In the study, people with higher vitamin D levels also tended to have better cardiorespiratory fitness, a measure of a person's aerobic fitness level. Indeed, the higher a person's vitamin D level was, the greater their cardiorespiratory fitness was, the researchers found.
However, the study found only an association between vitamin D and cardiorespiratory fitness (CRF) and could not prove that high vitamin D levels actually improve people's fitness.
"We don't know if higher vitamin D levels improved CRF or [if] higher CRF increased vitamin D levels," lead study author Dr. Amr Marawan, an assistant professor of internal medicine at Virginia Commonwealth University, told Live Science.
Still, the link between vitamin D and fitness level was strong and consistent among different groups of people, Marawan said in a statement. "This suggests that there is a robust connection and provides further impetus for having adequate vitamin D levels."
The study was published today (Oct. 30) in the European Journal of Preventive Cardiology.
It's well-known that vitamin D is important for healthy bones, but a growing body of research suggests that the vitamin may also affect the heart and the skeletal muscles.
In the new study, the researchers analyzed information from nearly 2,000 people in the U.S. ages 20 to 49 who participated in a national health survey from 2001 to 2004. The participants had blood samples taken to analyze the levels of vitamin D in their blood. They also underwent an exercise test on a treadmill to measure their VO2 max, a proxy for cardiorespiratory fitness. VO2 max refers to the maximum amount of oxygen that the body can use during exercise. A higher VO2 indicates greater cardiorespiratory fitness.
When participants were divided into five groups based on their vitamin D levels, those in the group with the highest vitamin D levels had a VO2 max that was about 3 units higher than those in the group with the lowest vitamin D levels. (VO2 max is measured in units of milliliters of oxygen consumed per kilogram of body weight per minute.)
In addition, the researchers also found that as vitamin D levels increased, VO2 max increased as well.
The findings held even after the researchers considered factors that could affect people's vitamin D levels or their cardiorespiratory fitness, including their age, sex, race and body mass index (BMI), as well as whether they smoked or had high blood pressure or diabetes.
The findings were consisted among men and women, as well as among people in different age groups or ethnicities.
While the researchers didn't specifically look for the mechanism linking vitamin D and improved cardiorespiratory fitness, there is a biologically plausible way that vitamin D could affect people's fitness levels, the researchers said. Vitamin D receptors are found on many types of cells in the body, including heart muscle cells, so the vitamin could bind to these cells. Vitamin D could help with muscle-protein synthesis or energy production in cells, the researchers said.
Still, the study was not able to take into account people's vitamin D consumption or their physical activity levels, both of which could affect the link found in the study, the researchers said. For example, it could be that people who are fitter are more likely to take vitamin D supplements or to have higher vitamin D levels due to sun exposure.
"We expect to see higher vitamin D levels in people spending more time outdoors," Marawan said.
But regardless of the reason for the link, people should make sure to get adequate amounts of vitamin D, Marawan said. However, they shouldn't go overboard. "It is not the case that the more vitamin D, the better," Marawan said. Taking very high levels of vitamin D supplements, called "megadoses," is tied to toxicity.
More research is also needed to examine whether taking vitamin D supplements at certain doses affects cardiorespiratory fitness, the researchers said.
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