The study found that people with the highest levels of vitamin D had a 4.3-fold higher cardiorespiratory fitness than those in the bottom quartile.
Cardiorespiratory fitness is a reliable surrogate for physical fitness, and is a measure of the ability of the heart and lungs to supply oxygen to the muscles during exercise.
Scientists from Virginia Commonwealth University in Virginia added that even after adjusting for factors that could influence the association such as age, sex, race, body mass index, smoking, hypertension, and diabetes, the highest vitamin D quartile still showed a 2.9-fold greater cardiorespiratory fitness.
The data, which is published in the European Journal of Preventive Cardiology, shows correlation and not causation, but lead author Dr Amr Marawan noted that the association was strong, incremental, and consistent across groups.
“This suggests that there is a robust connection and provides further impetus for having adequate vitamin D levels, which is particularly challenging in cold, cloudy places where people are less exposed to the sun,” he said.
“Our study shows that higher levels of vitamin D are associated with better exercise capacity. We also know from previous research that vitamin D has positive effects on the heart and bones. Make sure your vitamin D levels are normal to high. You can do this with diet, supplements, and a sensible amount of sun exposure.”
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. Both D3 and D2 precursors are transformed in the liver and kidneys into 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D).
While our bodies do manufacture vitamin D on exposure to sunshine, the radiation levels in some northern countries are so weak during the winter months that our body makes no vitamin D at all, meaning that dietary supplements and fortified foods are seen by many as the best way to boost intakes of vitamin D.
Cardiorespiratory fitness, as measured by maximal oxygen consumption during exercise (VO2 max), was assessed in 1,995 participants with a mean age of 33 years. Forty-five percent of the participants were women, and 49% were white. The mean vitamin D levels were 58 nmol/L and the mean VO2 max was 40 ml/kg/min.
“Participants in the highest quartile for vitamin D levels had a significantly higher VO2 max than participants in the lowest quartile (41.4 vs. 37.7; P < 0.001),” wrote Dr Marawan. And his co-authors. “There was a significant positive correlation between vitamin D and VO2 max across all participants.”
The association between vitamin D levels and exercise capacity held for men and women, and across the young and middle age groups. It was also unaffected by ethnicities, body mass index or smoking status. Hypertension and diabetes also did not affect the results, said the researchers.
Each 10 nmol/L increase in vitamin D was associated with a statistically significant 0.78 mL/kg/min increase in VO2 max. “This suggests that there is a dose response relationship, with each rise in vitamin D associated with a rise in exercise capacity,” said Dr Marawan.
“Our findings have important clinical, research, and public health implications,” wrote the researchers. “Because of the association between vitamin D and CRF, identifying suboptimum levels of vitamin D should prompt an investigation of CRF. An important research question is identifying the optimum levels of vitamin D needed for cardiovascular health.
“Further research needs to be conducted into the biological pathways responsible for this observed association. Randomized clinical trials to examine the effect of vitamin D supplementation on CRF over a longer period of time are warranted. The public health benefits beyond bone health of the supplementation of food products with vitamin D need to be examined.”
Source: European Journal of Preventive Cardiology
Published online ahead of print, doi: 10.1177/2047487318807279
“Association between serum vitamin D levels and cardiorespiratory fitness in the adult population of the USA”
Authors: A. Marawan, et al.