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Cancer Myths (8 posts)  RSS

Can Vitamin D prevent cancer?

June 16, 2015

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.

Observational Studies

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.

Indoors or outdoors, there's no such thing as a safe tan

May 22, 2014

By Gery P. Guy Jr., PhD, MPH

If you read no further, know this: there is no such thing as a safe tan. Indoor tanning is just as dangerous, if not more, than tanning outside in the sun. In fact, indoor tanning injures thousands of people each year badly enough to go to the emergency department. Indoor tanning can cause sunburn and damage to your eyes that could lead to vision loss. Indoor tanning can also cause premature skin aging, including loss of elasticity, wrinkling, age spots, and changes in skin texture.

Most dangerous of all, indoor tanning is a recognized cause of skin cancer, including deadly melanoma. Skin cancer is the most common form of cancer in the United States. Approximately 3.5 million cases of non-melanoma skin cancers are treated each year, and more than 70,000 melanomas are diagnosed yearly. While many cancers have been on the decline in recent years, rates of melanoma, which causes the most skin cancer-related deaths, have been on the rise. Increased exposure to ultraviolet radiation (UV) through indoor tanning may be partially responsible for the continued increase in melanoma, especially among young women. Indoor tanning is particularly dangerous for younger and more frequent users.

Tanning myths

There are a lot of misconceptions about indoor tanning, so it's important to know the following:

  • Tanned skin is not healthy skin. That "healthy glow" from the tanning bed indicates damage to your skin. Whether tanning or burning, you are exposing yourself to harmful UV rays. In fact, every time you engage in indoor tanning, you increase your risk of melanoma. The truly healthy glow is your natural skin color. More...

Cell Phones, Bras, and Breast Cancer Risk

May 13, 2014

By Ted Gansler, MD, MPH


Like other contributors to the Expert Voices blogs, I am occasionally asked to reply to questions from journalists about various cancer-related topics. The most recent question I received is whether it is true that women who carry a cell phone in their bras are at increased risk for developing breast cancer.

This kind of question is surprisingly difficult to answer. It's relatively easy to write about things that are known to cause cancer. It's more difficult to be confident that something does not cause cancer, but one can still provide some guidance if there have been at least a few carefully-conducted epidemiologic studies with negative results. The most challenging requests we receive are often about questions that researchers have not addressed by scientific studies of humans populations. This is one such question.

Cause or Coincidence?

There are a few known instances of breast cancer in young women who have kept cell phones in their bras. (Even when cell phones are not being used for conversation or texting, if they are on then they still periodically emit low energy electromagnetic signals to stay in touch with nearby cell towers.) Because breast cancer is an uncommon and tragic occurrence among young women, these cases have received significant attention on television and on the Internet. But it is the nature of these media to emphasize unusual events, so of course we don't hear much about the millions of women and men who carry phones close to various organs and still remain healthy. More...

The Bottom Line on Soy and Breast Cancer Risk

August 02, 2012

By Marji McCullough, ScD, RD


EDITOR'S NOTE: Dr. McCullough added the following statement 4/8/14 in response to questions related to sources of isoflavones:

Research on soy and cancer is highly complex, controversial, and evolving.

When concerns about soy are raised, they generally focus on findings from rodent models of cancer which tend to use isolated soy compounds like soy protein isolate or high doses of isoflavones (compounds found in soy).  However, soy is metabolized differently in humans than it is in mice and rats, so findings in rodents may not apply to people. (See: for more on this.)(Setchell, AJCN, 2011).  There is no evidence in the medical literature that soy protein isolate is bad for humans, compared to other forms of soy. Soy protein isolate is often used as a supplement in randomized studies of the effects of soy on health and none of these studies have shown harm.

Most of the studies suggesting benefits of soy consumption in people have measured how much soy foods people are eating, including tofu, soybeans, and soy milk.  These foods are more commonly eaten in Asian countries. In the U.S., purified forms of soy are used in the food supply, including in energy bars and soy hot dogs.  The few US studies that have measured these forms of soy do not suggest harm.    

More research is needed to understand the relationship between specific forms of soy and doses of isoflavones on cancer risk and recurrence. We also need to learn more about childhood exposure to isoflavones and risk of cancer. Until more is known, if you enjoy eating soy foods, the evidence indicates that this is safe, and may be beneficial (but note that miso, a fermented soy product, is high in sodium.)  It is prudent to avoid high doses of isolated soy compounds found specifically in supplements, as less is known about their health effects. As for other "hidden" sources of soy proteins, the evidence to date does not suggest harm or benefit. However, if you are concerned about these products, you can choose to avoid them.  


Before writing a blog about soy and breast cancer, I took an informal poll of a few friends to get a sense of what women believe about soy.  I asked them, "What do you know about eating soy food?  Is it good for you? Not good for you?" (I didn't even mention breast cancer.)  The responses I got were,  "I think it acts like estrogen in the body"; "Consuming any soy products increases the risk of breast cancer"; "I don't eat it a lot because I heard something negative but I can't remember what it was;" and "I've heard you should only have it in moderation."  Apparently, people are hearing that soy may not be good. But what's the truth? In this blog I'll walk you through what we know and what we don't know about soy and breast cancer, and give you some practical tips on eating soy. More...

Breast Cancer Myths: Separating Fact from Fiction

October 24, 2011

By Ted Gansler, MD, MBA, MPH

You have probably seen and heard a lot about breast cancer during the past few weeks, but as we approach the end of this year's breast cancer awareness month this is a good time to ask how much of the information you encountered is actually true. See if you know which of the following statements are true and which are false... More...

Will a vitamin a day keep cancer away?

August 16, 2011

By Marji McCullough, ScD, RD


Editor's note: Dr. McCullough added the following statement 12/20/13 in response to new studies being released:

Recent findings on multivitamin supplements published after the posting of this blog deserve mention. In 2012, the results of a randomized controlled trial (RCT) of a daily multivitamin supplement were published, showing a small but statistically significant lower risk of all cancers combined in male physicians followed over 11 years.  The supplement included 30 nutrients at levels similar to that found in a regular diet in the United States (≤100% recommended daily allowance (RDA)).  The U.S. Preventive Services Task Force (USPSTF) recently updated their review of vitamin and mineral supplements for the primary prevention of cancer and cardiovascular disease (CVD) and found there isn't much evidence that multivitamins or supplements help prevent cancer or CVD. But they add that a small benefit from multivitamin supplements on cancer in men was found, based on this study and an earlier RCT in France. The study in France found lower rates of cancer in men, but not women. The authors speculate that this may have been due to worse nutritional status - a generally less nutritious diet in the men - but more research is needed. In the study with the male physicians mentioned above, they all were well nourished but still saw some benefit.

There are still questions to answer about multivitamins and cancer from the RCTs: why only in men? Does whether you have a nutritious diet matter, and to what degree? Are these findings able to be replicated? Longer follow-up from these trials (to see long term effects) may provide more clues. In the meantime, these studies show that lower-dose multivitamins (with ≤100% RDA) appear safe and don't raise risk of cancer or CVD. Still, the best way to get nutrients is through a healthy diet, and the cost of multivitamins can add up. People should discuss the pros and cons of supplement use with their health care professional.


Original blog, 8/16/2011

Can popping vitamin pills prevent cancer? The simple answer is no, based on what we know so far. In fact, some vitamin supplements have even shown harm. What I'm talking about mostly are pills containing individual nutrients in amounts that are greater than that found in food. Before you stop reading, thinking this is simply another "just eat your vegetables" message, let me give you a little history.

Toward the end of the last century, scientists observed that people with healthy diets, and with higher levels of certain phytochemicals ("phyto" for plant) in their bloodstream, such as beta-carotene, had lower rates of cancer. But observations don't prove cause and effect.

So, after careful evaluation of promising dietary compounds, the scientists began planning randomized, placebo controlled clinical trials ("RCTs") with tens of thousands of healthy people to see if taking supplements of individual phytochemicals could actually prevent cancer. RCTs are considered by most to be the gold standard for proving something works. Most of the supplements tested were antioxidants, which are chemical compounds that combat "free radicals" in the body that can damage DNA and possibly lead to cancer. More...

Is your car killing you with benzene?

July 19, 2011

By Ted Gansler, MD, MBA

An e-mail message that may have come into your inbox recently claims that dangerous levels of a cancer-causing chemical (benzene) are released from the plastic surfaces of automobile interiors. The e-mail recommends opening the vehicle's windows to remove the benzene before using the air conditioner.


Although benzene is linked to leukemia, very little research has looked at whether the interior surfaces of cars release dangerous amounts of benzene, and the information that is available does not support the e-mail's claims. More...

Filed Under:

Cancer Myths | General | Ted Gansler

Busting Clinical Trials Myths

April 26, 2011

By Katherine Sharpe, MTS

"It might be time to consider a clinical trial." I have heard this many times in my work with the American Cancer Society. Unfortunately, in most cases, people think of clinical trials as the option of last resort, so they consider one only when all other treatment options have failed.


But the truth is that clinical trials should always be considered as a treatment option. In fact, there are clinical trials for almost every type of cancer and stage of disease - there are even clinical trials for cancer prevention! Without clinical trials, we would see virtually no advances in cancer treatment. 


The good news is that more and more people are considering a cancer clinical trial when they are first diagnosed - and that helps speed up breakthroughs in cancer care. But there is clearly a need for more people to learn about and consider this option.


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