We have been bombarded over the past couple of years with scientific articles suggesting that vitamin D is the key to improving many aspects of our health, including reducing the risks of dying from cancer.
An article in this week’s Journal of the National Cancer Institute reminds us that perhaps we should be a bit cautious in embracing vitamin D as “the answer” before we do more research.
The report, from the National Cancer Institute and the Centers for Disease Control and Prevention, concluded that vitamin D levels in the blood were not related to overall cancer mortality.
However, the study did find that higher levels of vitamin D were associated with a substantial decrease in the risk of dying from colorectal cancer, and possibly with a reduction in the risk of dying from breast cancer.
The study was performed between 1988 and 1994 and was designed to examine the health and nutritional status of the noninstutionalized United States population.
As part of the study, the researchers took baseline blood samples, which included a measurement of serum vitamin D levels.
16,818 people were part of the study which continued with follow-up through the end of 2000.
The researchers also monitored a number of factors including race/ethnicity, the latitude where the people lived (which would be expected to influence vitamin D levels through sun exposure), smoking, educational levels, and physical activity among other variables.
When they analyzed the data, they found that 536 people had died from cancer.
Higher levels of vitamin D were found in men, whites and those who were more highly educated. Women, African and Mexican Americans, and less educated participants had lower levels of vitamin D in their blood tests.
Those who were more overweight and obese had lower blood levels of vitamin D than those who were thinner. We know that fat tends to store vitamin D and reduce serum levels so this was no surprise.
Greater degrees of physical activity were also associated with higher vitamin D levels.
The key finding of the study was that there was no impact of vitamin D levels on the overall risk of dying from cancer, when comparing groups based on where they lived or what season their blood test was drawn (spring and summer would be expected to increase vitamin D levels, compared to winter).
Vitamin D had no impact on cancer deaths when various racial/ethnic groups were examined.
When the researchers broke down the risks of cancer deaths based on a number of cancer sites, the only significant reduction they found was for colorectal cancer. In this cancer, those people with higher levels of vitamin D had a risk of dying from this disease that was 72% less than people with lower levels of vitamin D.
Although the data for breast cancer was suggestive of a protective effect of vitamin D, the numbers were insufficient to rule out other possible explanations for the decreased risk of death from breast cancer noted in the study.
Why would this study find results that appear to contradict the several other studies that have recently reported decreased risks of cancer deaths for a variety of cancers?
First, let me be clear: no study is perfect—even this one. Many other studies that have been reported to show a decrease in cancer deaths related to higher vitamin D intake or sun exposure have been done by excellent researchers from highly regarded institutions.
This study stands out because it was done prospectively. That means the participants were followed looking forward, and there were actual blood tests which measured vitamin D in the blood.
Many of the other studies have tried to infer vitamin D levels through a variety of means, such as asking about dietary habits or inferring a vitamin D level based on descriptions of outdoor activities.
That doesn’t mean that one study is right and the other is wrong. It simply means that different researchers have reached different conclusions based on the analysis of different types of information.
For example, in this JNCI study, the numbers of certain cancer deaths may have been too small to reach an accurate conclusion. Or, perhaps the follow-up wasn’t long enough, or perhaps the impact of vitamin D would have been greater if the blood test had been performed in younger people who were then followed for many more years.
The authors note that their results do not support the theory that low vitamin D levels contribute to the higher cancer deaths rates among African Americans in this country.
The researchers also write that their study had only small numbers of cancer deaths for certain cancer for certain ethnic and season/latitude subgroups.
They concluded with a statement that “additional studies with large numbers of samples of measured (vitamin D) levels, preferably at multiple time points, are needed to confirm the total cancer mortality findings of this paper and to obtain more accurate risk estimates for mortality from specific cancers.”
An editorial in the same issue of the Journal agrees.
The editorialists point out that it may take longer than 6-12 years of this current study to see the benefits of higher vitamin D levels, especially since it can take many years for a cancer to develop. They also point out that it would have been better to have blood samples done at multiple points in time, since a single sample may not reflect the actual vitamin D levels over time.
They point out that we need to know more about how vitamin D levels change from season to season, and how that impacts our health. We need to have a better understanding about the relationship between skin pigmentation and the response to ultraviolet B radiation to increase blood levels of vitamin D.
I particularly appreciated the conclusion of the editorial:
“Whether vitamin D reduces cancer risks and, if it does, whether these amounts suffice are actively being debated. Randomized clinical trials of the effects of vitamin D on the incidence of colonic polyps and invasive cancer are needed. While vitamin D may well have multiple benefits beyond bone, health professionals and the public should not in a rush to judgment assume that vitamin D is a magic bullet and consume high amounts of vitamin D. More definitive data on both benefits and potential adverse effects of high doses are urgently needed.”
I couldn’t agree more.
Since vitamin D has come into the spotlight, I have transitioned from being a skeptic to believing there may in fact be a role of vitamin D in reducing the risks of a variety of cancers. The evidence has been inferential, but I can’t ignore the weight of that evidence suggesting that such a relationship exists.
But the American Cancer Society has resisted the temptation to draw the conclusion that we currently know enough to make a general recommendation to the more than 300 million people in this country (and throughout the world) that they should markedly and routinely increase their vitamin D intake.
(I will say that we recommend the safer route of dietary supplements as opposed to increasing sun exposure for those who make their own decision to pursue a higher level of vitamin D. Seeking the sun for this purpose is, in our opinion, not the way to go to accomplish this goal, especially when supplements are safer and avoid the long term cumulative risks of skin-aging and skin cancer.)
We have consistently called for more research into this topic. This is especially important given our past experience with other vitamins, such as vitamin C and beta-carotene, where well-qualified experts touted the benefit of those vitamins in reducing cancer risk.
When the studies were actually done, we discovered that the vitamins had either no effect or, for some people, may have actually increased their risk of cancer.
So what is the bottom line? Is this study the end of the vitamin D debate (except perhaps for colorectal cancer and breast cancer)?
I don’t think this study should end the discussion of the possible role of vitamin D in cancer prevention, any more than I think the other studies answer the question that we need to embark on a major nationwide effort to increase vitamin D intake to supernormal levels.
We need more research, we need more information, and we need to answer the question definitively.
Until we get those research studies—and right now no one to my knowledge has undertaken the massive clinical trial it would take to answer the question—it is essentially an individual decision what to do, perhaps in consultation with your medical professional.
We do not have all the answers about the possible benefits of vitamin D in preventing cancer (or lack thereof), and we don’t have all the information about the risks.
Hopefully, as this debate continues, we will be able to move the research agenda forward and provide solid answers and guidance to replace the uncertainty which many of us currently face regarding the role of vitamin D in cancer prevention.
As I have said previously, this isn’t over. Stay tuned….
