Importance of Vitamin D for Adult Health

By Michael F. Holick, PhD, MD

Vitamin D is taken for granted and its role in health is underappreciated by both physicians and patients. Health care practitioners may be aware that vitamin D is associated with bone health in growing children, but little attention has been paid to the important role that vitamin D plays in maximizing and maintaining bone health in adults. Additionally, vitamin D ingestion may have implications for the prevention of many common cancers, hypertension, and Type 1 diabetes.1

Unlike most fat-soluble and water-soluble vitamins that are plentiful in a healthy diet, very few foods naturally contain vitamin D. Consumption of oily fish, such as salmon or mackerel, three to four times a week, or ingestion of cod liver oil on a daily basis provides vitamin D from natural sources. Some foods, including milk, some breads, and some cereals, also are fortified with vitamin D. A majority of our vitamin D requirement (80-95%) comes not from dietary sources but from sunlight exposure.1,2

Prevalence of Vitamin D Deficiency

Vitamin D deficiency is extremely common in the U.S. adult population.1-7 In the epidermis, concentrations of provitamin D3 are inversely related to age. Not surprisingly, more than half of elders (both free-living and those in nursing homes) are deficient in vitamin D. It has been assumed that young and middle-aged adults are not at risk for vitamin D deficiency. However, the lifestyle of many young and middle-aged adults is such that they are constantly at work indoors.

In addition, heightened awareness about sun exposure and skin cancer has resulted in the use of a sunscreen before exposure to sunlight. A sunscreen with an SPF of 8 will reduce vitamin D synthesis by 97.5%.8 The CDC recently reported that 41% of African-American women ages 15-49 years throughout the United States were vitamin D deficient at the end of the winter.9,10 Thirty-two percent of medical students and residents at our hospital were found to be vitamin D deficient at the end of the winter.6

Consequences of Vitamin D Deficiency

Chronic vitamin D deficiency has subtle and insidious consequences for both bone health and overall health and well-being. Vitamin D deficiency results in decreased efficiency of intestinal calcium absorption. The body responds by increasing the production of parathyroid hormone, which in turn mobilizes precious calcium stores from the skeleton. Thus, vitamin D deficiency can precipitate and exacerbate osteoporosis. In addition, vitamin D deficiency causes osteomalacia, a mineralization defect of the skeleton. Unlike osteoporosis, which is a silent disease, osteomalacia often presents as isolated or generalized muscle aches, muscle weakness, and bone pain. It has been estimated that upward of 60% of patients with symptoms consistent with fibromyalgia (i.e., muscle and bone aches and pains) actually may have vitamin D deficiency.1,11,12

Exposure to tanning bed radiation resulting in a more than 100% increase in circulating concentrations of 25-hydroxyvitamin D [25(OH)D] was effective in treating hypertension.13 It now is recognized that 1,25-dihy-droxyvitamin D (the biologically active form of vitamin D) can down-regulate the renin-angiotensin system. It also is recognized that most organs in the body have vitamin D receptors (VDRs),11-16 and that 1,25(OH)2D is a potent down-regulator of cell growth. Furthermore, the prostate, colon, and breast, as well as other tissues, can make 1,25(OH)2D3; the most likely purpose is for regulating cell growth.17-19 There is strong evidence that vitamin D deficiency, especially in those who live at higher latitudes, increases the risk of death from many common cancers, including colon, breast, prostate, and ovarian cancer.20-22 Garland et al observe that the risk of death from colon cancer was reduced by 50% if 25(OH)D was greater than 20 ng/mL.20

Vitamin D deficiency may predispose children to diabetes. The ß-islet cells of the pancreas have VDRs, and a recent study reported that the risk of developing Type 1 diabetes decreased by 80% in children who received vitamin D supplementation.23

The best method to determine vitamin D deficiency is to measure circulating concentrations of 25(OH)D. Patients who are vitamin D deficient will have a normal serum calcium due to their secondary hyperparathyroidism. Thus, the only measure of vitamin D status is a 25(OH)D level. 1,25(OH)2D is of no value and indeed can be low, normal, or even elevated in a vitamin D insufficient patient.1

Treatment of Vitamin D Deficiency

The best method to treat vitamin D deficiency is to fill the vitamin D tank quickly by giving the patient 50,000 IU of vitamin D once a week for eight weeks.8 A repeat 25(OH)D after the therapy often shows an increase of at least 100%. However, if this does not occur, the treatment should be repeated for an additional eight weeks. For those who cannot absorb vitamin D due to chronic malabsorption syndromes (e.g., chronic liver disease, cystic fibrosis, Crohn’s disease, Whipple’s disease, sprue), exposure to natural sunlight or tanning bed radiation24,25 that has a component of ultraviolet-B radiation is suggested.26,27

The National Academy of Sciences recently recommended that the adequate intake of vitamin D be increased in middle-aged adults and elders. Current recommended dietary allowances are 200 IU for ages 1-50, 400 IU for ages 51-70, and 600 IU for those older than 70 years. We, and others, have estimated that in the absence of exposure to sunlight, the requirement is closer to 1,000 IU of vitamin D daily.1,28-31 The goal is to maintain circulating concentrations of 25(OH)D of at least 20 ng/mL and preferably, between 30 and 50 ng/mL.

Because vitamin D deficiency is so common, I have recommended that as a preventive health measure, similar to measuring blood lipids, it is reasonable to check 25(OH)D annually. This will ensure that patients are vitamin D sufficient, thus maximizing their bone health, as well as their overall health and well-being.1,32,33

Dr. Holick is Director of the Bone Health Care Clinic, Director of the General Clinical Research Center, and Professor of Medicine, Dermatology, Physiology, and Biophysics, Boston University Medical Center, Boston, MA.


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