Long known as an important factor in bone health, a quickly growing body of evidence now also shows that vitamin D may help lower the risk of cancer, heart disease, and even premature death.^1,2 Not surprisingly, scientists and the public have started to take note, particularly of D’s potential to protect against cancer.

Often referred to as the “sunshine vitamin” because direct exposure to ultraviolet B (UV-B) 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 40s 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.^{3, 4} 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 to drive down cancer rates in areas with higher sun exposure.

Since the early studies 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 as well as from food and supplements). The last five years, especially, have seen a number of large well-designed studies on the issue, and they 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.^5-10 And one National Cancer Institute study of close to 17,000 people nationwide found that those with high D levels had an almost 75 percent lower risk of dying from the disease compared with those with low to moderates levels.¹¹

The cancer prevention benefits of vitamin D, though, seem to reach well beyond colon cancer. 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 not only cut the risk of colon cancer, but also prostate, breast, and ovarian cancers. ¹² 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. ⁶

Despite such positive findings, it’s important to note that most of the evidence compiled to date of vitamin D’s benefits comes from what are called observational studies—studies that follow a free-living group of people over time. While large, well-designed observational studies can provide very good data on the links between risk factors and diseases, they aren’t usually the “gold standard” when looking at things like vitamin D and cancer. For this, randomized, controlled trials are usually held up as 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 However, this wasn’t the resounding downfall of vitamin D you might think. The results have been roundly criticized by researchers in the field because the amount of vitamin D used in the study (400 IU per day) was well below the amount where past research show benefits likely begin (around 1000 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 would likely still have been hard to see a difference between the two groups.

The second trial, however, published in 2007, called for the study group to take a significantly larger dose of vitamin D—1100 IU a day—and the results were much more positive. The study followed approximately 1200 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.¹⁴

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 to vitamin D levels could go a long way toward lowering the cancer risk of much of the population.  Over half of all women, and over 40 percent of all men, in the United States have less than optimal blood levels of vitamin D (often suggested to be 75 nmol/dL of a type of circulating vitamin D called 25(OH)D).^{15, 16} 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 climes (where vitamin D-producing UV-B sunlight can be low for much of the year).

To reach beneficial blood levels of vitamin D, most people would need to take about 1000 IU per day, with those prone to lower levels taking perhaps 1500 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.¹⁷ But, in moves acknowledging the growing evidence on the health benefits of vitamin D, 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 1000 IU a day during fall and winter, when UV-B 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.^{18, 19}

References

1. Heaney RP. Vitamin D in health and disease. Clin J Am Soc Nephrol. Sep 2008;3(5):1535-1541.

2. Melamed ML, Michos ED, Post W, Astor B. 25-hydroxyvitamin D levels and the risk of mortality in the general population. Arch Intern Med. Aug 11 2008;168(15):1629-1637.

3.  Peller S, Stephenson C, S.,. Skin irritation and cancer in the United States. Am J Med Sci. 1937;194:326-333.

4.  Apperly F, L.,. The relation of solar radiation to cancer mortality in North America. Cancer Research. 1941;1:191-195.

5.  Feskanich D, Ma J, Fuchs CS, et al. Plasma vitamin D metabolites and risk of colorectal cancer in women. Cancer Epidemiol Biomarkers Prev. Sep 2004;13(9):1502-1508.

6.  Giovannucci E, Liu Y, Rimm EB, et al. Prospective study of predictors of vitamin D status and cancer incidence and mortality in men. J Natl Cancer Inst. Apr 5 2006;98(7):451-459.

7.  Giovannucci E, Liu Y, Willett WC. Cancer incidence and mortality and vitamin D in black and white male health professionals. Cancer Epidemiol Biomarkers Prev. Dec 2006;15(12):2467-2472.

8.  Bostick RM, Potter JD, Sellers TA, McKenzie DR, Kushi LH, Folsom AR. Relation of calcium, vitamin D, and dairy food intake to incidence of colon cancer among older women. The Iowa Women’s Health Study. Am J Epidemiol. Jun 15 1993;137(12):1302-1317.

9.  Garland CF, Comstock GW, Garland FC, Helsing KJ, Shaw EK, Gorham ED. Serum 25-hydroxyvitamin D and colon cancer: eight-year prospective study. Lancet. Nov 18 1989;2(8673):1176-1178.

10.  Wu K, Feskanich D, Fuchs CS, Willett WC, Hollis BW, Giovannucci EL. A nested case control study of plasma 25-hydroxyvitamin D concentrations and risk of colorectal cancer. J Natl Cancer Inst. Jul 18 2007;99(14):1120-1129.

11.  Freedman DM, Looker AC, Chang SC, Graubard BI. Prospective study of serum vitamin D and cancer mortality in the United States. J Natl Cancer Inst. Nov 7 2007;99(21):1594-1602.

12.  Garland CF, Garland FC, Gorham ED, et al. The role of vitamin D in cancer prevention. Am J Public Health. Feb 2006;96(2):252-261.

13.  Wactawski-Wende J, Kotchen JM, Anderson GL, et al. Calcium plus vitamin D supplementation and the risk of colorectal cancer. N Engl J Med. Feb 16 2006;354(7):684-696.

14.  Lappe JM, Travers-Gustafson D, Davies KM, Recker RR, Heaney RP. Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial. Am J Clin Nutr. Jun 2007;85(6):1586-1591.

15.  Zadshir A, Tareen N, Pan D, Norris K, Martins D. The prevalence of hypovitaminosis D among US adults: data from the NHANES III. Ethn Dis. Autumn 2005;15(4 Suppl 5):S5-97-101.

16.  National Institutes of Health: Office of Dietary Supplements. Dietary Supplement Fact Sheet: Vitamin D. http://ods.od.nih.gov/factsheets/vitamind.asp, 2008.

17.  Institute of Medicine. Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride; 1997: http://www.nap.edu/catalog.php?record_id=5776. Accessed.

18.  Wagner CL, Greer FR. Prevention of Rickets and Vitamin D Deficiency in Infants, Children, and Adolescents. http://www.aap.org/new/VitaminDreport.pdf.

19.  Canadian Cancer Society. Canadian Cancer Society Announces Vitamin D Recommendation. http://www.cancer.ca/Canada-wide/About%20us/Media%20centre/CW-Media%20releases/CW-2007/Canadian%20Cancer%20Society%20Announces%20Vitamin%20D%20Recommendation.aspx?sc_lang=en.