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New research strongly suggests that people with too little vitamin D in their systems are more likely to get colorectal cancer (McCullough et al, Journal of the National Cancer Institute, June 14, 2018). The researchers compared vitamin D blood levels of 5,700 colorectal cancer patients and 7,100 healthy volunteers. People deficient in vitamin D were 31 percent more vulnerable to colorectal cancer. Unfortunately, the study did not establish whether vitamin D pills can reverse this risk.

Should You Take Vitamin D Pills to Compensate for Sunscreen?

Q. My dermatologist says I should use sunscreen every time I step outside, even if I’m just going to the grocery store. When I ask about vitamin D, she says I should take a supplement.

I have read that vitamin D can help protect against cancer. Will the supplement protect me?

A. Although sun exposure can increase the risk of skin cancer, it also raises levels of vitamin D. This in turn reduces the risk of many other cancers (Anticancer Agents in Medicinal Chemistry, Jan. 2013). As a result, patients who have had skin cancer face a dilemma: should they rely on vitamin D pills or risk limited sun exposure? There is no easy answer.

Do Vitamin D Pills Prevent Cancer?

Vitamin D deficiency increases the risk for certain cancers, but a review of studies on vitamin D supplements for preventing cancer was inconclusive (Cochrane Database of Systematic Reviews, June 23, 2014). Scientists have had difficulty establishing a standard dose for research purposes.

We are sending you our Guide to Vitamin D Deficiency, which describes how to find out about optimal vitamin D levels. It also discusses the pros and cons of sunshine and vitamin D pills to get the vitamin you need.

How Much Vitamin D Should You Get from Pills?

The US RDA for vitamin D is 600 IU for peopler up to age 70 and 800 IU for those over 70. It is not clear, however, if vitamin D pills at this dose will bring circulating vitamin D (25-hydroxyvitamin D) into the range that is often considered optimal, from 50 to 100 nmol/L.

Just a few years ago, British health authorities suggested that all Britons take vitamin D. Public health experts recognized that in Great Britain people don’t always get enough sunshine to make adequate vitamin D. So they recommended a supplement of 400 International Units per day for everyone. Here in the US, the Institute of Medicine suggests a daily supplement of 600 International Units for most people.

Is a 600 IU Pill Enough Vitamin D?

Some experts believe that dose is too low. They argue that people out in the sun all day would make 5,000 to 10,000 IU daily through exposed skin. They think that 5,000 IU would be a more appropriate level for vitamin D pills unless people can get their vitamin D the old fashioned way by going out in the sun.

Bottom Line from The People’s Pharmacy

We are not at all convinced that 600 IU is an adequate intake of vitamin D. We await large, well-controlled trials that compare vitamin D3 pills to a placebo. Ideally, these would track a number of health conditions for a long period of time. Such a study is underway under the supervision of Harvard public health experts. Until the research is completed, we will not know the benefits or risks of supplementation.

Low levels of vitamin D circulating in the body are clearly linked to a number of serious health problems including cancer. You can learn more about measuring vitamin D levels and optimal target levels from our Guide to Vitamin D Deficiency. You may also be interested in our Show 945: Vitamin Pills and Aspirin for Prevention–A Smart Move or a Waste of Money?

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  1. Kristin

    Some people (I am one) are naturally VERY low in vitamin D. For years my bones weakened as a result, and my doctor only prescribed Fosamax which gave me very serious side effects (locking jaw) so I stopped. I changed doctors, discovered my D deficiency, and was instructed by the new doctor to take 10,000 iu’s of vitamin D3 every day. Since then my bone density has actually increased slightly each time it is tested. My vitamin D level has increased from 19 to about 65. Unfortunately, I cannot spend much time in the sun because I had a melanoma removed several years ago, so I must rely on supplements to get sufficient D.

  2. Jean

    I have read that too much Vitamin D contributes to too much calcium in the blood? That article suggested limiting Vitamin D to 400 IUD. What is your take on this?

  3. Marciab
    Houston, Tx

    There is so much information out there you dont know what to believe. I take 6-8,000 i.u. a day. When I was tested as part of a normal workup, I was right in the middle so I guess what I take is ok. I do live in Tx so I think I get enough sun. I do wear sunscreen all the time though. The daily recommended is simply not enough. If I took that my labs would not have been in the middle.

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