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What you need to know about vitamin d

Author: Kari Bohlke, ScD

Vitamin D is in the spotlight: the news is full of stories about the potential health benefits of vitamin D along with worrisome reports that many of us may have less-than-optimal levels. A committee of experts convened by the Institute of Medicine (IOM), however, offers a somewhat different perspective. The committee’s report states that while vitamin D provides clear bone benefits, evidence of many other benefits—including cancer prevention— is still inconclusive. The report also notes that the frequency of vitamin D insufficiency may have been overestimated and that a fairly modest level of vitamin D intake—600 international units (IU) per day—is adequate for a large majority of the population.1

The IOM report generated a great deal of debate among experts, and research on the topic continues. In the meantime it’s up to each of us to learn what we can about the topic and to work with our health care providers to make decisions that fit our individual needs.

Vitamin D: The Basics

The discovery of vitamin D began with research on the causes and the prevention of rickets in children. Rickets— once common, particularly among children living in cities—is characterized by the softening of bones. This can result in delayed growth and skeletal deformities such as bowed legs. Cod liver oil and sun exposure were each found to prevent the condition, and further research uncovered vitamin D as the active agent.

Vitamin D can be produced in the skin in response to ultraviolet B (UVB)radiation from the sun or obtained from dietary sources. Few foods naturally contain large amounts of vitamin D, but vitamin D can be obtained from fatty fish such as salmon, fortified foods such as milk, and dietary supplements. Once in the body, vitamin D is converted in the liver to a form called 25-hydroxy vitamin D (25[OH ]D). This is the form of vitamin D that can be measured in the blood to determine whether someone has adequate levels. 25(OH )D is then converted in the kidneys to the active form of vitamin D.2

A lthough sun exposure can produce high levels of vitamin D, organizations such as the American Academy of Dermatology do not recommend sun exposure as a source of vitamin D because of the link with skin cancer.3 Dietary supplements can provide an identical form of vitamin D without the skin cancer risk. It’s also important for people to realize that at high latitudes (regions farther from the equator, including the northern part of the United States), sun exposure does not produce any vitamin D during the winter months.4

Vitamin D and health

The link between vitamin D and bone health is well established. Adequate vitamin D can prevent rickets and osteomalacia (bone disease in adults caused by vitamin D deficiency) and contributes to bone mineralization, growth, and repair.5 Vitamin D can also help reduce inflammation, increase muscle strength, and improve immune function.

More-recent research on the health effects of vitamin D has focused on its potential role in cancer prevention. Some studies have produced promising results, but the evidence is still limited. The most compelling evidence for a benefit has been found in studies of colorectal cancer; several (but not all) have reported a substantial reduction in risk among those with the highest vitamin D levels.6 At the other end of the spectrum, however, is a study that reported an increased risk of pancreatic cancer among those with the highest vitamin D levels.7

An important point to keep in mind is that many of these studies were observational. In an observational study, researchers do not control the vitamin D intake of study participants. Rather they measure existing levels and assess how they relate to subsequent health outcomes. When done well, observational studies can provide important information; they are, however, more prone to bias than studies in which researchers are able to randomly assign subjects a particular level of vitamin D exposure.

Relatively little information is available from randomized, controlled trials about the effects of vitamin D on cancer. One important study that addressed this question—the Women’s Health Initiative trial of calcium and vitamin D supplementation— did not find a reduced risk of colorectal cancer among subjects taking a vitamin D supplement. Certain aspects of the study were criticized, however, including the low dose of vitamin D that was used (400 IU per day).

After reviewing the available evidence, the IOM committee concluded that there is still no definitive evidence that vitamin D reduces the risk of cancer or other common chronic conditions such as heart disease and diabetes. Other experts, however, maintain that the evidence of a benefit is strong, particularly for colorectal cancer.8

An ongoing study that may resolve some of the uncertainties about the nonskeletal health benefits of vitamin D is the VI Tamin D and OmegA-3 TriaL (VI TAL). Study participants will be assigned to receive either 2,000 IU of vitamin D per day or a placebo. Outcomes of interest include cancer, heart disease, and stroke. The study is also evaluating the health effects of fish oil.

How Much Is Enough? How Much Is Too Much?

For children more than 12 months old and adults up to age 70, the IOM set the recommended dietary allowance at 600 IU per day. For adults over 70, the recommended dietary allowance is 800 IU per day. These levels assume little sun exposure and are intended to maintain bone health in a large majority of the population.

For older children and adults, the IOM set the tolerable upper level of intake at 4,000 IU per day. This is not the recommended intake; it is the maximum intake that is expected to be safe. When discussing very high intake of calcium and vitamin D, the IOM notes, “Higher levels of both nutrients have not been found to confer greater benefits, and in fact, they have been linked with other health problems, challenging the concept that ‘more is better.’”

Some experts feel that 600 IU per day is too little and that higher levels are necessary to maximize health benefits. As additional information becomes available about vitamin D, the recommended levels may be revisited.

For people with limited sun exposure, a vitamin D supplement may be necessary to achieve the desired level of vitamin D. Few of us get large amounts of vitamin D from food.

Vitamin D testing

Testing for vitamin D has increased dramatically in recent years, but at this point there is no clear evidence that everyone needs to be tested.9 Use of the test is probably best reserved for those who have symptoms of vitamin D deficiency or who are at high risk of having low vitamin D levels (such as the elderly). Even among people at risk of low vitamin D, however, testing may not be necessary; simply taking a vitamin D supplement may be sufficient.

In addition, although the idea of vitamin D testing may seem straightforward, there is disagreement among experts about how to interpret the results. Specifically, there is debate about what level of vitamin D should be considered “insufficient.” Some studies have defined levels of 25(OH )D below 30 nanograms per milliliter (ng/mL) as insufficient. Using this cut-point, a majority of the US population would be classified as having a suboptimal vitamin D level.10 The 2010 IOM report, however, concluded that blood levels should not be considered insufficient unless they are lower than 20 ng/mL. Using this lower cut-point, roughly one-third of the US population would be considered to have low vitamin D levels.11 While this point continues to be debated, it makes sense to be aware of the disagreement and to discuss with your physician the different possible interpretations of test results.

Stay tuned

In summary, vitamin D provides clear bone benefits. The effect of vitamin D on other health outcomes—including cancer—remains uncertain, but it is an active area of research. In the meantime if you’re uncertain about your vitamin D status and need for supplementation, talk with your doctor. Each person’s situation is different.


  1. Institute of Medicine of the National Academies. Report Brief: Dietary Reference Guidelines for Calcium and Vitamin D. November 30, 2010. http://www.iom. edu/Reports/2010/Dietary-Reference-Intakes-for-Calcium- and-Vitamin-D/Report-Brief.aspx.
  2. Thacher TD, Clarke BL. Vitamin D Insufficiency. Mayo Clinic Proceedings. 2010;86(1):50—60. doi:10.4065/ mcp.2010.0567.
  3. Be Sun Smart. American Academy of Dermatology website. Available at: skin-cancer-prevention/be-sun-smart. Accessed May 18, 2011.
  4. Webb AR , Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. Journal of Clinical Endocrinology and Metabolism. 1988;67(2):373— 78. doi:10.1210/jcem-67-2-373.
  5. Vitamin D and Cancer Prevention: Strengths and Limits of the Evidence. National Cancer Institute website. Available at prevention/vitamin-D. Accessed May 18, 2011.
  6. World Health Organization/International Agency for Research on Cancer. Vitamin D and Cancer. IARC Working Group Reports, Volume 5, 2008. Available at http:// VitD.pdf. Accessed May 18, 2011.
  7. Helzlsouer KJ. Overview of the Cohort Consortium Vitamin D Pooling Project of Rarer Cancers. American Journal of Epidemiology. 2010;172(1):4—9. doi:10.1093/aje/ kwq119.
  8. Bischoff-Ferrari H, Willett. Comment on the IOM vitamin D and calcium recommendations: for adult bone health, too low on vitamin D—and too generous on calcium. The Nutrition Source. Harvard School of Public Health. Available at what-should-you-eat/vitamin-d-fracture-prevention. Accessed May 18, 2011.
  9. Kennel KA, Drake MT, Hurley DL. Vitamin D deficiency in adults: when to test and how to treat. Mayo Clinic Proceedings. 2010;85:752—58.
  10. Ginde AA , Liu MC, Camargo CA. Demographic differences and trends of vitamin D insufficiency in the US population, 1988—2004. Archives of Internal Medicine. 2009;169:626—32.
  11. Looker AC , Johnson CL, Lacher DA , et al. Vitamin D status: United States, 2001—2006. NCHS Data Brief, No. 59. March 2011. Available at data/databriefs/db59.htm. Accessed May 18, 2011.
  12. Dietary Supplement Fact Sheet: Vitamin D. Health professional version. NIH Office of Dietary Supplements website. Available at HealthProfessional. Accessed May 18, 2011.