Eliud Kipchoge's gut microbiome looks statistically different from a recreational runner's — not because of genetics, but because of specific bacterial populations shaped by years of high-volume training and dietary patterns. One of those bacteria converts lactate directly into a fuel source. This is the emerging performance variable that your nutrition label cannot measure.
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The Veillonella Finding
A 2019 study by Scheiman et al. published in *Nature Medicine* is now considered a landmark in sports microbiome research. Researchers collected stool samples from Boston Marathon participants before and after the race, and from sedentary controls. The finding:
Veillonella atypica was present in dramatically higher concentrations in marathon runners compared to sedentary controls — and spiked further in the 30 minutes post-race, precisely when blood lactate is also elevated.
The mechanism: Veillonella is one of the few bacteria that uses lactate as its primary carbon source, converting it via propionate metabolism to short-chain fatty acids (SCFAs) — specifically propionate, which can cross the gut barrier, enter systemic circulation, and be oxidised by muscle and cardiac tissue as an additional fuel substrate.
When the researchers gavaged mice with Veillonella, run-time-to-exhaustion improved by 13%. When they gavaged mice with Veillonella's metabolic product — propionate — alone, the same improvement occurred. The bacteria-to-fuel chain is mechanistically established, not just correlational.
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Intestinal Permeability: The Performance Limiter No One Talks About
High-intensity exercise — particularly running at threshold or above — transiently increases intestinal permeability. The mechanism: splanchnic vasoconstriction (blood redirected from gut to working muscle) reduces intestinal blood flow, compromising tight junction integrity between enterocytes.
The result is a transient "leaky gut" state where lipopolysaccharide (LPS) — fragments of gram-negative bacterial cell walls — enter systemic circulation. LPS triggers an inflammatory cascade via TLR4 receptors. In athletes with chronic training-induced permeability and low microbiome diversity, this systemic LPS load is associated with:
- Elevated baseline CRP and IL-6
- Increased upper respiratory tract infection (URTI) frequency — the well-documented "open window" immune suppression post-race
- GI distress during race conditions — nausea, cramping, diarrhoea in 30–50% of marathon runners
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Dietary Fibre as the Primary Lever
Microbiome composition is profoundly shaped by diet — and the most reliable intervention with consistent evidence is dietary fibre diversity. Different fibre types selectively feed different bacterial populations:
- Inulin and FOS (fructooligosaccharides): selectively feeds Bifidobacterium and Lactobacillus — associated with reduced gut permeability
- Resistant starch (found in cooled cooked potatoes, green banana, oats): primary substrate for butyrate-producing Firmicutes — butyrate is the primary energy source for colonocytes, maintaining barrier integrity
- Arabinogalactan (found in legumes, root vegetables): feeds Prevotella — a genus associated with carbohydrate utilisation efficiency in athletes
- Polyphenols (found in berries, dark chocolate, olive oil): selective prebiotics that reshape microbial communities independently of fibre fermentation
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Probiotic Evidence: Specific Strains, Specific Outcomes
Not all probiotics are equal, and most commercial probiotic products contain strains with no sport-specific evidence base. The strains with the strongest RCT data in athletes:
- Lactobacillus rhamnosus GG: reduced URTI duration and frequency in multiple trials in athletes and military personnel under high physical stress
- Lactobacillus acidophilus NCFM + Bifidobacterium lactis Bi-07: combined strain supplementation reduced intestinal permeability markers (lactulose-to-mannitol ratio) in endurance athletes over 12 weeks
- Streptococcus thermophilus: associated with improved lactose digestion and reduced GI distress — relevant for athletes using whey-based nutrition strategies
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Practical Nutrition Implications
For athletes looking to support microbiome diversity through diet:
- Aim for 30+ different plant food types per week — the threshold associated with measurably higher microbiome diversity in the American Gut Project dataset
- Prioritise fibre diversity over fibre quantity — different sources feed different populations
- Include fermented foods daily if tolerated: kefir, kimchi, plain live yogurt, sauerkraut — each contributes distinct live cultures alongside dietary fibre
- Structure carbohydrate sources across training days to include resistant starch alongside immediately available glucose sources
Do you currently advise athletes on microbiome diversity strategies alongside their macronutrient targets — or does gut health remain a separate conversation from performance nutrition?