Long known as the “sunshine vitamin” because the body makes it when skin is exposed to sunlight, vitamin D might now be dubbed the “superstar vitamin,” judging by the amount of attention it has garnered of late from researchers and the general public. In the last two decades vitamin D has gone from being recognized for its importance in bone health to being investigated for its roles in a wide range of physiological functions—and for its potential as a preventive or treatment for more than 100 disorders, ranging from cancer and diabetes to multiple sclerosis and depression.
Amazingly, thousands of studies and other scientific papers on vitamin D have been published since our last general article on the subject just a few years ago, when the influential Institute of Medicine released its long-awaited report and recommendations about the vitamin. Many doctors now routinely test blood levels of the vitamin in their patients and recommend supplements when levels are found to be low. By many estimates, at least half of people in the U.S. and many other countries have blood levels of vitamin D considered either deficient or insufficient. No wonder, then, that vitamin D has become one of the best-selling supplements, that more and more foods are being fortified with it, and that some experts are even advising less sun avoidance.
Are we any closer to knowing if vitamin D can really help keep us healthy in ways not related to bone health? Are people jumping the gun when they take large doses of it—and perhaps even harming themselves? Here’s some background, plus an evaluation of where things stand in the delirious world of vitamin D.
Not an ordinary vitamin
Vitamin D is actually a hormone that your body makes, starting when your skin is exposed to the sun’s ultraviolet rays. If you’re in the sun enough, you probably don’t need to consume any vitamin D from food or supplements. But many people don’t spend much time in the sun (or wear sunscreen), often because of concern about skin cancer, and some don’t synthesize enough D even when they do go out in the sun, as we discuss below. And few foods are good sources of it.
Vitamin D was once thought of only as a nutrient to prevent rickets, a childhood disease characterized by softening and weakening of the bones. (Vitamin D deficiency in adults causes a related disease called osteomalacia.) Cod liver oil, which is rich in the vitamin, was prescribed in the early 19th century for the treatment of rickets. To eradicate the disease, milk was fortified with D in the U.S. starting in the 1930s. By the 1940s, a dramatic decline in rickets had occurred, and most people believed that the major health problem resulting from vitamin D deficiency was resolved.
Vitamin D acts in the small intestine to increase calcium absorption. It acts on bone cells to release calcium and maintain blood calcium levels and is needed for bone growth and bone remodeling. Moreover, we now know that most human cells have vitamin D receptors, suggesting that it plays other roles in the body as well. The vitamin also helps regulate many genes.
Today, many researchers believe that rickets and osteomalacia are only the tip of iceberg—just the most obvious of many disorders that can result from low levels of vitamin D. The term vitamin D insufficiency is often used to describe blood levels of vitamin D not low enough to cause such deficiency diseases, but low enough to potentially cause other health problems.
See a summary of recent research at the bottom of this article.
Though observational studies have linked low blood levels of vitamin D to a wide variety of diseases and conditions, it is very possible that low vitamin D is not a cause of these conditions but rather an effect of them. Learn more from our experts.
The many guises of vitamin D
Vitamin D is actually a group of related compounds with a mind-numbing array of names and a complex metabolism. The vitamin produced in the skin or consumed from food or supplements is converted in the liver to a compound known as calcidiol or 25-hydroxyvitamin D. This is the form of vitamin D usually measured in the blood to determine vitamin D status. In the kidneys, it is converted to the biologically active form of vitamin D, called calcitriol or 1,25-dihydroxyvitamin D3.
The vitamin D we consume exists in two forms:
Vitamin D3 (cholecalciferol) is the form the skin produces in response to sunlight and is also in certain foods derived from animals. The D3 used for food fortification and in most supplements is made from lanolin (from sheep’s wool).
Vitamin D2 (ergocalciferol) comes from non-animal sources (fungi and plants) exposed to ultraviolet light to convert cholesterol-like substances into vitamin D. Most studies have found that vitamin D2 is much less potent than D3 in raising blood levels of vitamin D.
Whether you get vitamin D from sun exposure or from food or supplements, the effects in your body are the same. Since vitamin D is fat-soluble, your body can store it for when you don’t get any sun.
Causes of D-ficiency
Many people today get little or no direct exposure to sunlight and thus have low blood levels of vitamin D. This is especially a problem for housebound people, as well as people trying to reduce the risk of skin cancer.
Even when people do spend time in the sun, the amount of vitamin D their bodies make can vary greatly, depending on the following factors:
- Skin pigmentation. Dark skin reduces penetration of ultraviolet rays from sunlight and thus lowers vitamin D production.
- Northern latitudes and season. In most parts of the U.S., blood levels of vitamin D drop markedly in the winter, when days are shorter, the sun is weaker, and we spend less time outside and wear more clothes. The farther north you live, the longer the winter and the greater the reduction in vitamin D production. People who live in the northern third of the continental U.S. and in Canada synthesize little or no vitamin D from sun exposure from November through March. In contrast, those living in the southern third can produce the vitamin pretty much year-round.
- Time of day and cloud cover. The sun’s ultraviolet rays are strongest between 10 a.m. and 3 p.m. The intensity can vary, depending on cloud cover or smog.
- Age. The skin’s ability to manufacture vitamin D declines with age. On average, a 70-year-old synthesizes only about one-quarter as much vitamin D as a 20-year-old from the same sun exposure. This is partly due to changes in the structure of the skin. In addition, the liver and kidneys are not as efficient in converting vitamin D to its active form, and vitamin D is not absorbed as well from the intestine.
- Sunscreen use. Sunscreens filter out or block ultraviolet rays and prevent the production of vitamin D. A sunscreen with a sun protection factor (SPF) of 15 reduces the production of vitamin D by 99 percent.
- Sitting behind a window. Glass filters out most of the ultraviolet rays that trigger vitamin D production in the skin.
Other factors that contribute to low levels of vitamin D in the body include:
- Low intake of foods containing D. People rarely get enough vitamin D from their diet, since few foods contain it.
- Obesity. Obesity is linked with lower blood levels of D because some of this fat-soluble vitamin is trapped in fat tissue and because of other changes in vitamin D metabolism.
- Medications. Certain drugs—notably anticonvulsants, corticosteroids, and some HIV medications—reduce vitamin D levels.
- Disorders that impair fat absorption or liver or kidney function. Vitamin D needs fat to be absorbed in the intestines, so disorders that affect fat absorption (such as Crohn’s or celiac disease and cystic fibrosis) can reduce vitamin D levels. Some types of gastrointestinal surgery (such as gastric bypass for obesity) can also lead to D deficiency.
- Genetics. Various genetic factors influence the body’s ability to produce and utilize vitamin D.
Getting Vitamin D from Food
Few foods naturally contain significant amounts of vitamin D, including oily fish, egg yolks, and some mushrooms. But many foods are fortified with the vitamin, notably milk and breakfast cereals. Here's a handy chart to guide you.
Testing for vitamin D
Blood levels of vitamin D are measured in nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L) of 25-hydroxyvitamin D, with 1 ng/mL equaling 2.5 nmol/L. There’s disagreement about who should get their blood levels measured and how to define vitamin D deficiency or insufficiency. Moreover, laboratory tests lack standardization and therefore reliability.
Another problem with testing is seasonal variability. Blood levels of vitamin D tend to be lowest in late winter and early spring and highest in summer and fall, and this variability can range from 25 to 50 percent. If you’re going to be tested, it’s generally recommended to have it done in the winter.
Perhaps most important, it’s not known whether widespread vitamin D testing in healthy people without deficiency symptoms, followed by supplementation when levels are low, will prevent diseases.
What’s an optimal level?
During the past decade many vitamin D researchers and expert groups have argued that a blood level of at least 30 ng/mL is optimal; some advise even higher goals— 40 or 50 ng/mL. But in its major report on vitamin D in 2010, the Institute of Medicine concluded that blood levels starting at 20 ng/mL would be adequate in the vast majority of people.
Laboratories vary in how they define “optimal” levels of vitamin D for their blood tests. Some labs classify 30 to 100 ng/mL as optimal and 12 to 29 ng/mL as being mild to moderately deficient.
Can blood levels be too high? People who are in the sun a lot, such as lifeguards and farmers, often have very high levels of vitamin D, sometimes over 100 ng/mL. Documented vitamin D toxicity is rare and is believed to occur at blood levels above 150 ng/mL—typically the result of long-term, very-high-dose supplement use. This can cause high blood calcium levels, which in turn can lead to calcification of tissues, damage to the kidneys and cardiovascular system, and a whole array of symptoms.
Vitamin D Supplements 101
Vitamin D (typically D3) is found in multivitamin/mineral formulations but is also sold as a single supplement or combined with calcium. Until recently most supplements contained 400 IU, the Daily Value used on labeling. But doses have been increasing.
The IOM's recommendations
The Institute of Medicine (IOM) devises the recommended nutrient intakes for the U.S. and Canadian governments, among other tasks. For years many experts complained that the IOM’s recommended intakes for vitamin D from 1997 were too low. In 2010 an IOM committee released its long-awaited 1,000-page report on vitamin D (and calcium). Its recommended intakes pertain only to vitamin D’s role in bone health and fracture reduction and assume minimal sun exposure. The committee concluded that the evidence for all other proposed benefits was inconsistent, inconclusive, or insufficient to set recommended intakes for vitamin D. It called for large well-designed clinical trials.
Contrary to research suggesting that vitamin D deficiency and insufficiency are widespread, the IOM concluded that most Americans are getting adequate amounts of vitamin D, based on its conclusion that 20 ng/mL is the acceptable cutoff for blood levels. It defined levels lower than 12 ng/mL as deficient, 12 to 20 ng/mL as insufficient (for many people), and above 20 ng/mL as sufficient in terms of bone health.
The IOM bumped up the daily recommended dietary intake from 400 to 600 IU (international units) for people up to age 70, and to 800 IU starting at age 70. The Upper Limit—the level at which health risks are thought to begin to increase—was set at 4,000 IU a day for adults.
Many vitamin D researchers and some health organizations (such as the Endocrine Society and the International Osteoporosis Foundation) have disagreed with the IOM guidelines. They set higher goals and recommend supplements of 800 to 2,000 IU a day, at least for people known or likely to have low blood levels. Some also argue that 10,000 IU should be considered the Upper Limit, 2.5 times the level set by the IOM.
While vitamin D’s role in bone health is clearly established, the debate continues about whether higher doses might prevent, treat, or delay a wide variety of conditions. On one side are some experts who point to the massive body of research, much of it promising, and advocate higher intakes and blood levels of vitamin D. On the other side are researchers who fear that the vitamin D bandwagon has gotten far ahead of the science and who worry that it may crash—as did the bandwagon for antioxidant supplements when long-awaited clinical trials failed to find benefit and sometimes even suggested harm.
One thing is clear: Blood levels of vitamin D are a good barometer of overall health. But whether high levels are the cause or result of good health will remain an open question until the results of several large important clinical trials come out in a few years.
Meanwhile, until the answers are in, should you have your vitamin D measured to find out if you should take supplements? You can discuss this with your health care provider. If you have some of the risk factors mentioned above, it’s likely that your blood level is low, making you a candidate for supplementation. Keep in mind, too, that even younger, thin, active, light-skinned people can have low levels of D.
Unless you’ve been tested and know that your vitamin D level is adequate—at least 20 ng/mL, though we think 30 ng/ mL is a better target—consider taking a supplement, especially if and when you get little or no sun exposure.
We recommend 800 to 1,000 IU a day for most people—preferably vitamin D3. Your health care provider may advise a higher dose (usually 1,000 to 3,000 IU) if your blood level of D is very low or if you have osteoporosis or milder bone loss (osteopenia), inflammatory bowel disease, or certain other disorders.
If you don’t like to take supplements or would rather wait for the results of the big studies, try to consume more foods that are high in vitamin D and get some (but not too much) sun exposure. It’s impossible to know how much sun is “enough” in general for D production because, as we’ve said, so many variables are involved. For some people, getting 10 to 20 minutes of midday sun on their arms and legs two or three times a week much of the year will result in fairly high blood levels of vitamin D; others will need longer sun exposure. Because of the risk of skin cancer, supplements are a safer bet.
Vitamin D Research: A Round-Up
Vitamin D has certainly kept a lot of researchers busy, as evidenced by the multitude of studies published in recent years. Berkeley Wellness experts break down the latest findings on the vitamin for preventing fractures, heart disease, and more.