Vitamin D is both a nutrient we eat and a hormone our bodies make. It is a fat-soluble vitamin that has long been known to help the body absorb and retain calcium and phosphorus; both are critical for building bone. Also, laboratory studies show that vitamin D can reduce cancer cell growth, help control infections and reduce inflammation. Many of the body’s organs and tissues have receptors for vitamin D, which suggest important roles beyond bone health, and scientists are actively investigating other possible functions.
Few foods naturally contain vitamin D, though some foods are fortified with the vitamin. For most people, the best way to get enough vitamin D is taking a supplement because it is hard to eat enough through food. Vitamin D supplements are available in two forms: vitamin D2 [“ergocalciferol” or pre-vitamin D] and vitamin D3 [“cholecalciferol”]. Both are also naturally occurring forms that are produced in the presence of the sun’s ultraviolet-B [UVB] rays, hence its nickname, “the sunshine vitamin,” but D2 is produced in plants and fungi and D3 in animals, including humans. Vitamin D production in the skin is the primary natural source of vitamin D, but many people have insufficient levels because they live in places where sunlight is limited in winter, or because they have limited sun exposure due to being inside much of the time. Also, people with darker skin tend to have lower blood levels of vitamin D because the pigment [melanin] acts like a shade, reducing production of vitamin D [and also reducing damaging effects of sunlight on skin, including skin cancer].
Recommended Amounts
The Recommended Dietary Allowance for vitamin D provides the daily amount needed to maintain healthy bones and normal calcium metabolism in healthy people. It assumes minimal sun exposure.
RDA: The Recommended Dietary Allowance for adults 19 years and older is 600 IU [15 mcg] daily for men and women, and for adults >70 years it is 800 IU [20 mcg] daily.
UL: The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause harmful effects on health. The UL for vitamin D for adults and children ages 9+ is 4,000 IU [100 mcg].
Many people may not be meeting the minimum requirement for the vitamin. NHANES data found that the median intake of vitamin D from food and supplements in women ages 51 to 71 years was 308 IU daily, but only 140 IU from food alone [including fortified products]. [1] Worldwide, an estimated 1 billion people have inadequate levels of vitamin D in their blood, and deficiencies can be found in all ethnicities and age groups. [2-4] In industrialized countries, doctors are seeing the resurgence of rickets, the bone-weakening disease that had been largely eradicated through vitamin D fortification. [5-7] There is scientific debate about how much vitamin D people need each day and what the optimal serum levels should be to prevent disease. The Institute of Medicine [IOM] released in November 2010 recommendations increasing the daily vitamin D intake for children and adults in the U.S. and Canada, to 600 IU per day. [1] The report also increased the upper limit from 2,000 to 4,000 IU per day. Although some groups such as The Endocrine Society recommend 1,500 to 2,000 IU daily to reach adequate serum levels of vitamin D, the IOM felt there was not enough evidence to establish a cause and effect link with vitamin D and health benefits other than for bone health. Since that time, new evidence has supported other benefits of consuming an adequate amount of vitamin D, although there is still not consensus on the amount considered to be adequate.
Vitamin D and Health
The role of vitamin D in disease prevention is a popular area of research, but clear answers about the benefit of taking amounts beyond the RDA are not conclusive. Although observational studies see a strong connection with lower rates of certain diseases in populations that live in sunnier climates or have higher serum levels of vitamin D, clinical trials that give people vitamin D supplements to affect a particular disease are still inconclusive. This may be due to different study designs, differences in the absorption rates of vitamin D in different populations, and different dosages given to participants. Learn more about the research on vitamin D and specific health conditions and diseases:
Bone health and muscle strength
Several studies link low vitamin D blood levels with an increased risk of fractures in older adults. Some studies suggest that vitamin D supplementation in certain amounts may prevent such fractures, while others do not. [8-11]
- A meta-analysis of 12 randomized controlled trials that included more than 42,000 people 65+ years of age, most of them women, looked at vitamin D supplementation with or without calcium, and with calcium or a placebo. Researchers found that higher intakes of vitamin D supplements—about 500-800 IU per day—reduced hip and non-spine fractures by about 20%, while lower intakes [400 IU or less] failed to offer any fracture prevention benefit. [12]
- A systematic review looked at the effect of vitamin D supplements taken with or without calcium on the prevention of hip fractures [primary outcome] and fractures of any type [secondary outcome] in older men and postmenopausal women 65+ years of age. It included 53 clinical trials with 91,791 participants who lived independently or in a nursing home or hospital. It did not find a strong association between vitamin D supplements alone and prevention of fractures of any type. However, it did find a small protective effect from all types of fractures when vitamin D was taken with calcium. All of the trials used vitamin D supplements containing 800 IU or less. [13]
- The VITamin D and OmegA-3 TriaL [VITAL] double-blind placebo-controlled randomized trial of 25,871 women and men, 55+ years and 50+ years of age, respectively, did not find a protective effect from vitamin D supplements on bone fractures. [79] The participants were healthy at the start of the study—representative of the general population and not selected based on low bone mass, osteoporosis, or vitamin D deficiency—and were given either 2,000 IU of vitamin D or a placebo taken daily for about five years. Vitamin D did not lower the incidence of total bone fractures or fractures of the hip or spine.
Vitamin D may help increase muscle strength by preserving muscle fibers, which in turn helps to prevent falls, a common problem that leads to substantial disability and death in older people. [14–16] A combined analysis of multiple studies found that taking 700 to 1,000 IU of vitamin D per day lowered the risk of falls by 19%, but taking 200 to 600 IU per day did not offer any such protection. [17] However, the VITAL trial following healthy middle-aged men and women did not find that taking 2,000 IU of vitamin D daily compared with a placebo pill reduced the risk of falls. [80]
Though taking up to 800 IU of vitamin D daily may benefit bone health in some older adults, it is important to be cautious of very high dosage supplements. A clinical trial that gave women 70+ years of age a once-yearly dosage of vitamin D at 500,000 IU for five years caused a 15% increased risk of falls and a 26% higher fracture risk than women who received a placebo. [18] It was speculated that super-saturating the body with a very high dose given infrequently may have actually promoted lower blood levels of the active form of vitamin D that might not have occurred with smaller, more frequent doses. [13]
JoAnn Manson, MD, DrPH, leader of the main VITAL trial and coauthor of the report on fracture, commented:
“We conclude that, in the generally healthy U.S. population of midlife and older adults, vitamin D supplementation doesn’t reduce the risk of fractures or falls. This suggests that only small-to-moderate amounts of vitamin D are needed for bone health and fall prevention, achieved by most community-dwelling adults. Of course, vitamin D deficiency should always be treated and some high-risk patients with malabsorption syndromes, osteoporosis, or taking medications that interfere with vitamin D metabolism will benefit from supplementation.”
Cancer
Nearly 30 years ago, researchers noticed an intriguing relationship between colon cancer deaths and geographic location: People who lived at higher latitudes, such as in the northern U.S., had higher rates of death from colon cancer than people who lived closer to the equator. [19] Many scientific hypotheses about vitamin D and disease stem from studies that have compared solar radiation and disease rates in different countries. These studies can be a good starting point for other research but don’t provide the most definitive information. The sun’s UVB rays are weaker at higher latitudes, and in turn, people’s vitamin D blood levels in these locales tend to be lower. This led to the hypothesis that low vitamin D levels might somehow increase colon cancer risk. [3]
Animal and laboratory studies have found that vitamin D can inhibit the development of tumors and slow the growth of existing tumors including those from the breast, ovary, colon, prostate, and brain. In humans, epidemiological studies show that higher serum levels of vitamin D are associated with substantially lower rates of colon, pancreatic, prostate, and other cancers, with the evidence strongest for colorectal cancer. [20-32]
However, clinical trials have not found a consistent association:
The Women’s Health Initiative trial, which followed roughly 36,000 women for an average of seven years, failed to find any reduction in colon or breast cancer risk in women who received daily supplements of 400 IU of vitamin D and 1,000 mg of calcium, compared with those who received a placebo. [33,34] Limitations of the study were suggested: 1] the relatively low dose of vitamin D given, 2] some people in the placebo group decided on their own to take extra calcium and vitamin D supplements, minimizing the differences between the placebo group and the supplement group, and 3] about one-third of the women assigned to vitamin D did not take their supplements. 4] seven years may be too short to expect a reduction in cancer risk. [35,36]
A large clinical trial called the VITamin D and OmegA-3 TriaL [VITAL] followed 25,871 men and women 50+ years of age free of any cancers at the start of the study who took either a 2,000 IU vitamin D supplement or placebo daily for a median of five years. [37] The findings did not show significantly different rates of breast, prostate, and colorectal cancer between the vitamin D and placebo groups. The authors noted that a longer follow-up period would be necessary to better assess potential effects of supplementation, as many cancers take at least 5-10 years to develop.
Although vitamin D does not seem to be a major factor in reducing cancer incidence, evidence including that from randomized trials suggests that having higher vitamin D status may improve survival if one develops cancer. In the VITAL trial, a lower death rate from cancer was observed in those assigned to take vitamin D, and this benefit seemed to increase over time since starting on vitamin D. A meta-analysis of randomized trials of vitamin D, which included the VITAL study, found a 13% statistically significant lower risk of cancer mortality in those assigned to vitamin D compared to placebo. [38] These findings are consistent with observational data, which suggest that vitamin D may have a stronger effect on cancer progression than for incidence.
Heart disease
The heart is basically a large muscle, and like skeletal muscle, it has receptors for vitamin D. [39] Immune and inflammatory cells that play a role in
cardiovascular disease conditions like atherosclerosis are regulated by vitamin D. [40] The vitamin also helps to keep arteries flexible and relaxed, which in turn helps to control high blood pressure. [41]
In the Health Professionals Follow-up Study nearly 50,000 healthy men were followed for 10 years. [42] Those who had the lowest levels of vitamin D were twice as likely to have a heart attack as men who had the highest levels. Meta-analyses of epidemiological studies have found that people with the lowest serum levels of vitamin D had a significantly increased risk of strokes and any heart disease event compared with those with the highest levels. [40;43-46]
However, taking vitamin D supplements has not been found to reduce cardiovascular risk. A meta-analysis of 51 clinical trials did not demonstrate that vitamin D supplementation lowered the risk of heart attack, stroke, or deaths from cardiovascular disease. [47] The VITamin D and OmegA-3 TriaL [VITAL] came to the same conclusion; it followed 25,871 men and women free of cardiovascular disease who took either a 2,000 IU vitamin D supplement or placebo daily for a median of five years. No association was found between taking the supplements and a lower risk of major cardiovascular events [heart attack, stroke, or death from cardiovascular causes] compared with the placebo. [37]
Type 2 diabetes
Vitamin D deficiency may negatively affect the biochemical pathways that lead to the development of Type 2 diabetes [T2DM], including impairment of beta cell function in the pancreas, insulin resistance, and
inflammation. Prospective observational studies have shown that higher vitamin D blood levels are associated with lower rates of T2DM. [48]
More than 83,000 women without diabetes at baseline were followed in the Nurses’ Health Study for the development of T2DM. Vitamin D and calcium intakes from diet and supplements were assessed throughout the 20-year study. [49] The authors found that when comparing the women with the highest intakes of vitamin D from supplements with women with the lowest intakes, there was a 13% lower risk of developing T2DM. The effect was even stronger when vitamin D was combined with calcium: there was a 33% lower risk of T2DM in women when comparing the highest intakes of calcium and vitamin D from supplements [>1,200 mg, >800 IU daily] with the lowest intakes [