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Medically reviewed by Dr. C.H. Weaver M.D. Medical Editor 1/2022

Vitamin D is a fat-soluble vitamin that comes from dietary supplements, foods such as fortified milk and cereal, certain kinds of fish (including salmon, mackerel, and tuna), and exposure to sunlight. Sunshine is a main source of vitamin D and as winter approaches your vitamin D level is likely to be lower.

The main function of vitamin D is to regulate calcium balance in your body. Vitamin D regulates how well calcium is absorbed from the intestine. With adequate vitamin D, you absorb about 35% of the calcium that you take in from foods, drinks, and supplements. If your vitamin D level is low, the efficiency of calcium absorption drops to 10 to 15 percent. When not enough calcium is consumed, bone releases calcium to keep everything running and this can result in the gradual development of osteoporosis.

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Benefits beyond Bone Health

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 from cancer, diabetes, autoimmune diseases, and infectious respiratory diseases. Not surprisingly, scientists and the public have started to take note, particularly of vitamin D’s potential to protect against cancer.

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 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 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 evaluating whether 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 dying from advanced colon cancer—the third most common cancer among both men and women in the United States. A number of 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 vitamin D levels had an almost 75% lower risk of dying from the disease compared with those with low to moderates levels.11

Women with early breast cancer who have low blood levels of vitamin D also have a worse outcome than those with adequate levels of vitamin D. Researchers from Toronto, Canada, conducted a clinical study that included 512 women who were diagnosed with early breast cancer between 1989 and 1995. Women with low levels of vitamin D had a 94% increased risk of cancer and a 73% increased risk of death than those with adequate vitamin D levels.21

Does the Type of Study Matter?

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.

What do Randomized Trials Say About Vitamin D and Cancer? 

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). A second trial, 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.14

The pivotal study designed to answer this question, Vitamin D and Omega-3 Trial (VITAL), which concluded in 2018 however did not find that vitamin D or omega 3s reduced the risk of developing cancer. Additional analyses of the study however reported that vitamin D was associated with a 38% risk reduction of developing advanced cancer in individuals with a normal body mass index.

VITAL was designed to test the independent effects of vitamin D and omega-3 supplements as well as to test for synergy between the two. The VITAL study was a rigorous, placebo-controlled study that took place over a span of more than five years in men who were 50 or older and women 55 or older who did not have cancer when the trial began.

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Study participants were received one of four treatments.

  • Vitamin D (2000 IU/day) plus omega-3
  • Vitamin D plus placebo
  • Omega-3s plus placebo
  • Placebos for both.

The initial analyses of the VITAL study did not find a statistical difference in cancer rates suggesting that neither vitamin D or omega 3s at the evaluated dose can prevent cancer. Researchers however did observe a reduction in cancer-related deaths.

Of the 13,000 participants who received vitamin D, 226 were diagnosed with advanced cancer compared to 274 who did not receive vitamin D. Of the 7,843 participants with a normal body mass index (non-obese) taking vitamin D, only 58 were diagnosed with advanced cancer compared with 96 taking the placebo suggesting vitamin D may help prevent advanced cancers in individuals who are not overweight.

There is previous evidence that body mass may affect vitamin D action. Randomized trials of vitamin D in type 2 diabetes have found greater benefits of vitamin D in people with normal weights and no benefit among those with obesity.

While the VITAL study did not confirm the benefits of vitamin D or omega 3’s cancer prevention benefits in all patients, non-obese patients may benefit from vitamin D supplementation. Unfortunately, additional studies will be required to confirm a benefit in non-obese individuals.22

SUNSHINE: Vitamin D Slows Colon Cancer Progression

Doctors from Dana Farber report that Vitamin D slows colon cancer progression. The first-ever completed comparative clinical trial of the use of vitamin D as a colorectal cancer therapy has shown that disease progression is slowed with high-dose supplements according to lead author Kimmie Ng, MD, of the Dana Farber Cancer Institute in Boston, Massachusetts, who presented the study results at the American Society of Clinical Oncology (ASCO) 2017 Annual Meeting.24

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The SUNSHINE clinical trial was conducted in patients with previously untreated metastatic colorectal cancer. All participants received standard treatment with the FOLFOX chemotherapy regimen (folinic acid, fluorouracil, and oxaliplatin) plus Avastin (bevacizumab) and either a high or low dose of supplemental vitamin D.Patients in the high-dose group received a loading dose of 8000 IU/day of vitamin D3 orally for 2 weeks followed by 4000 IU/day. Those in the low-dose group received a standard vitamin D3 dose of 400 IU/day. The higher dose of vitamin D supplementation was found to significantly improve progression-free survival (PFS) by about 2 months compared to the lower dose.

The Final Word

How Much Vitamin D Do You Need?

How much vitamin D you need is widely debated with different groups providing different recommendations. In light of growing evidence of widespread vitamin D deficiency, the International Osteoporosis Foundation (IOF) has released a new position statement with increased vitamin D recommendations for older men and women. The statement was published in Osteoporosis International.23

Based on a growing body of evidence, the IOF has made the following recommendations:

  • The estimated average vitamin D requirement for older adults to reach appropriate blood levels of the nutrient is 800 to 1,000 international units (IU) per day.
  • Individuals who are obese, have osteoporosis, have limited sun exposure, or suffer from malabsorption may need to increase their intake to 2,000 IU/day.
  • High-risk individuals are encouraged to undergo testing to measure blood levels of vitamin D and then supplement accordingly.

Because there is such a widespread deficiency of vitamin D, the IOF hopes that the new recommendations will help to prevent falls and fractures in the older population.

People who have questions about the level of vitamin D that’s right for them are advised to talk with their physician.

Food Sources of Vitamin D

While you might assume that food would be the ideal source of vitamin D, the reality is that few foods naturally contain vitamin D. The main natural food sources of vitamin D are oily fish, like salmon, mackerel, and sardines. Egg yolks contain only small amounts of vitamin D. In the United States, all milk is fortified with vitamin D. An 8-ounce glass of milk contains 100 IU. Other fortified foods include cereals and some brands of orange juice and yogurt.

Can You Take Too Much Vitamin D?

Vitamin D is a fat-soluble vitamin that is stored in fat. Vitamin D taken at recommended doses is generally not of a concern. Classic toxicity with high blood levels of calcium and kidney and liver damage occurs with blood levels in the 200 to 400 ng/mL range as a result of high vitamin D intake. In general, daily intake up to 10,000 IU is thought to be safe. You should consult your healthcare provider to individualize the right dose for you.

Should You Be Screened for Vitamin D?

Most organizations do not recommend universal screening for assessing vitamin D blood levels. The US Preventive Services Task Force concluded this year that the benefits as well as any potential harm from vitamin D screening and early interventions cannot be determined. Individuals at high risk of low vitamin D, however, such as those with obesity, osteoporosis, celiac disease, or Crohn’s disease, are advised to have vitamin D levels checked. Winter and early spring, when sun exposure is lowest, is the best time of year to check. The best indicator of vitamin D levels is the serum 25-hydroxyvitamin D (25OHD), which is determined by sun exposure and vitamin D intake.

People who have questions about the level of vitamin D that’s right for them are advised to talk with their physician.

​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.

Sources of Vitamin D

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).16-20

Sources of Vitamin D 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) Supplements (depending on formulation) 400–1,000

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.


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