’t dramatically change your gut bacteria in the short term. A regular balanced diet may cause more noticeable shifts. However, this was a very small study, so athletes should talk to a sports nutritionist before making major diet changes
The Research Details
This was a randomized controlled trial, which is one of the strongest ways to test if something works. Researchers took 10 professional handball players and randomly split them into two groups. One group ate a Paleo diet (focusing on meat, fish, vegetables, nuts, and healthy fats while avoiding grains, dairy, and processed foods) while the other group ate a regular balanced diet. Both diets had the same number of calories so the comparison would be fair. The study lasted 8 weeks, with researchers collecting samples of the athletes’ poop at the start, after 4 weeks, and after 8 weeks to analyze their gut bacteria.
The researchers used advanced laboratory techniques to identify which types of bacteria were present and what functions those bacteria could perform. They looked at how diverse the bacterial communities were and whether the two diet groups showed different patterns over time. They also carefully analyzed what each group actually ate to make sure the diets were different in the ways they expected.
Understanding how different diets affect gut bacteria is important because these bacteria help athletes digest food, absorb nutrients, and even support immune function. By studying professional athletes specifically, researchers can see how diet changes affect people who have very controlled training and nutrition routines. This helps separate the effects of diet from other lifestyle factors
This study has both strengths and weaknesses. The strength is that it was a randomized controlled trial with meals prepared by a professional catering company, so researchers knew exactly what participants ate. The main weakness is the very small sample size, only 10 people total. This makes it hard to know if the results would apply to other athletes. The study was also relatively short (8 weeks), so longer-term effects remain unknown. Individual athletes showed very different responses, which suggests that gut bacteria changes are highly personal
What the Results Show
The most important finding was that the regular balanced diet caused specific changes to the types of bacteria present and their functions, while the Paleo diet did not produce significant changes to gut bacteria over the 8-week period. This was somewhat surprising because researchers expected both diets to affect the microbiome.
When researchers looked at the nutritional content of each diet, they found that the Paleo group ate much less carbohydrate (about 40% less) and more fat, including more healthy unsaturated fats. Both groups ate plenty of fiber, which is good for gut health. However, both groups weren’t getting enough vitamin D, and the Paleo group was also low in calcium, likely because they weren’t eating dairy products.
Another important finding was that individual athletes responded very differently to the same diet. Some people’s gut bacteria changed a lot, while others’ barely changed at all, even when eating the same food. This suggests that genetics and other personal factors play a big role in how your gut bacteria respond to diet changes.
The overall diversity and richness of gut bacteria (meaning how many different types were present) stayed stable in both groups over the 8 weeks. This is actually a good sign, as it suggests neither diet caused harmful bacterial depletion. The rational diet group showed more specific changes in which bacteria were abundant and what functions they could perform, suggesting this diet created a more active shift in the bacterial community. The Paleo group’s microbiome remained relatively stable, which could mean either that the diet wasn’t different enough to trigger changes, or that athletes’ gut bacteria are naturally resistant to dietary shifts
Previous research has shown that restrictive diets can sometimes reduce the diversity of gut bacteria, which is generally considered unhealthy. This study suggests that the Paleo diet, despite being more restrictive in some ways (no grains, no dairy), didn’t reduce bacterial diversity in these athletes. The finding that a regular balanced diet caused more changes aligns with some previous research showing that nutrient-rich, varied diets can shift bacterial communities. However, most previous studies looked at non-athletes or different populations, so direct comparisons are limited
This study has several important limitations. First, it only included 10 people, all male professional handball players, so results may not apply to female athletes, non-professional athletes, or people who aren’t athletes at all. Second, 8 weeks is relatively short, we don’t know what would happen if people followed these diets for months or years. Third, the study didn’t measure how the athletes actually performed or felt, only their gut bacteria. Fourth, there was huge individual variation, meaning some athletes responded very differently from others, making it hard to draw general conclusions. Finally, the researchers note that larger studies are needed to confirm these findings
The Bottom Line
Based on this small pilot study, there’s no strong evidence that switching to a Paleo diet will significantly change your gut bacteria if you’re an athlete. A regular balanced diet may cause more noticeable bacterial shifts. However, both diets should include adequate vitamin D and calcium (especially important for the Paleo diet). Athletes should focus on eating plenty of fiber and varied whole foods. Confidence level: Low to moderate, due to the small sample size and short duration
Professional and serious amateur athletes interested in optimizing their nutrition may find this relevant. However, this study is too small to make specific recommendations for individuals. Anyone considering a major diet change should consult with a sports nutritionist or doctor. This study doesn’t apply to people with digestive disorders or those taking antibiotics, which would dramatically affect gut bacteria
If you switched to a Paleo diet, based on this study, you probably wouldn’t see major changes to your gut bacteria within 8 weeks. Changes that do occur might take 4-8 weeks to develop. However, individual responses vary widely, so some people might see changes faster or slower. Long-term effects beyond 8 weeks remain unknown
Frequently Asked Questions
Does the Paleo diet change your gut bacteria?
In this 8-week study of 10 athletes, the Paleo diet did not produce significant changes to gut bacteria composition or function. However, individual athletes responded differently, and the study was small, so results may not apply to everyone. Longer studies are needed.
Which diet is better for athlete gut health, Paleo or regular balanced diet?
This study found that a regular balanced diet caused more measurable changes to gut bacteria, while Paleo didn’t significantly alter the microbiome. Neither diet reduced bacterial diversity. The best diet depends on individual response, which varies greatly among athletes.
How long does it take for diet changes to affect gut bacteria?
In this study, measurable changes appeared within 4-8 weeks for some athletes following a regular diet, but not for Paleo dieters. However, individual responses vary widely. Some people may see changes faster or slower depending on their genetics and current diet.
Is the Paleo diet safe for professional athletes?
This study found the Paleo diet was safe and maintained healthy bacterial diversity in athletes over 8 weeks. However, it was low in calcium and vitamin D, so athletes should supplement these nutrients. Consult a sports nutritionist before making major dietary changes.
What nutrients are missing from a Paleo diet for athletes?
This study found that athletes following a Paleo diet had insufficient vitamin D and calcium intake compared to recommended levels. Both groups also lacked adequate vitamin D. Athletes on Paleo should supplement these nutrients or work with a nutritionist to ensure adequate intake.
Want to Apply This Research?
- Track daily fiber intake (aim for 25-35 grams), vitamin D intake, and calcium intake. Log which diet pattern you’re following and note any digestive changes, energy levels, or athletic performance weekly
- If trying a Paleo diet, add a vitamin D supplement (1000-2000 IU daily) and consider a calcium source like fortified plant-based milk or supplements. Ensure you’re eating at least 25 grams of fiber daily from vegetables, fruits, nuts, and seeds
- Track gut health markers weekly: digestive comfort, energy levels, and athletic performance. Every 4 weeks, reassess whether you’re meeting vitamin and mineral targets. Consider testing gut bacteria composition at 8-week intervals if available through your healthcare provider
This research is a small pilot study (10 participants) and should not be used as the sole basis for major dietary changes. Individual responses to diet vary significantly. Athletes considering switching to a Paleo diet or making other major nutritional changes should consult with a registered sports nutritionist, registered dietitian, or physician. This article is for informational purposes only and does not constitute medical advice. The findings may not apply to non-athletes, female athletes, or people with digestive conditions. Always seek professional medical guidance before starting new dietary patterns, especially if you have existing health conditions or take medications.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.
