What is Vitamin D Good For?

Many studies that track healthy people for years have reached the same con­clusion: “People with higher levels of vitamin D have a lower risk of developing type 2 diabetes in the future,” says Tufts Medical Center’s Anastassios Pittas, who is also a professor of medicine at Tufts University’s Sackler School of Graduate Biomedical Sciences.

But those studies can’t tell if something else about people with low vitamin D increases their risk. So researchers have looked further.

“We and others have done short ­term intervention studies looking at insulin sensitivity, insulin secretion, or similar out­comes,” says Pittas. “Some have not shown any difference, but others show promise.”


For example, Pittas and his colleagues gave 92 overweight or obese adults with prediabetes vita­min D (2,000 IU a day), calcium (400 mg twice a day), both, or a placebo for four months. Those taking vitamin D (either alone or with calcium) had better pancreatic beta­ cell function than those who didn’t take vita­min D.

“Beta­ cell function is one of the best measures we have for predicting the future risk of getting type 2 diabetes,” notes Pittas.

How might vitamin D work?

“There’s preliminary evidence that it may improve insulin secretion,” says Pittas.

“The enzyme that converts vitamin D to its active form is expressed in the pancreas’ beta cells, which means that the beta cell needs an adequate vitamin D supply to function. Only a few organs outside of the kidney have the ability to activate vitamin D so they don’t have to depend on the circulat­ing levels of the active vitamin.”

Many trials have revolved around the question, “What is Vitamin D good for?”

Vitamin D and your skeleton

Bess Dawson-Hughes, director of the Bone Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston, Massachusetts, and a professor of medicine at Tufts, tells us that vitamin D is essential for absorbing calcium. People with low vitamin D blood levels have more rapid bone loss and more fractures.

“Many have found that taking both vitamin D and calcium lowers the risk of fractures. It’s hard to tell if vitamin D alone would lower the risk, because trials that used higher doses of vitamin D also used calcium. You can’t tell if it’s the combination or the higher dose of vitamin D that did it,” says Dawson-Hughes.

Vitamin D lower the risk of falls

“We know that vitamin D lowers the risk of falling,” says Dawson-Hughes. “Falls are almost always the precipitating cause of hip fractures. So lowering the risk of falls is hugely important.”

Vitamin D helps build muscle. “We did one study on healthy but frail older women with low starting vitamin D levels. We gave them vitamin D or a placebo for four months. We saw that the women who got vitamin D had larger cross-sectional muscle area than the placebo takers, suggesting that vitamin D increased their muscle mass. We and others have also found vitamin D receptors in muscle biopsy studies. That also suggests that vitamin D matters for muscles,” says Dawson-Hughes.

How to get more vitamin D

Dawson-Hughes says, “There’s not much vitamin D in our diet. It’s in foods that people don’t tend to eat daily, like salmon.” It’s also in other fatty fish, and from fortified foods like milk, breakfast cereals, and some brands of yogurt, margarine, and orange juice.

“So supplements are pretty much the way to go,” she says. “And I would take them with food, not on an empty stomach. Also, if you take vitamin D with a meal that has some fat in it, your absorption is greater than if the meal is fat-free.”

What to do

Shoot for the Recommended Dietary Allowance for vitamin D (600 IU a day for adults up to age 70 and 800 IU a day for people over 70).

Sources: Am. J. Clin. Nutr. 94: 486, 2011.. doi:10. J. Clin. Endocrin. Metab. 98: E1927, 2013.

10 Replies to “What is Vitamin D Good For?”

  1. I’m always wary when a health professional recommends a supplement. While vitamin D may not be as widespread as some nutrients, the idea that we need a pill to fulfill our needs is counterintuitive.

    I also wonder if the results of these studies were impacted by baseline vitamin D status. For instance, we know that obesity negatively impacts vitamin D status and thus that group is likely to be deficient to start with. But it’s very possible only those who are already deficient or low will see any benefit from taking a supplement and those who aren’t may actually create an imbalance.

    Given the supplement craze, as health advocates, I think we should be encouraging people to first determine if they are even deficient. Then we should suggest dietary strategies (which may include losing weight among other things). Then in some cases we can recommend supplements and monitor. Interestingly, many of the foods high in vitamin D are also high in omega 3 fatty acids, another nutrient promoted and consumed in supplement form.

    Simply supplementing does also not help to displace other unhealthy food choices which may be affecting our overall health and nutrient status.

    This article also does not mention a number of studies/meta-analysis that don’t support all the proposed benefits of vitamin D. In that sense, I think it is a little unbalanced. It seems we are learning more and more that the supplement form of nutrients misses the mark and is a back-up option. We are also learning that too much of a good thing, even just a little too much can have negative health consequences over the long haul.

    1. That may certainly be, but the study is simple in its design and in its recommendation which is valuable in itself. Losing weight, while it can be good as a general idea, adds a lot more confounding variables into the mix. A person can be eliminating both good and bad fats. They can not be complying with the diet. In some cases, such as elderly frail woman, weight loss may not even be indicated. There’s even some budgetary issues. We are in an economy where a daily Vitamin D pill is cheaper than eating salmon every day. Supplements are not a silver bullet but neither should they dismissed. The biggest issue I have with supplements is that there is not enough regulation to insure that what the bottle saying you are getting is in fact what you are getting.

  2. shaunb:
    While I agree with you that most people should be tested for Vitamin D deficiency, your comments reduce some interesting questions to the usual “eat better food instead of supplementing” argument. It really is more complicated and more interesting than that.

    (My personal experience is that Vitamin D testing only occurs when I ask for it rather than as routine wellness check. Lack of data on the scope of the deficiency maybe the root of that problem.)

    Your comments underscore the huge number of “we really don’t know” items related to human health and biochemistry. (Remember, fat used be just an energy storage system.) Some physicians refer to Vitamin D as a hormone because of the many complex metabolic functions that it serves. In addition, no one has found an upper limit that produces toxicity from Vitamin D. While that doesn’t make overdose impossible, it may indicate that there is a high (even amazingly high) upper limit.

    Your limited focus on food versus supplements misses an important distinction for Vitamin D. The human body can make sufficient amounts of Vitamin D when skin is exposed to sunlight. This process works much better at lower latitudes, in the summer and during peak sun hours. Unfortunately, many people live like vampires. We use sunscreen to reduce our risk of skin cancer, and work indoors during most daylight hours. For my part, I’ll happily keep my sunscreen and take a supplement.

    Another question we be should be asking is: Is there enough fatty fish available in all food markets and at an affordable price to provide enough Vitamin D and omega-3 fatty acids to keep the population healthy without supplements? Getting enough of any nutrient in food is much more complicated than just “personal choices.” In some cases, supplements may be the cost effective, if less than perfect, solution.

    Finally, using a supplement as a therapy to correct an imbalance from whatever source is a valid course of action for an acute condition. It simply doesn’t make sense to avoid that correction while someone adjusts eating habits or body mass.

    I hope to see more clarification from future research on Vitamin D, and I suspect that it will follow a path much like fat. There will be much more to it than any facile explanation provides..

  3. The health benefits of vitamin D encompass reduced risk of many types of cancer, cardiovascular disease, diabetes mellitus, infectious diseases, Alzheimer’s disease, Parkinson’s disease, as well as muscle and bone condition. The evidence comes from ecological studies of ultraviolet-B dose, observational studies of vitamin D levels, randomized controlled trials, and laboratory studies of mechanisms. Unfortunately, many randomized controlled trials were not properly designed, so missed finding beneficial effects. The interested reader should search pubmed.gov, vitamindcouncil.org, grassrootshealth.net, vitamindsociety.org, and vitamindwiki.com for information on UVB and vitamin D.

  4. I was wondering how much sun is needed each day to get the daily requirements. Even in winter to get outside is not impossible, does it have to be in direct sun? What are the effects of over dosing?

    1. Carol – you only need 10-15 minutes a day to optimize your vitamin D synthesis but it needs to be when the sun is fairly high (a rule of thumb is that your shadow should be shorter than you). That limits it to a few hours around sun-noon during spring and fall with longer times during the summer. Both UVA and UVB cause sunburn but only UVB will create vitamin D and being longer wavelength is filtered out by the atmosphere at lower angles. See my blog posts for a more detailed answer.

  5. Although the text here only refers to “Vitamin D,” it is important to note that there are two kinds of the Vitamin and there is a BIG difference between the two. Vitamin D3 or cholecalciferol is the form produced naturally by the body via the sun and when we eat certain foods. There is also Vitamin D2 or ergocalciferol which is synthetic and does not have nearly the same benefits of the natural form nor is it as efficient in keeping levels of VD in the body. These two forms were used and prescribed interchangeably decades ago because both address the deficiency that caused rickets. But all the long-term health benefits discussed here are NOT attributed to cholecalciferol (vD2). In fact, I found this in google:
    “According to the latest research, D3 is approximately 87 percent more potent in raising and maintaining vitamin D concentrations and produces 2- to 3-fold greater storage of vitamin D than does D2. Regardless of which form you use, your body must convert it into a more active form, and vitamin D3 is converted 500 percent faster than vitamin D2. Vitamin D2 also has a shorter shelf life, and its metabolites bind poorly with proteins, further hampering its effectiveness.” “A recent meta-analysis by the Cochrane Database looked at mortality rates for people who supplemented their diets with D2 versus those who did so with D3,
    The analysis of 50 randomized controlled trials, which included a total of 94,000 participants, showed: A six percent relative risk reduction among those who used vitamin D3, but
    A two percent relative risk increase among those who used D2” found on Mercola.com

    So obviously it makes a big difference which form one takes as a supplement and so it should be clearly stated here.

    1. From Nutrition Action Healthletter: Most vitamin D in the diet is D3. Most of the vitamin D in supplements is D3. D2 is preferred by vegetarians. The Cochrane Review did not find any effect, positive or negative, of D2 on mortality.

  6. I’m 62 and for the past 5 years my vitamin D blood test #’s have been low. I swim 1 mile every other day (40″ of sunlight) and take a vitamin D supplement (5,000 IU) every day. This has been the drill for the past 5 years and the vitamin D blood test #’s have not increased.

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