Vitamin D Supplements for Reducing Cancer Mortality

It all started with a famous study entitled “Do sunlight and vitamin D reduce the likelihood of colon cancer?” that was published in 1980. Johns Hopkins University researchers were trying to figure out why states like New Mexico and Arizona have only about half the colon cancer rates of states like New York, New Hampshire, and Vermont. Could it be because New Mexicans and Arizonans get so much more sun? The researchers proposed that perhaps vitamin D, known as the sunshine vitamin, is a protective factor against colon cancer. Since then, sun exposure has also been associated with lower rates of 14 other types of cancer.

As I discuss in my video Do Vitamin D Supplements Reduce the Risk of Dying from Cancer?, vitamin D may also affect cancer survival. Higher blood levels of vitamin D were associated with lower mortality of patients with colorectal cancer. How much lower? Nearly half the mortality. And, the higher the vitamin D levels, the lower the death rate appeared to fall. This may explain why the survival rate from colon cancer may depend in part “on the season of diagnosis.” The risk of rapid death is lowest if you’re diagnosed in the fall after you’ve spent the summer building up your vitamin D stores. Other risk factors could be seasonal, too. For example, perhaps people are taking advantage of the fall harvest and eating healthier, which might explain lower risk in the autumn. Additionally, “[a]lcohol intake is a risk factor and may be highest in the winter season…” What about physical activity? In the summer, we may be more likely to be outside running around, not only getting more sun, but also getting more exercise, which may itself be protective.

These kinds of studies just provide circumstantial evidence. Establishing a cause-and-effect relationship between colon cancer and vitamin D deficiency using observational studies is challenging because of confounding factors, such as exercise habits—so-called lurking variables. For example, there may be a tight correlation between ice cream sales and drowning deaths, but that doesn’t mean ice cream causes drowning. A more likely explanation is that there is a lurking third variable, like hot weather in summertime, that explains why drowning deaths are highest when ice cream consumption is also at its highest.

This actually happened with hormone replacement therapy. Women taking drugs like Premarin appeared to have 50 percent less risk of heart disease, so doctors prescribed it to women by the millions. But, if we dig a little deeper into the data, we find that, indeed, women taking estrogen had 50 percent lower risk of dying from heart disease, but they also had a 50 percent lower risk of dying from accidents and homicide, so it probably wasn’t the drug. The only way to know for sure is to put it to the test in a randomized, clinical trial, where half the women are given the drug and we see what happens. A decade later, such a study was done. Instead of having a 50 percent drop in risk, within a year of being given the hormone pills, heart attack and death rates shot up 50 percent. In retrospect, the lurking variable was likely socioeconomic class. Poor women are less likely to be prescribed hormone replacement therapy and more likely to be murdered and die of heart disease. Because of the lurking variable, a drug we now know to be dangerous had initially appeared to be protective.

Besides lurking variables, there’s also the possibility of reverse causation. Perhaps low vitamin D levels didn’t worsen the cancer. Instead, maybe the cancer worsened the vitamin D levels. This may be unlikely, since tumors don’t appear to directly affect vitamin D levels, but cancer treatment might. Even simple knee surgery can cause vitamin D levels to drop dramatically within hours, thought to be due simply to the inflammatory insult of cutting into someone. So, maybe that could help explain the link between lower vitamin D and lower survival. And, cancer patients may be spending less time running around the beach. So, yes: Higher vitamin D levels are associated with improved survival in colorectal cancer, and the same has been found for breast cancer. In fact, there is about double the risk of breast cancer recurrence and death in women with the lowest vitamin D levels. What’s more, higher vitamin D levels are associated with longer survival with ovarian cancer, as well as having better outcomes for other cancers like lymphoma. But, the bottom-line, as we learned with hormone replacement, is that we have to put it to the test. There weren’t a lot of randomized controlled trials on vitamin D supplements and cancer, however…until now.

We now have a few randomized controlled trials, and vitamin D supplements do indeed appear to reduce the risk of dying from cancer! What dose? Researchers suggest getting blood levels up to at least about 75 nanomoles per liter. These levels are not reached by as many as three-quarters of women with breast cancer nor achieved by a striking 97 percent of colon cancer patients .

Getting up to these kinds of levels—75 or, perhaps even better, 100 nanomoles—might require about 2000 to 4000 IU of vitamin D a day, levels of intake for which there appear to be no credible evidence of harm. Regardless of what the exact level is, the findings of these kinds of studies may have a profound influence on future cancer treatment.


What about just getting sun instead? Be sure to check out my six-part video series:

It’s better, of course, to prevent colon cancer in the first place. See, for example:

For more on that extraordinary story about Premarin and hormone replacement therapy, see How Did Doctors Not Know About the Risks of Hormone Therapy?

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Cow Cancer Virus Implicated in Breast Cancer

Up to 20 percent of all cancers in general are linked to infections, particularly viruses, and the list of potentially carcinogenic infectious agents is growing. It would be great if we could find a virus that contributed to breast cancer risk, because then we might have new ways to prevent and treat it. Currently, the dietary link between breast cancer and consumption of meat and dairy is considered a saturated fat effect, but there is a cancer-causing bovine virus that infects the mammary gland cells of cows. The infectious virus is then released into the milk supply. Since most U.S. dairy herds are infected, scientists posit that Americans are often exposed to this bovine leukemia virus (BLV), which I discuss in my video The Role of Bovine Leukemia Virus in Breast Cancer.

We didn’t have proof of this until 2003, 34 years after the virus was first identified. Early on, our best available tests failed to find antibodies to BLV in human blood. When our immune system is exposed to a virus, it creates antibodies to attack it. No antibodies, no exposure. “This led to the prevailing opinion that…the virus is not a public health hazard.” Though those tests “were state of the art at that time, they are extremely insensitive compared to more modern techniques.” As a result, researchers decided to re-examine the issue now that we have better tests. They took blood from about 250 people simply to address the question: “Do any humans have antibodies to BLV?” The answer? Yes, 191 of them did––74 percent. That shouldn’t have come as a surprise, however: By then, nearly 90 percent of American dairy herds were infected, and, according to the latest national survey, 100 percent of the big factory dairy farms were infected, as determined by testing the milk coming from those operations. Given this, why isn’t there an epidemic of cancer of the udder? Dairy cattle are slaughtered so young that there isn’t a lot of time for them to develop gross tumors, but that’s how most women may be getting infected. Although pasteurization should knock out the virus, who hasn’t eaten a rare, pink-in-the-middle burger at some point?

The bottom line is that the “long-held assumption that BLV is not a public health hazard…is no longer tenable…” This whole field of investigation needs to be reopened, with the next step determining whether humans are actually infected. “The presence of antibodies to particular viruses in human sera is generally interpreted as an indicator of a present or past infection with the virus.” But, theoretically, we might have developed antibodies to the dead viruses we ate, viruses that had been killed by cooking or pasteurization. Just because three-quarters of us have been exposed doesn’t mean we were actively infected by the virus.

How do we prove this? We would need to find the retrovirus actively stitched into our own DNA. Well, millions of women have had breast surgery, so why not just look at the tissue? Researchers finally did just that and published their findings in the Centers for Disease Control and Protection’s emerging infectious diseases journal: Forty-four percent of samples tested positive for BLV, proving for the first time that humans can be infected with bovine leukemia virus. The final step? Determine whether the virus is actually contributing to disease. In other words, are the bovine leukemia viruses we’re finding in human breast tissue cancer-causing or just “harmless passengers”?

One way to make that determination is to see whether the virus is more often present in those with breast cancer. No one had ever looked for the virus in breast tissue from people with cancer…until now. The “[p]resence of BLV-DNA in breast tissues was strongly associated with diagnosed and histologically confirmed breast cancer…” As many as 37 percent of human breast cancer cases may be attributable to exposure to bovine leukemia virus.


For some historical background leading up to these shocking findings, see my video Is Bovine Leukemia Virus in Milk Infectious?.

I couldn’t wait to read the meat and dairy industry journals to see how they’d try to spin this. Find out what I discovered in my final video in this series Industry Response to Bovine Leukemia Virus in Breast Cancer.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Should You Be Concerned about Bovine Leukemia Virus in Milk?

Decades ago, concern was raised that the milk of dairy cows frequently contains a leukemia-causing virus—more specifically, bovine leukemia virus (BLV), the leading cancer killer among dairy cattle. Most U.S. dairy herds are infected with the cancer virus. “Thus the question of whether dairy cows naturally infected with BLV release infectious virus into milk is an important public health consideration” and the subject of my video Is Bovine Leukemia Virus in Milk Infectious?.

Researchers at the University of Pennsylvania decided to put it to the test. And indeed, infectious virus was demonstrated in the milk of 17 of the 24 cows tested, indicating that “humans are often orally exposed to BLV.” Just because we’re exposed to it doesn’t mean it’s causing human disease, though. How do we know BLV can even infect human cells? We didn’t until 1976 when it was discovered that BLV can indeed infect human, chimpanzee, and rhesus monkey cells. Nevertheless, that still doesn’t mean BLV necessarily causes cancer in other species.

Researchers can’t lock human infants in a cage and feed them infected milk, but they can cage infant chimpanzees. Chimps Bois and Roger were fed infected milk, developed leukemia, and died. Until then, we didn’t even know chimps could get leukemia. The fact that BLV-infected milk appeared to transmit or induce leukemia in our closest living relatives certainly did raise the stakes, but human beings are not chimpanzees. Yes, our DNA may be 98 percent identical, but we may share 60 percent of our DNA with a banana. We need human studies.

We can’t do interventional trials in this case, thanks to those pesky Nuremberg principles, but what about observational studies? Do cattle farmers have higher rates of cancer? Apparently so. This finding led some to suggest that “milk- and egg-borne viruses may be highly important in the pathogenesis [or development] of human leukemia and lymphoma,” but farmers may be exposed to all sorts of potential carcinogens, such as pesticides. Large animal veterinarians may also have more leukemia and lymphoma, but some are also “particularly lax in the use of X-ray protective equipment,” so it didn’t necessarily have anything to do with viruses.

We needed so-called serology studies, testing people’s blood for antibodies against the virus, which would prove human exposure, and we got them. Ten different studies looked for BLV antibodies in cancer patients and non-cancer patients, creamery employees versus office employees, veterinarians, unpasteurized milk drinkers, and more. “Not one of these studies found a single individual with antibodies to BLV…” As a result, in 1981, the case was closed: “Therefore, there is strong serological evidence to indicate that BLV is not transmissible to man.” However, the strength of the evidence is only as strong as the strength of the test. Chimpanzees Bois and Roger didn’t develop detectable antibodies either, and they died from BLV.

The tests available a handful of decades ago were not really sensitive. “Clearly, the question of whether BLV poses a public health hazard deserves thorough investigation” using highly sensitive molecular probes. It would take a few decades for us to get such an examination, and I discuss those landmark findings in my videos The Role of Bovine Leukemia in Breast Cancer and Industry Response to Bovine Leukemia Virus in Breast Cancer.


Thankfully, feline leukemia virus does not appear to be transmissible. See Pets and Human Lymphoma.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations: