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

Aims 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). Methods 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. Results 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 (beta = 0.78 ml/kg/min, 95% CI 0.55-1.01; P < 0.001; beta = 0.51 ml/kg/min, 95% CI 0.23-0.79; P = 0.001, respectively). Conclusions 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.