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Studies have reported sex and age differences in self-rated health. On average, women rate their health as being poorer compared to men, and older individuals report poorer health than younger individuals. The current study evaluated sex and age differences for self-reported immune fitness, i.e. the capacity of the body to respond to health challenges (such as infections) by activating an appropriate immune response in order to promote health and prevent and resolve disease. Data from different survey studies (N = 8586) were combined for the current analyses. N = 8064 participants (93.3%) completed the single-item scale to assess momentary immune fitness (mean (Standard deviation, SD) age of 32.4 (16.7) years old, range: 18 to 103, 68.0% women) and N = 4263 participants (49.7%) completed the Immune Status Questionnaire (ISQ) to assess past year's immune fitness (mean (SD) age of 40.9 (17.1) years old, range: 18 to 103, 61.1% women). The analyses revealed that women rated their momentary and past year's immune fitness significantly lower than men ( p  < 0.001). A small but significant decline in momentary immune fitness when aging was found (r = -0.073, p  < 0.001). In contrast, past year's immune fitness steadily improved with progressing age (r = 0.295, p  < 0.001), and for each age group the difference from the 18-24 years old group was statistically significant ( p  < 0.001). When using age as covariate, the sex differences in immune fitness remained significant for both momentary immune fitness ( p  < 0.001) and past year's immune fitness ( p  < 0.001). In conclusion, women report a poorer momentary and past year's immune fitness than men. The sex effects in immune fitness are robust and seen across all age groups except the elderly. A relative stable momentary immune fitness was found across the age groups. However, past year's immune fitness (assessments with the ISQ) improved with age. This observation may be related to the fact that the studies comprised convenience samples. Therefore, the observed age effects should be interpreted with caution and require further investigation in nationally representative samples., Competing Interests: Over the past 3 years, J.C.V. has acted as a consultant/advisor for 10.13039/501100003769Eisai, KNMP, Med Solutions, Red Bull, Sen-Jam Pharmaceutical, and Toast! J.G. is part-time employee of Nutricia Research and received research grants from 10.13039/501100001720Nutricia research foundation, 10.13039/501100004522Top Institute Pharma, Top Institute Food and Nutrition, GSK, 10.13039/501100003958STW, NWO, Friesland Campina, 10.13039/100020214CCC, Raak-Pro, and 10.13039/100006939EU. The other authors have no potential conflicts of interest to disclose., (© 2024 The Authors.)

Purpose: It is vital that Immune fitness, i.e., how well the immune system functions and reacts to challenges, can be reliably be examined. The current study aimed to compare immune fitness with assessments of saliva biomarkers of systemic inflammation., Methods: N = 108 healthy young adults (18-30-year-old students of Utrecht University, the Netherlands) participated in the study. A saliva sample was collected for biomarker assessment (Interleukin (IL)-1β, IL-6, IL-8, IL-10, immunoglobulin A (IgA), and tumor necrosis factor-alpha (TNF-α), and c-reactive protein (CRP). Additionally, a survey was completed to assess immune fitness, mood, mental resilience, and quality of life. The correlations between the biomarker assessments, immune fitness and mood were determined., Results: No significant correlations between immune fitness and biomarkers of systemic inflammation were found. Significant sex differences in correlations with immune fitness were demonstrated for loneliness (significant only in men) and fatigue (significant only in women). For both sexes, immune fitness correlated significantly with anxiety, mental resilience, and quality of life., Conclusion: No significant correlations were found between immune fitness and saliva biomarkers of systemic inflammation. Immune fitness correlated significantly with anxiety, mental resilience, and quality of life. Sex differences were demonstrated in the relation of immune fitness with loneliness and fatigue. Future research should further investigate factors that may influence the relationship between immune fitness, mood, and biomarkers of systemic inflammation, including underlying psychological mechanisms of possible sex differences., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Over the past 3 years, Joris Verster has acted as a consultant/advisor for Eisai, KNMP, Red Bull, Sen-Jam Pharmaceutical, and Toast!. Aletta Kraneveld has held research grants from H2020, Nutricia-Danone, Netherlands Center of Translational Research, Lung fund, SGF/Health Holland, and NWO. Johan Garssen is a part-time employee of Nutricia Research and received research grants from Nutricia research foundation, Top Institute Pharma, Top Institute Food and Nutrition, GSK, STW, NWO, Friesland Campina, CCC, Raak-Pro, and EU. The other authors have no potential conflicts of interest to disclose., (© 2023 The Authors.)