Confirmation of the POLA Index and Its Association with COVID-19 and Influenza Incidence, Gut Microbiota, and Mucosal Immunity Markers in Young Women: A Pilot Cohort Study

Evidence for diet-driven immunomodulation remains limited in terms of practical tools. This pilot study prospectively evaluated the dietary POLA index in 51 women aged 25–45 years and living in Kraków, Poland. At baseline (2022), 7-day dietary intake and physical activity were recorded; stool samples were analyzed for gut microbiota composition; short-chain fatty acids were quantified using high-performance liquid chromatography; and fecal secretory IgA (sIgA), human β-defensin-2 (HBD-2), and calprotectin levels were measured by the enzyme-linked immuno-sorbent assay. In June 2023, post-baseline incidence of COVID-19 and influenza was self-reported. Participants were categorized into a group with beneficial immunomodulation (BIM, POLA score ≤ 5) and a group with unbeneficial/highly unbeneficial immunomodulation (UBIM + HUBIM, POLA score > 5). The incidence of COVID-19 or influenza was 7.7% (1/13) in the BIM group vs. 36.8% (14/38) in the UBIM + HUBIM group. After adjusting for age and smoking, the UBIM + HUBIM group had higher odds of infection compared with the BIM group (adjusted OR = 6.53; 95% CI: 1.02–129.85), corresponding to a higher absolute risk of 47.9% (95% CI 26.1–70.6) versus 12.3% (95% CI 1.4–58.1). The BIM group more often met the fiber and micronutrient adequacy targets and showed a higher proportion of sIgA levels within the reference range (92.3% vs. 60.5%), along with lower fecal succinic acid concentrations (median 3.27 mg/g vs. 4.32 mg/g). In this cohort, a favorable POLA score was associated with lower self-reported COVID-19 or influenza incidence and an sIgA profile suggestive of intestinal immune homeostasis. As this pilot study is underpowered, findings should be interpreted as exploratory and hypothesis-generating. Nonetheless, results support the POLA index as a practical diet-quality metric with potential immunomodulatory relevance.