Author(s): FRACZEK, B., PIETA, A., ZIELINSKA, K., HUMINSKA-LISOWSKA, K., KARPECKA-GALKA, E., SWITALA, K., Institution: UNIVERSITY OF PHYSICAL EDUCATION, Country: POLAND, Abstract-ID: 1145

INTRODUCTION:
Research on athletes’ gut microbiome and RED-S risk is limited. This study aimed to analyze the structure and functional potential of the gut microbiome in elite cross-country skiers in relation to RED-S risk.
METHODS:
The study included 18 participants: 9 elite Polish National Team cross-country skiers (CCS) and 9 young, healthy, physically inactive women as controls (CON). During the peak-load preparatory season, energy intake and energy expenditure were tracked. The CSS group underwent RED-S screening. Post-microcycle fecal samples underwent metagenomic profiling.
RESULTS:
Based on the IOC RED-S CAT2, two skier subgroups were identified: high/moderate (HM_RED-S; n=6) and low (L_RED-S; n=3) risk of RED-S. The CSS group demonstrated higher energy expenditure (2961 vs. 1885 kcal) and energy intake (2975 vs. 1814 kcal) than CON, along with greater macronutrient intake, specifically carbohydrates (7 vs. 4 g/kg bm), protein (2.1 vs. 1 g/kg bm), and fat (1.5 vs. 1.4 g/kg bm). Fiber consumption (23 vs. 16 g) and most micronutrient intakes were also higher, with the exception of vitamin K, omega-3 fatty acids, and selenium. No significant differences in energy (38.75 vs. 38 kcal) or carbohydrate availability (7.5 vs. 6.6 g/kg bm) were observed between the HM_RED-S and L_RED-S groups. CSS was further characterized by greater gut species richness and distinct taxonomic and functional profiles, including higher C. sp. AF34_13 and R. lactatiformans abundance and more degradation pathways in athletes, whereas controls showed higher levels of L. lactis and R. callidus. Paradoxically, HM_RED-S athletes exhibited significantly higher species richness compared to CON, suggesting a compensatory expansion of niche-specific taxa. While CON was enriched in core metabolic pathways (TCA cycle, glycolysis) and butyrate producers (L. lactis, C. butyricum), HM_RED-S athletes showed a functional shift toward specialized degradation (e.g., gallate, catechol) and increased vitamin biosynthesis (B1, K2). HM_RED-S groups were also characterized by taxa associated with high protein/aromatic turnover (R. lactatiformans, P. gallinarum) and depletion of the health-associated D. welbionis, signaling an adaptation to an energy-restricted environment despite higher overall richness. This indicates a metabolic pivot toward scavenging energy from complex secondary compounds due to primary fuel insufficiency.
CONCLUSION:
Increased microbial diversity, accompanied by a reduction in taxa commonly associated with gut health, may represent a compensatory response to high training demands and potentially suboptimal carbohydrate availability in RED-S athletes. The observed functional shift toward specialized degradation pathways further suggests microbial adaptation to more complex energy substrates. Accordingly, higher diversity in RED-S could reflect a metabolically “scavenging-like” state rather than superior gut health, even when overall energy availability appears adequate.