VO2 max, or maximal oxygen uptake, is the measure of how efficiently your body uses oxygen during exercise. It's one of the most reliable indicators of cardiovascular fitness available, reliable enough that the American Heart Association has recommended it be treated as a vital sign alongside blood pressure and heart rate.
But research published in a peer-reviewed journal found something that stopped exercise scientists in their tracks: VO2 max may account for around 22% of the variation in a person's gut bacteria composition, specifically the ratio of Firmicutes to Bacteroidetes, two of the dominant bacterial families in the gut. That's a significant share, even after controlling for diet and body composition.
Put simply: how fit you are aerobically appears to have a measurable relationship with the make-up of your gut microbiome. Not just what you eat. Not just your age. How fit you are.
The Two-Way Street Between Fitness and Your Microbiome
So what's actually going on? Researchers believe the relationship runs in both directions.
When you exercise regularly, particularly cardio, your gut microbiome appears to become more diverse. Diversity is generally considered a marker of a healthy gut; a wider range of bacterial species is associated with better immune function, lower systemic inflammation, more stable energy, and faster recovery from training. Studies suggest that increasing moderate exercise frequency may enhance beneficial bacterial populations and contribute to a more resilient gut environment.
But it also works the other way. A healthier gut microbiome may support your ability to perform and recover from exercise. Your gut bacteria are involved in nutrient absorption, the production of short-chain fatty acids (which appear to reduce inflammation and fuel your gut lining), and the regulation of your immune system, all of which feed directly into how your body responds to physical effort.
There's also emerging evidence that the gut-brain axis, the communication network between your gut and your central nervous system, may influence motivation, mental sharpness, and perceived effort during exercise. If your gut is struggling, your performance may feel harder than it should, even when the numbers look fine.
What This Means in Practice
The gut is still largely an afterthought for most men, something to think about when something goes wrong. But the research suggests it may be worth paying attention to long before symptoms appear.
Some practical things that appear to support both gut diversity and cardiovascular fitness simultaneously:
Eat a wide variety of plants. Emerging research around eating 30 different plants per week is gaining serious traction. Variety appears to matter as much as volume when it comes to feeding a diverse microbiome.
Prioritise consistency over intensity. Regular moderate exercise appears to do more for gut diversity than sporadic high-intensity sessions. Showing up steadily may matter more than going hard occasionally.
Manage your stress. Chronic stress appears to suppress both gut diversity and recovery from exercise. If you're running on empty, your gut likely knows it.
Stay well hydrated. Hydration plays a role in gut motility and overall digestive function, and may support the kind of consistent training that improves VO2 max over time.
Track your gut patterns, not just your symptoms. Most people only engage with their gut health reactively. But if your gut and your fitness are genuinely connected, understanding your baseline, what's normal for you, may give you a meaningful edge in both
Tracking the Connection
You can't feel your Firmicutes-to-Bacteroidetes ratio shifting. You can't see microbiome diversity declining. By the time symptoms like bloating, sluggish digestion, fatigue, or inconsistent energy become noticeable, something may have been off for a while.
That's where passive, consistent gut tracking may offer something that a one-off test can't: pattern recognition over time. Gutsi is designed to help you understand what's normal for your gut, quietly, in the background, so that changes become visible before they become problems.
Because if your gut and your fitness are in conversation, it's probably worth knowing what they're saying.
Want to understand your gut patterns better? Explore how Gutsi works, no stool samples, no disruption to your routine.
This content is for informational purposes only and does not constitute medical advice. If you have concerns about your digestive health, please speak to a healthcare professional.
References:
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Allen, J.M. et al. (2018). Gut Microbiota Composition Is Related to Cardiorespiratory Fitness in Healthy Young Adults. International Journal of Sport Nutrition and Exercise Metabolism. https://pubmed.ncbi.nlm.nih.gov/29989465/
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Allen, J.M. et al. (2017). Exercise Alters Gut Microbiota Composition and Function in Lean and Obese Humans. Medicine & Science in Sports & Exercise. https://pubmed.ncbi.nlm.nih.gov/28085071/
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Clarke, S.F. et al. (2014). Exercise and associated dietary extremes impact on gut microbial diversity. Gut, 63(12), 1913-1920. https://pubmed.ncbi.nlm.nih.gov/25021423/
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American Heart Association (2016). Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign. Circulation. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000461
