Purpose: Low cardiorespiratory fitness (CRF), vitamin D deficiency, and metabolic syndrome (MetSyn) are prevalent among U.S. adults. Joint associations among these variables are not well described.
Methods: Between 2006 and 2018, 14,353 apparently healthy men completed a comprehensive health examination, including CRF based on a maximal treadmill test, components of MetSyn, and serum vitamin D levels [25(OH)D]. Participants were classified into groups of low (category 1), moderate (categories 2-3), and high (categories 4-5) CRF by age-group, as well as by clinical cut points for MetSyn and 25(OH)D. We calculated odds ratios (OR) of MetSyn across levels of CRF and 25(OH)D and also examined joint associations among these three variables.
Results: Mean 25(OH)D levels were 30.9 ± 11.6 and 26.3 ± 10.7 ng·mL in men without and with MetSyn, respectively (P < 0.001). The prevalence of MetSyn was inversely associated with ordered categories of CRF and 25(OH)D (Ptrend < 0.001 for both). Men with normal 25(OH)D had significantly lower odds of MetSyn than men who were vitamin D deficient (OR = 0.29, 95% confidence interval = 0.26-0.33). Men with moderate (OR = 0.31, 0.27-0.35) or high CRF (OR = 0.08, 0.07-0.09) had significantly lower odds of MetSyn than men with low CRF. Joint associations between CRF, 25(OH)D, and MetSyn revealed significantly greater prevalence of MetSyn in unfit men compared with fit men within each category of 25(OH)D (P < 0.001). Each 5 ng·mL increment of 25(OH)D and 1 MET increment of CRF was associated with a 16.0% and 31.3% lower prevalence of MetSyn, respectively.
Conclusion: There are strong individual and joint associations between CRF, 25(OH)D, and MetSyn. Prospective studies are needed to evaluate these joint associations with regard to mortality outcomes.