Gut microbiome and VO2max: does fitness shape your gut?

Your gut holds trillions of bacteria that digest fibre, make vitamins and talk to your immune system. Over the past decade, researchers have asked whether aerobic fitness and these microbes are linked. The short answer: fitter people tend to have a richer, more diverse gut microbiome, and exercise can shift it. But this field is young, most of the data are associative, and the jump from "linked" to "causes" has not been made in humans.

The key study: Estaki et al., 2016

Researchers in Canada sequenced the stool bacteria of 39 healthy adults with similar age, BMI and diets but different levels of cardiorespiratory fitness, measured as VO2peak in the lab. They report that:

• VO2peak accounted for more than 20% of the variation in bacterial richness, after accounting for other factors including diet.
• Fitter participants had more faecal butyrate, along with more butyrate-producing bacteria such as Clostridiales, Roseburia, Lachnospiraceae and Erysipelotrichaceae.
• The differences centred on what the microbes do (their predicted functions) more than on specific species.

Butyrate is a short-chain fatty acid that feeds the cells lining the colon and helps keep the gut barrier intact. The study was cross-sectional and small, so it shows a correlation, not that fitness built the microbiome.

Can exercise change the microbiome? Allen et al., 2018

To test cause and effect, Allen and colleagues took 32 previously sedentary adults (18 lean, 14 with obesity) through six weeks of supervised endurance training, three days a week, building from 30 to 60 minutes at moderate to vigorous intensity. Diet was kept stable. Then everyone stopped training for six weeks.

• Training changed the composition and function of gut bacteria.
• Faecal short-chain fatty acids rose in lean participants but not in those with obesity.
• Most changes reversed once training stopped.

That mirrors what happens to VO2max itself: gains fade when you stop, as covered in detraining. The microbiome effects seem to need ongoing exercise.

The marathon microbe: Scheiman et al., 2019

This Nature Medicine paper made headlines. The team found more Veillonella in marathon runners' stool after a race. Veillonella uses lactate as its only carbon source and turns it into propionate. When the researchers gave a strain of Veillonella atypica to mice, the mice ran significantly longer on a treadmill, and putting propionate directly into the colon reproduced the effect. They also showed that blood lactate can cross into the gut.

It is a fascinating mechanism, but the performance data come from mice, not people. No human trial has shown that taking Veillonella raises VO2max or race times.

What the studies show and what they do not

Study	Design	Main finding	Limitation
Estaki 2016	Cross-sectional, 39 adults	Higher VO2peak linked to more diversity	Correlation only
Allen 2018	6-week training trial	Exercise shifted gut bacteria, reversed after stopping	Small, short
Scheiman 2019	Runners plus mouse experiments	Veillonella turns lactate into propionate	Performance effect shown in mice
Barton 2018	Rugby players vs controls	Athletes' microbes make more short-chain fatty acids	Diet differs a lot

Why the link might exist

Several plausible routes have been proposed, mostly from animal work:

• Diet travels with fitness. Active people often eat more fibre and protein, and diet is the strongest known driver of the microbiome.
• Gut transit and blood flow change with regular exercise.
• Lower body fat and better insulin sensitivity, both linked to fitness, are tied to different microbial profiles (see VO2max and diabetes).
• Lactate and other exercise metabolites may feed specific bacteria, as with Veillonella.

The arrow could also run both ways: a healthier gut might support training and recovery. That question is explored in gut microbiome and athletic performance.

Keep the bigger picture in mind

Even if the microbiome turns out to matter, it is a small piece next to what VO2max already predicts. Low fitness is one of the strongest risk factors for early death, at any body weight, as covered in fit but fat and VO2max and longevity. Gut bacteria may turn out to be one of many routes by which fitness protects health, rather than a shortcut to fitness.

What it means for you

1. Train for VO2max first. Consistent aerobic work, mostly easy with some intervals, raises fitness in weeks. See how to improve VO2max and zone 2 training.
2. Feed your microbes. A varied diet with plenty of plants is the best-supported way to support diversity. More in fibre and gut health for endurance athletes.
3. Keep it going. Like fitness, gut changes from exercise fade if you stop. Exercise and gut health covers the dose.
4. Be sceptical of "performance probiotics". Evidence is strain-specific and modest; see our probiotics guide.
5. Track your fitness with the VO2max calculator rather than chasing a stool test.

Persistent bloating, a change in bowel habit, unexplained weight loss or blood in your stool are reasons to see a doctor, not to buy a supplement.

Sources

• Estaki M, et al. Cardiorespiratory fitness as a predictor of intestinal microbial diversity and distinct metagenomic functions. Microbiome, 2016.
• Allen JM, et al. Exercise alters gut microbiota composition and function in lean and obese humans. Med Sci Sports Exerc, 2018.
• Scheiman J, et al. Meta-omics analysis of elite athletes identifies a performance-enhancing microbe that functions via lactate metabolism. Nat Med, 2019.
• Barton W, et al. The microbiome of professional athletes differs from that of more sedentary subjects in composition and particularly at the functional metabolic level. Gut, 2018.
• Mailing LJ, et al. Exercise and the gut microbiome: a review of the evidence, potential mechanisms, and implications for human health. Exerc Sport Sci Rev, 2019.