The effects of ageing, BMI and physical activity on blood IL-15 levels: A systematic review and meta-analyses

Aim: The purpose of the study was to test the effect of ageing, BMI, physical activity and chronic exercise on IL-15 blood concentration by meta-analyses of the literature.Methods: The search was performed on PubMed/MEDLINE, Web of Science, ProQuest, Embase and Cochrane databases. First meta-analysis compared blood IL-15 of healthy adults across three age groups (<35 years, 35-65 years, and >65 years), considering BMI as confounding factor; the second compared IL-15 levels between physically active and non-physically active individuals (cross-sectional studies); and the third tested the effect of chronic exercise interventions on blood IL-15 levels on participants of any age, sex, and health condition.Results: From 2582 studies retrieved, 67 were selected for the three meta-analyses (age effect: 59; physical ac-tivity cross-sectional effect: 5; chronic exercise effect: 14). Older adults had lower blood IL-15 than young and middle-aged adults (5.30 pg/ml [4.76; 5.83]; 7.11 pg/ml [6.33; 7.88]; 7.10 pg/ml [5.55; 8.65], respectively). However, the subgroup of overweight older adults had higher IL-15 than young and middle aged overweight adults; Habitual physical activity did not affect blood IL-15 (standardized mean difference [SMD] 0.61 [-0.65; 1.88], p = 0.34); Chronic exercise reduced blood IL-15 in short-term interventions (<16 weeks) (SMD-0.14 [-0.27;-0.01], p = 0.04), but not studies of >16 weeks of intervention (SMD 0.44 [-0.26; 1.15], p = 0.22).Conclusion: The present meta-analyses highlight the complex interaction of age, BMI and physical activity on blood IL-15 and emphasize the need to take these factors into account when considering the role of this myokine in health throughout life.