By Marji McCullough, ScD, RD
You may be aware that Vitamin D is important for helping make strong bones. But vitamin D often appears in the media because of its potential role in a host of other health effects, from preventing cancer, diabetes, and heart disease to simply living longer. However, most of these "non-skeletal" (not having to do with your bones) roles of vitamin D are not clearly established and remain a topic of active investigation and debate. To add to the confusion, several recent scientific reviews of the vast data on vitamin D arrived at different conclusions about whether it helps prevent disease or not.
In this blog, I am going to focus on the evidence on vitamin D and cancer prevention, highlight some key unresolved questions, and give some advice to consider while we await more solid answers (which may take a while).
Where does vitamin D come from?
People can get vitamin D from exposure to sunlight, from certain foods, and from supplements.
Current vitamin D recommendations from the Institute of Medicine (IOM), the organization tasked with developing the Recommended Dietary Allowances (RDAs), are 600 International Units (IU) per day for most adults, and 800 IU of vitamin D per day for those over age 70. Very high doses of vitamin D over a long period of time can be toxic, so the IOM recommends that no one should exceed 4,000 IU/day.
Vitamin D is found naturally in very few foods, including cod liver oil, fatty fish like sardines or salmon, and in smaller amounts in eggs and leafy greens. In the U.S., vitamin D is added to milk, some yogurts, orange juices, and cereals. One cup of milk or yogurt contains about 100 IU, whereas fatty fish contains about 500 IU per serving. Read about other sources of vitamin D. Vitamin D supplements are available in a range of doses.
Sun exposure can provide a wallop of vitamin D, depending on amount of skin exposed, skin tone, time of day, time of year, location, and a variety of other factors. For example, a Caucasian adult wearing a bathing suit exposed to enough sun to have a light pink sunburn has received roughly 10,000-20,000 IU of vitamin D. It takes dark-skinned individuals 5-10 times longer to form the vitamin because of higher concentrations of melanin in the skin, and dark-skinned individuals often have lower vitamin D levels. Other factors that can influence vitamin D formation in the skin include use of sunscreens and sun-protective clothing (because they block UV rays).
But don't look to the sun as a source of vitamin D because the same UV radiation that forms vitamin D in the skin also burns the skin and can lead to skin cancer. Studies have not identified a level of sun exposure that is safe for avoiding skin cancer. The American Cancer Society recommends limiting sun exposure to prevent skin cancer, including melanoma (the most serious type of skin cancer).
What the studies say
It has been suggested that vitamin D itself may prevent, or even increase the risk for, some forms of cancer. Different types of studies are used to understand if this is true. All have their strengths and weaknesses and add a piece to the puzzle.
Laboratory studies provide some strong biological evidence to support a role for vitamin D in cancer prevention. Vitamin D can "turn on" or "turn off" a host of genes, including some that regulate cell growth, limit inflammation, and reduce levels of a signaling protein that can allow cancer cells to spread. The exact role of vitamin D in these processes is a very active area of research.
In humans, the idea that vitamin D might help protect against cancer first came from studies that mapped cancer death rates in the US by region. These studies showed that Northern states, where sun exposure was lowest, had higher death rates from several different cancers compared to the Southern, sunnier states. Because the sun is a source of vitamin D, scientists thought that vitamin D might protect against cancer. However, different cancer rates by region also may be due to other factors that vary among people living in different parts of the country.
So far, the most support for a role of vitamin D comes from prospective studies of colorectal cancer (includes both colon and rectal cancer). In several studies, compared to people with low blood vitamin D levels, people with higher blood levels have a significantly lower risk of colorectal cancer. However, there are inconsistencies in results across studies, potentially due to different methods used, such as how vitamin D was measured. An ongoing study, called the "Vitamin D Pooling Project of Breast and Colorectal Cancer" is carefully measuring blood levels of vitamin D and examining their association with colorectal and breast cancer in 21 prospective studies, using the same methods. These findings should be published in the next year.
Observational studies of vitamin D and risk of other cancers do not provide clear evidence of benefit. In a combined analysis from 10 prospective cohort studies, the amount of vitamin D in the blood before diagnosis was not associated (no link was found) with the risk of kidney, lymphoid, ovarian, endometrial, or upper gastrointestinal cancers like stomach or esophagus. For other cancers, including prostate and pancreas, studies have had inconsistent results, some even suggesting increased risk of cancer in those with the highest levels.
A strength of observational prospective cohort studies is that they can typically examine wide ranges of vitamin D in the blood that occur naturally in a population. Their main limitation is that they cannot prove that it's the vitamin D in the blood that really prevents cancer. For example, people with low vitamin D levels may also tend to be less physically active and more overweight or obese, both of which are risk factors for certain cancers. For this reason, researchers conducting observational studies collect detailed information on other risk factors and account for them when they're examining the results.
Randomized Controlled Trials (RCTs)
When conducted well, RCTs can prove if something prevents disease because people are randomly assigned to get vitamin D or a placebo (sugar pill). The randomization helps to ensure that on average people are more alike in other ways (e.g. body weight, physical activity, and other known or unknown risk factors), so researchers can isolate the effect of the vitamin D supplement. But these studies are very expensive and logistically difficult, and may not study the right dose, for the right amount of time, in the right (susceptible) population.
So far, a large RCT did not find that 400 IU vitamin D combined with calcium lowered colorectal or breast cancer risk. In this study, women were allowed to take their own supplements, and by the end of the trial, most were. In other words, the placebo group as well as the intervention group were exposed to vitamin D, increasing the risk of results that aren't significant (null results).
Other RCTs of vitamin D that reported on development of cancer or death from cancer had very small sample sizes and did not provide conclusive results. For some of these trials, the initial goal was to study bone health, not cancer, increasing the likelihood of "chance" findings. There are currently a handful of larger trials underway, including the large U.S. VITAL trial, which will examine vitamin D and fish oil supplements in relation to cancer outcomes and heart disease. Study results are not expected for several years.
What to do in the meantime?
The key for research will be to identify the amount of vitamin D that may lower the risk of certain diseases, but not increase the risk of others. Until we know more, make sure you meet the IOM recommendations for bone health of 600 IU for most adults or 800 IU of vitamin D/day for those over age 70. Even for people who are not exposed to the sun, the recommended doses are thought to be enough for 97.5% of people in the US. Depending on your health status, your doctor may choose to measure how much vitamin D you have in your blood, but routine vitamin D measurements are currently not recommended by any agency for cancer prevention or to avoid other serious illnesses.
Bottom line: we don't know yet if vitamin D can help prevent cancer or other diseases, but we're working on finding out. In the meantime, make sure to meet the IOM recommendations for bone health through food choices as much as you can, and discuss with your health care provider whether you need a supplement to help.
McCullough is strategic director of nutritional epidemiology for the American Cancer Society.