By Graham Colditz, MD, DrPH, and Hank Dart, MS
Vitamin D’s days of obscurity seem pretty much over. Once just an afterthought to most people—relegated to the sides of milk cartons and the pages of medical texts—it’s now on the cusp of becoming a full-fledged disease prevention star. Although vitamin D has long been known as an important factor in bone health, a quickly growing body of evidence now shows that it may also help lower the risk of cancer, heart disease, and even premature death., Not surprisingly, scientists and the public have started to take note, particularly of vitamin D’s potential to protect against cancer.
The Cancer Connection: Research Shows the Link
Often referred to as the “sunshine vitamin” because direct exposure to ultraviolet B (UVB) sunlight causes the vitamin to naturally form in the skin, vitamin D’s cancer prevention pedigree actually began well before it was a direct subject of research. Key studies in the 1930s and 1940s found clear links between rates of skin cancer or level of sun exposure and rates of non-skin cancers. As skin cancer rates or sun exposure went up, rates of non-skin cancer went down., These findings caused considerable confusion among researchers, to say the least. That sunlight or skin cancer could ward off non-skin cancers seemed a strange idea; and, however important these studies were to become, they stayed largely curiosities until the early 1980s, when researchers started building the hypothesis that vitamin D and sunlight could be the main factor helping drive down cancer rates in areas with higher sun exposure.
Since the early research connecting vitamin D and cancer, an increasing number of studies each year have looked at the links between cancer rates and the vitamin (both from sun exposure and from food and supplements). The past five years especially have seen a number of large, well-designed studies on the issue that provide compelling arguments that vitamin D could be an important player in cancer prevention.
The Most Compelling Results
By far the strongest evidence to date supports a link between vitamin D and colon cancer—the third most common cancer among both men and women in the United States. A number of very good studies have shown that people with higher vitamin D levels can have as little as half the risk of developing colon cancer as those with lower vitamin D levels. And one National Cancer Institute study of close to 17,000 people nationwide found that those with high vitamin D levels had an almost 75 percent lower risk of dying from the disease compared with those with low to moderates levels.
The cancer prevention benefits of vitamin D seem to reach well beyond colon cancer, however. Studies hint that high vitamin D levels may substantially lower the risk of oral and esophageal cancers as well as pancreatic cancer and leukemia.6 And a detailed 2006 review of research findings on the links between vitamin D and cancer risk concluded that increased vitamin D levels could cut the risk of not only colon cancer but also prostate, breast, and ovarian cancers. The paper also suggested that high vitamin D levels could even help prolong survival in those who went on to develop certain cancers. Furthermore, it didn’t seem to matter where the vitamin D came from—food and supplements or sun exposure. Both sources had benefits.
Does the Type of Study Matter?
With such a wide potential impact on so many different cancers, it’s not too surprising that a recent study of nearly 48,000 male health professionals also found that, compared with those with lower predicted levels of vitamin D intake, those with higher levels had a nearly 20 percent lower risk of developing cancer overall and a nearly 30 percent lower risk of dying from the disease.
Despite such positive findings, it’s important to note that most of the evidence of vitamin D’s benefits compiled to date comes from what are called observational studies—studies that follow a free-living group of people over time. Such large, well-designed observational studies can provide very good data on the links between risk factors and diseases, but they aren’t usually the “gold standard” when looking at things like vitamin D and cancer. For this, randomized, controlled trials are usually considered the study of choice.
Unlike a lot of strange-sounding types of studies, randomized, controlled trials adhere pretty closely to what most people think of when they picture a scientific study: One group of people is given an intervention (say, vitamin D pills), and one group (the control group) is not. Then rates of disease are compared after a period of time to see if one of the groups fared better or worse than the other. If done well, these types of studies can provide some of the most reliable scientific data. Unfortunately, as you’ll read, they’re not immune to some frailties either.
So what do the randomized trials have to say about vitamin D and cancer? Well, not a great deal right now. There have been only two published trials to date. The first, from the well-known Women’s Health Initiative, found no cancer prevention benefit from vitamin D supplementation.13 This wasn’t, however, the resounding downfall of vitamin D as you might think. Researchers in the field have roundly criticized the results because the amount of vitamin D used in the study—400 IU (international units) per day—was well below the amount where past research shows benefits likely begin (around 1,000 IU per day).
Further shaking confidence in the results was the fact that about 40 percent of the women in the study group didn’t regularly take their vitamin D pills over the course of the study, and many women in the control group—who got placebo pills—kept taking vitamin D independent of the study. This introduced a lot of room for error in the results. So even if the dose had been large enough to see benefits, it likely would have still been hard to see a difference between the two groups.
The second trial, published in 2007, called for the study group to take a significantly larger dose of vitamin D—1,100 IU per day—and the results were much more positive. The study followed approximately 1,200 postmenopausal women over four years and found that the risk of cancer overall in the vitamin D group was cut by more than half compared with that of the control group.
What Does the Research Mean to Me?
So what does this all mean? When all the evidence is looked at together, it likely means that a boost in vitamin D levels could go a long way toward lowering the cancer risk of much of the population. More than half of all women, and over 40 percent of all men, in the United States have less than optimal blood levels of vitamin D; the optimum is often suggested to be 75 nmol/dL (nanomoles per deciliter) of a type of circulating vitamin D called 25(OH)D (see sidebar “Vitamin D by Any Other Name”)., Those prone to very low sun exposure or low vitamin D levels are even worse off: the elderly, those who are very overweight, people with dark skin, and those who live in northern climates (where vitamin D–producing UVB sunlight can be low for much of the year; see sidebar “A Moving Target: Factors Affecting Vitamin D Levels”).
To reach beneficial blood levels of vitamin D, most people would need to take about 1,000 IU per day, with those prone to lower levels taking perhaps 1,500 IU per day. Right now this is well above the current U.S. guidelines for adults—200 to 600 IU per day—which is too low to provide cancer prevention, and possibly even bone health, benefits. In moves acknowledging the growing evidence of the health benefits of vitamin D, however, the American Academy of Pediatrics recently doubled the recommended intake of vitamin D for children, and the Canadian Cancer Society boosted its general recommendation for adults to 1,000 IU per day during fall and winter, when UVB levels in that far northern land are too low to create vitamin D even if one is brave enough to expose bare skin to the cold.
The “Safe Sun” Issue: Weighing the Risks and the Benefits
The issue of whether to encourage people to spend a little time in the sun to reap the benefits of vitamin D production is a bit of double-edged sword for the public health community. While sun is in fact a potent (and free) source of vitamin D—producing all you’d need in a day in about 10 minutes of unprotected exposure, depending on where you live—it’s also a clear cause of skin cancer, including deadly melanoma. “Safe sun” proponents feel that the benefits of a small amount of sun exposure could far outweigh the risks. And the numbers can make the point compelling. About 8,500 people die of melanoma every year, whereas increased vitamin D levels, partially boosted by sun exposure, could possibly prevent more than 50,000 cancer deaths annually overall.
Still it’s hard to find the right balance of risk and benefits, especially when vitamin D supplements are so easy to come by and relatively cheap. Is it okay to promote something that causes melanoma when there’s a safer alternative that has the same benefits? For now most experts lean toward boosting vitamin D levels with pills rather than sun exposure, but the discussion continues, and newer data may provide some clarity in the future on the “safe sun” issue.
The Final Word
Unlike many factors that make a splash in the news and then fade away—remember cancer-causing French fries?—the evidence on the cancer prevention benefits of vitamin D should hold up pretty well over time. In fact, so compelling is the current batch of evidence that many health organizations are reassessing their current intake guidelines and seem poised to kick up their recommendations, if they haven’t already. So, while the final say on the topic will likely come down to the results of the next round of randomized, controlled trials, adding a little extra vitamin D to your day may be just what the doctor ordered.
A Moving Target: Factors Affecting Vitamin D Levels
One of the things keeping scientists on their toes as they try to devise updated recommendations for vitamin D intake is that many different factors affect an individual’s blood level of the vitamin.
- Where you live. Northern parts of the country get less vitamin D–producing ultraviolet B (UVB) sunlight than do southern parts. Above about 37 degrees latitude (around San Francisco, California, and St. Louis, Missouri), UVB is too low in late fall through early spring to produce any vitamin D.
- How much sun you get. People who work or spend a lot of time outdoors typically have higher vitamin D levels than those who don’t often get outside.
- Your skin color. Darker skin produces less vitamin D because melanin, the substance that gives skin its color, basically acts as a UVB sunscreen.
- Your age. Vitamin D production in skin gets much less effective as you age.
- Your weight. Overweight, and especially obese, people have lower levels of vitamin D in the blood because the vitamin can be taken up and held by fat cells.
With so many factors playing a role in a person’s vitamin D levels, it can be hard to determine blanket recommendations for an entire population. Because about half or more of the U.S. population is low in the vitamin, shooting for 1,000 IU of vitamin D per day is likely a good, safe place to start. But some scientists looking at the data feel that a bit of personalization may eventually be in order. A simple blood test could show if someone is at or near levels that provide larger health benefits; after that, intake recommendations can be adjusted accordingly.
Such testing makes a lot of sense given the huge differences in potential vitamin D production in Fargo, North Dakota, and San Diego, California. But given the time and the cost of such a practice, it will take some time for the number-crunchers, policy makers, and researchers to sift through the data and settle on a cost-effective, evidence-based approach.
Vitamin D by Any Other Name
It’s easy to get overwhelmed by an alphabet soup of scientific notation when you delve into the topic of vitamin D, but it’s not as complicated as it may seem. Here’s a quick guide to the different forms of the vitamin and how it likely helps protect against cancer:
- Vitamin D3 and D2. These forms of the vitamin are produced in the skin and are found in food and supplements.
- 25(OH)D (calcidiol). Vitamin D3 and D2 are converted to this prohormone form of the vitamin, which is the major form found in the blood. 25(OH)D is most often used to determine someone’s vitamin D level.
- 1,25(OH)2D (calcitriol). One final conversion creates this “active” hormone form of the vitamin, blood levels of which rarely change, even as levels of other types of vitamin D go up and down. 1,25(OH)2D helps influence bone metabolism and promote healthy cell growth, which is the likely reason why the vitamin helps lower cancer risk and even prolong survival.
Save for those Scandinavians among us, it’s pretty hard to get close to 1,000 IU per day of vitamin D by eating a typical Western diet. Most foods, including fortified milk and orange juice, are relatively low in the vitamin. Fatty fish is one of the best sources, but that isn’t a regular part of many people’s diets. And although sun exposure can be an incredibly rich source of vitamin D, it’s not a reliable source year-round in many parts of the country and has other drawbacks, such as increased risk of melanoma. Vitamin D supplements are likely the safest and easiest way for most people to boost their vitamin D levels. The amount contained in most supplements varies depending on the type. Look for those that contain vitamin D3 because vitamin D2 isn’t as potent a source. Unless directed by a doctor, stay away from formulations with doses at or over 2,000 IU (international units).
Source IUs per Serving
Cod liver oil, 1 tablespoon 1,360
Salmon (cooked), 3.5 ounces 360
Mackerel (cooked), 3.5 ounces 345
Tuna (canned in oil), 3 ounces 200
Sardines (canned in oil, drained), 1.75 ounces 250
Milk, vitamin D–fortified (nonfat, reduced fat, and whole), 1 cup 98
Margarine, fortified, 1 tablespoon 60
Ready-to-eat cereal (typical) 40
Egg, 1 whole (vitamin D is found in yolk) 20
Liver, beef (cooked), 3.5 ounces 15
Cheese, Swiss, 1 ounce 12
Sun exposure on arms and legs (10 to 20 minutes)
(depending on time of day, year, skin color, and latitude) 3,000
Supplements (depending on formulation) 400–1,000