Another Link Between Vitamin D Levels and Cardiovascular Health

By Floyd Arthur

Researchers at the Intermountain Medical Center Heart Institute in Salt Lake City, Utah, have found a link between total and bioavailable vitamin D levels and cardiovascular health. A team led by Dr. Heidi May, a cardiovascular epidemiologist at the Institute, found that patients in whom both  total vitamin D and bioavailable vitamin D were low had the poorest cardiovascular outcomes.

Dr. May’s team measured the levels of various vitamin D metabolites in 4,200 patients aged 52 to 76. Seventy percent of the study population had preexisting cardiovascular disease, and about 25 percent had diabetes.

The research was presented at the American College of Cardiology Scientific Sessions in Chicago, Illinois, held April 1 through April 4, 2016.

Vitamin D Levels and Cardiovascular Disease

A growing body of research links low vitamin D levels to a higher incidence of heart attacks,  peripheral arterial disease, congestive heart failure and stroke. Additionally, low vitamin D levels are commonly linked to conditions associated with CVD, such as obesity, hypertension and diabetes.

Only about 10 to 15 percent of total vitamin D in the body is available to pursue target cells, Dr. May explains. The remaining metabolites are bound to proteins. Measuring both total levels and bioavailable vitamin D appears to be important to determining overall cardiovascular risk.

Predisposing Factors in Low Vitamin D Levels

“Vitamin D” is it is not a vitamin in the strict sense of the word. It is a fat-soluble secosteroid produced in the skin from the action of ultraviolet light on 7-dehydrocholesterol. Thus, low vitamin D levels are most often linked to decreased exposure to UV light.

Over the past several decades the American population has spent more time indoors and increased its use of sunscreen when outdoors in the sun. As a result, the incidence of low vitamin D levels has increased.

Vitamin D can be obtained from dietary sources such as fish oils, egg yolk, butter, liver and fortified foods. However, endogenous production is much more important to maintaining vitamin D levels in most individuals.

Other important factors in vitamin D absorption include body weight, skin pigmentation, sex and age.

• Fat cells absorb vitamin D, so obese individuals are more prone to vitamin D deficiency. This correlation may explain the link between vitamin D deficiency and conditions associated with obesity, such as Type 2 diabetes, CVD and hypertension.
• Fair skinned people absorb more vitamin D than those with darker skin due to lower levels of melanin in the skin.
• Women have lower vitamin D levels than men. This may be due to the fact that women generally have more body fat than men and traditionally spend more time indoors. Women also tend to wear sun protection more often than men.
• The ability to absorb vitamin D decreases as people age, so people over 55 tend to have lower levels of vitamin D. This effect may be intensified by the fact that older or medically frail individuals tend to spend less time outdoors.

Additionally, those who live at higher latitudes tend to have lower levels of vitamin D due to the decreased intensity of ultraviolet light.

What Are “Normal” Vitamin D Levels

Over the past several decades, optimal levels of vitamin D have been defined and redefined. Historically, appropriate levels were determined by those found in healthy populations, and so the lower end of the range was frequently set between 25 and 35 nmol/l. However, in the face of evidence linking levels below 50 nmol/l to greater all-cause mortality, those numbers have been refined.

Today in the United States, the lower level of the reference range typically sits between 40 nmol/ and 120 nmol/l, with the majority of researchers suggesting that levels of 75 nmol/l are optimal. However, there are virtually no clinical trials supporting supplementation for levels in this range.

Treating Low Vitamin D Levels

The logical clinical approach to treating suspected vitamin D deficiency is to measure vitamin D levels and supplement those whose levels fall in the suboptimal range. However, measuring vitamin D levels is expensive and inexact: Values from different laboratories can vary by as much as 40 percent.

Thus, many clinicians choose to prescribe universal supplementation for persons in “high-risk” groups, such as those who are elderly, medically frail, homebound or dark skinned and living at higher latitudes. According to one study published in the Journal of the American Geriatric Society, the cost-effectiveness of either approach in patients 65 to 80 is about the same.

If the clinician and patient choose supplementation, a dose of 500 to 1000 units per day or 50,000 units per month are typically sufficient to achieve vitamin D levels of greater than than 50 nmol/l. Some advocates of higher vitamin D levels suggest daily doses of 2,000 units, which will typically achieve levels of 100 nmol/l or higher. However, there is some evidence that vitamin D levels above 120 lead to a greater incidence of falls and fractures, so it is most likely safest to maintain levels at 100 nmol/l or below.

Conclusion

The relationship between low vitamin D levels and all-cause mortality is well-established, and a growing body of research suggests a link between vitamin D deficiency and cardiovascular risk. However, these correlations are confounded by comorbidities that often exist in persons with low vitamin D, such as obesity, low activity levels, dyslipidemia, hypertension and diabetes. More research is needed to establish whether a causative relationship between low vitamin D levels and cardiovascular disease actually exists.