Association between serum vitamin D levels and cardiorespiratory fitness in the adult population of the USA

The small number of studies that have investigated the relationship between serum vitamin D levels and cardiorespiratory fitness (CRF) have reported conflicting results. We investigated the association between vitamin D levels and CRF in a representative sample of the US population using data from the National Health and Nutrition Survey (2001–2004). We included participants between the ages of 20 and 49 years and excluded those with vitamin D levels at the 5% extremes of the distribution. We used survey-weighted linear regression without and with adjustment for age, sex, race, body mass index, hypertension, diabetes, smoking, C-reactive protein, hemoglobin, and glomerular filtration rate to examine the relationship between the maximal oxygen consumption (VO2 max) (as a surrogate for CRF) and vitamin D levels. Of the 1995 participants, 45.2% were women, 49.1% were white, 13% had hypertension, and 4% had diabetes. The mean？±？SD age was 33？±？8.6 years, with a mean？±？SD vitamin D level of 58？±？5.3？nmol/L and a mean？±？SD VO2 max of 40？±？9.7？ml/kg/min. Participants in the highest quartile of vitamin D levels had a significantly higher CRF than participants in the lowest quartile (difference 4.3, 95% confidence interval (CI) 3.0–5.5; P？<？0.001). After adjustment for potential confounders, the difference between the highest and lowest vitamin D quartiles remained significant (difference 2.9, 95% CI 1.6–4.1; P？<？0.001). In unadjusted and adjusted linear regression, each 10？nmol/L increase in vitamin D level was associated with a significant increase in VO2 max (β？=？0.78？ml/kg/min, 95% CI 0.55–1.01; P？<？0.001; β？=？0.51？ml/kg/min, 95% CI 0.23–0.79; P？=？0.001, respectively). We found an independent and robust association between serum vitamin D levels and CRF, but our results need to be validated with clinical trials examining the effect of vitamin D supplementation on CRF