The Bacteria That Thrive After a Hard Run

by Rafal Nazarewicz Ph.D.

There's a class of bacteria living in the guts of elite marathon runners that most athletes have never heard of. It doesn't help you absorb carbs. It doesn't calm inflammation. It does something stranger and more interesting: it feeds on the lactate your muscles produce during hard efforts and converts it into fuel your body can actually use again.

The Bacteria That Thrive After a Hard Run

In 2019, researchers at Harvard and the Joslin Diabetes Center collected stool samples from Boston Marathon runners before and after the race. One genus of bacteria showed up significantly higher after the marathon than before: Veillonella.

The same pattern appeared in Olympic-caliber rowers and ultramarathoners. The harder the effort, the more Veillonella were present. And when the researchers fed this bacterium to mice and put them on a treadmill, run time to exhaustion increased.

The mechanism was specific: Veillonella uses lactate as its sole carbon source. Lactate, the byproduct of hard anaerobic work that your muscles push into the bloodstream, crosses from the blood into the gut lumen. There, Veillonella metabolizes it into propionate, a short-chain fatty acid. Propionate then re-enters circulation and can be used as an energy substrate.

The research team demonstrated the loop directly, using labeled lactate in mice to trace the path from muscle to gut to blood. When they administered propionate rather than Veillonella, treadmill performance still improved. The short-chain fatty acid itself was the active agent.

Scheiman et al., Nature Medicine (2019)

A 2025 human randomized controlled trial extended this finding, showing that Veillonella atypica supplementation reduced fatigue interference and increased voluntary physical activity, with mechanistic replication in animal models.

What This Means for How You Fuel

The gut is not just processing the gels and drinks you take in during a race. It is, to some degree, recycling your own metabolic output. That's a different frame than most athletes apply to nutrition.

This loop depends on having Veillonella in your gut in sufficient abundance and a functioning gut environment for it to thrive. That's where fueling choices start to matter in ways that go beyond race-day GI comfort.

Research consistently links high intake of refined, ultra-processed carbohydrates to reduced microbial diversity and lower populations of beneficial short-chain fatty acid-producing bacteria. Maltodextrin specifically has been shown to promote endoplasmic reticulum stress, deplete the mucus layer, and increase intestinal permeability (Laudisi et al., Cell and Molecular Gastroenterology and Hepatology, 2019). Refined sugars like corn syrup and rice syrup increase gut permeability and endotoxin translocation, particularly under the conditions of exercise-induced splanchnic hypoperfusion (van Wijck et al., American Journal of Physiology, 2012).

When the gut barrier is compromised and microbial diversity drops, the microbial ecosystem that supports this kind of metabolic recycling gets disrupted. The Veillonella loop doesn't work in a vacuum. It works in the context of a healthy microbiome.

The Whole-Food Difference

Whole food-based carbohydrates carry structural features that refined ingredients strip out: soluble fiber, organic acids, polyphenols, and natural fats. These compounds moderate absorption speed, support microbial diversity, and maintain the mucosal environment where bacteria like Veillonella operate.

This doesn't mean athletes need to eat less carbohydrate. It means the form those carbohydrates come in affects more than blood glucose. It affects the ecosystem itself, and through it, the downstream metabolic machinery your body uses during long efforts.

At Spring Energy, every gel is built on real food: basmati rice, honey, fruit, natural fats. Not because it sounds cleaner on a label. Because the whole-food matrix does something for your performance that refined glucose polymers don't. It feeds the system.

Elite runners carry bacteria that convert exercise-induced lactate into a performance-enhancing short-chain fatty acid. That feedback loop exists in healthy, diverse microbiomes. Repeated use of ultra-processed fuels with minimal fiber and no protective bioactive compounds puts that ecosystem at risk.

The science is still emerging. But the direction is consistent: your performance ecosystem extends well beyond your muscles, and what you put into it shapes how well it functions when it matters most.

Scheiman J, et al. Meta-omics analysis of elite athletes identifies a performance-enhancing microbe that functions via lactate metabolism. Nature Medicine (2019).
Laudisi F, et al. The food additive maltodextrin promotes endoplasmic reticulum stress-driven mucus depletion and exacerbates intestinal inflammation. Cell and Molecular Gastroenterology and Hepatology (2019).
van Wijck K, et al. Physiology and pathophysiology of splanchnic hypoperfusion and intestinal injury during exercise. American Journal of Physiology (2012).
Liu P, et al. The role of short-chain fatty acids in intestinal barrier function, inflammation, oxidative stress, and metabolic diseases. Pharmacological Research (2021).