Elite Female Cyclists Gut Microbiome: Unique Composition Despite Normal Short-Chain Fatty Acids Original paper

What was studied?

This cross-sectional study investigated whether the unique nutritional and physiological demands of elite-level endurance cycling shape a specific gut microbiome profile in female World Tour cyclists. The researchers compared the fecal microbiota of 14 elite female cyclists during the off-season (a period of reduced training) with that of 13 age- and BMI-matched non-athlete controls. The study employed 16S rRNA gene sequencing to characterize bacterial community composition and diversity, along with quantitative analysis of fecal short-chain fatty acids (SCFAs)—specifically acetate, propionate, and butyrate—to assess functional metabolic output. The authors aimed to determine if the elite female cyclists gut microbiome differs from that of non-athletes, and whether such differences reflect adaptation to the high-carbohydrate, low-fiber dietary strategies typical of endurance sport, which prioritize rapid energy availability over gut microbial diversity.

Who was studied?

Fourteen elite female cyclists from a single UCI World Tour team were recruited, along with 13 healthy, non-endurance-trained female controls matched for age (cyclists: 26.4 ± 4.1 years; controls: 24.4 ± 4.6 years) and BMI (cyclists: 20.4 ± 1.3 kg/m²; controls: 21.3 ± 2.1 kg/m²). All cyclists were at the highest competitive level, with physiological markers including first lactate threshold at 3.83 W/kg and second lactate threshold at 4.71 W/kg. During the week before sampling, cyclists trained 16.4 hours on average, including cycling, running, and strength work. Controls reported only 3.1 hours of moderate-to-vigorous exercise weekly. Exclusion criteria included inflammatory bowel disease, substance abuse, smoking, and probiotic/antibiotic use within two weeks. Fecal samples were collected during the off-season to minimize acute training effects, though cyclists had already resumed substantial training (13.3 cycling hours/week). Dietary intake was assessed via two-day food diaries, and gastrointestinal symptoms were evaluated using the Bristol Stool Chart and a symptom questionnaire. This focused population allowed the authors to isolate the chronic effects of elite cycling lifestyle on the elite female cyclists gut microbiome.

Most important findings

The study revealed striking compositional differences between the elite female cyclists gut microbiome and that of controls, alongside unexpected functional similarity in SCFA production.

Key microbiome-specific findings: The cyclist gut was dominated by Bacteroidota (72.7%) with marked depletion of Firmicutes (22.1%), a reversal of the typical Firmicutes-to-Bacteroidota ratio seen in controls (62.5% vs. 15.3%). This shift was accompanied by significant reductions in fiber-fermenting families—Lachnospiraceae (log₂FC = -2.3), Ruminococcaceae (log₂FC = -1.5), and Bifidobacteriaceae (log₂FC = -5.8)—all of which are major producers of butyrate and other SCFAs. Despite this depletion, fecal SCFA concentrations did not differ between groups, suggesting functional redundancy or compensatory upregulation of alternative fermentation pathways (e.g., succinate pathway in Bacteroidota). The cyclists also consumed significantly higher total energy (2499 vs. 1453 kcal/day) and protein (17.6% vs. 14.8% of energy), with lower starch intake (45.9% vs. 66.3% of carbohydrates), consistent with a diet rich in simple sugars and ultra-processed sports nutrition products. Notably, the cyclists’ reduced alpha-diversity (Shannon index) persisted even after adjusting for fiber intake, indicating that exercise-related factors beyond diet contribute to this pattern. The study also found no significant differences in gastrointestinal symptoms (GISS) or stool form (BSC) between groups, suggesting that this “performance-adapted” microbiome may be functionally adequate in the absence of overt distress.

Key implications

For clinicians working with female endurance athletes, this study provides important evidence that the elite female cyclists gut microbiome exhibits a unique, low-diversity, Bacteroidota-dominated profile that appears to prioritize rapid energy extraction from simple carbohydrates over the degradation of complex fibers. This adaptation likely reflects the sport-specific nutritional strategy of high carbohydrate availability before and during exercise, often delivered via ultra-processed gels, bars, and sports drinks. While the absence of differences in fecal SCFAs suggests functional resilience, the long-term health consequences of reduced microbial diversity and depletion of butyrate-producing families (e.g., Lachnospiraceae, Ruminococcaceae) remain unknown. Clinicians should consider that such profiles may increase susceptibility to gut barrier dysfunction, low-grade inflammation, or metabolic disturbances, particularly when athletes transition to lower training loads or adopt higher-fiber diets. The findings underscore the need for personalized nutritional strategies that balance performance demands with gut health—such as strategic fiber reintroduction during off-season periods or use of prebiotics/probiotics to support beneficial taxa. Additionally, the underrepresentation of female athletes in microbiome research is partially addressed here, but larger longitudinal studies are needed to establish causality and explore interactions with menstrual cycle phase and hormonal contraceptive use. For now, clinicians can use these findings to counsel elite female cyclists that their gut microbiome may be “performance-adapted” rather than optimal for long-term health, and that periodic dietary diversification could support microbial resilience without compromising performance.