Vitamin D levels in blood tied to cardiorespiratory fitness

A new study published in the European Journal of Preventive Cardiology identified a strong link between vitamin D levels in the blood and cardiorespiratory fitness (CRF)—with individuals in the top quartile of vitamin D being almost three times more fit than those in the bottom quartile after adjustment for clinical characteristics.

The publication comes on the heels of another study which showed significant survival benefits among individuals with higher CRF. Some previous reports have linked vitamin D to CRF, while others found no significant association, noted lead author Amr Marawan, MD, and colleagues at Virginia Commonwealth University School of Medicine.

The researchers used National Health and Nutrition Survey (NHANES) data from 2001 to 2004 to identify 1,995 Americans between the ages of 20 and 49. Participants were 49.1 percent white and 45.2 percent women, and 13 percent of them had hypertension.

In an unadjusted analysis, participants with the most vitamin D in their blood (top quartile) had 4.3-fold higher CRF than those in the bottom quartile of vitamin D. CRF was defined as the estimated maximal oxygen consumption (VO2 max) during exercise, which was calculated from each participant’s performance on a standardized exercise test.

Even after adjusting for characteristics like age, sex, race, body mass index, smoking, hypertension and diabetes, high vitamin D levels were associated with 2.9-fold higher CRF.

“Because of the association between vitamin D and CRF, identifying suboptimum levels of vitamin D should prompt an investigation of CRF,” the researchers wrote. “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.”

Despite acknowledging that the link between vitamin D and CRF requires additional explanation, the authors ventured some guesses to explain the relationship. One rodent study they cited found vitamin D deficiency was associated with decreased myocardial contractility and reduced cardiac output, and other potential factors could be at play, too.

“About 3% of all genes are directly or indirectly affected by vitamin D levels and vitamin D receptors are expressed in a large variety of cells, including myocytes,” Marawan et al. wrote. “Vitamin D may affect myocytes by increasing muscle protein synthesis and calcium and phosphorus transport in energy production. In addition, vitamin D may increase the relative number of one type of fast-twitch muscle fibers (IIa) and decrease another type of fast-twitch muscle fibers (IIb), suggesting that vitamin D may improve aerobic fitness.”

Analyzing their data further, the authors found each 10 nmol/L jump in serum vitamin D was associated with a statistically significant 0.78 mL/kg/min increase in VO2 max, suggesting a dose-response relationship.

Vitamin D levels can be boosted through diet, supplements or sun exposure, although Marawan warned against taking too many supplements.

“Toxicity is caused by megadoses of supplements rather than diet or sun exposure, so caution is needed when taking tablets,” he said in a press release, noting that vitamin D toxicity can lead to excess amounts of calcium in the blood, prompting nausea, vomiting and weakness.

Because the study was limited to adults in their 40s or younger, with an average age of 33, the authors said their results shouldn’t be extrapolated to older populations. However, they believe their research should prompt further investigations into vitamin D’s role in heart health.

“We know the optimum vitamin D levels for healthy bones but studies are required to determine how much the heart needs to function at its best,” Marawan said. “From a public health perspective, research should look into whether supplementing food products with vitamin D provides additional benefits beyond bone health.”