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Why Don’t Vitamin D Supplements Work as Expected?

Vitamin D supplements are extremely popular. Millions swallow little golden pills daily. But why are clinical trials producing such disappointing results?

If you are taking big doses of vitamin D in the hopes that it will ward off all sorts of health problems, you may be disappointed. Most research has not demonstrated that taking high doses of vitamin D supplements will improve health. Why not?

How Weight Affects Response to Vitamin D Supplements:

In a large randomized controlled trial of vitamin D supplements, researchers found that heavier participants did not benefit as much as others (JAMA Network Open, Jan. 17, 2023). The VITAL study included more than 16,000 adults who took vitamin D or placebo for two years. Those who were overweight or obese had lower levels of circulating vitamin D when the study started. More importantly, their levels of vitamin D markers did not rise as much when they took the pills.

The researchers suggest that this may help explain why heavy people don’t seem to get protection from vitamin D supplements. Other participants had a lower risk of cancer or diabetes if they took vitamin D.

When normal-weight volunteers took pills providing 2000 IU of vitamin D daily, their vitamin D levels rose significantly. The investigators note that people carrying excess fat do not increase vitamin D blood levels as much after taking supplements.

One possible explanation is that vitamin D is a fat-soluble compound. As a result, the body’s adipose tissue stores it. Consequently, it is less available to other body tissues. As you will read, it plays a critical role in helping them stay healthy.

The Double Life of Vitamin D:

Vitamin D leads a double life. In addition to being a crucial vitamin, it is also a hormone that affects every cell in the body.

For years, health professionals thought that this nutrient was responsible primarily for building strong bones. When there was a deficiency in vitamin D, children developed rickets. In this condition, bones are fragile and can become deformed, resulting in bowed legs and thickened wrists.

Over the last several decades, scientists have discovered that there are vitamin D receptors throughout the body. People who are deficient in vitamin D are more susceptible to a wide range of disorders, from arthritis and heart disease to diabetes and cancer.

Researchers report that when circulating levels of vitamin D are low, people are more susceptible to respiratory infections and more likely to die prematurely (Circulation Journal, April 20, 2017). That research from Japan has since been confirmed in the UK, Norway and elsewhere.

Health Problems Linked to Low Vitamin D Levels:

  • Asthma in children
  • Autoimmune disorders such as multiple sclerosis
  • Cancer
  • Cardiovascular disease (heart attacks, etc)
  • Cognitive impairment (Alzheimer’s disease, dementia)
  • Diabetes
  • Erectile dysfunction
  • Fractures, osteopenia, osteoporosis
  • High blood pressure
  • Infections (pneumonia, colds, etc)
  • Mental illness (depression, schizophrenia)
  • Psoriatic arthritis

Vitamin D Supplements: A Puzzling Paradox

Scientists don’t understand why vitamin D supplements don’t reliably prevent these serious health conditions. Why aren’t vitamin D supplements working as expected?

Vitamin D and the Sun:

For thousands of years, humans got their vitamin D by exposing their skin to the sun. When ultraviolet rays hit the skin, they help form a precursor of the hormone (cholecalciferol). After the skin absorbs this compound, the kidney converts it into the active form of vitamin D.

There is tremendous variability in the amount of vitamin D the skin will make. It depends in part on the intensity of the sun, the time of day and skin type. Your body can make between 10,000 and 25,000 international units of vitamin D in less time than it takes to develop a sunburn. To a large extent, sunscreen may block the natural formation of vitamin D from the sun (PLoS One, Jan. 29, 2016).

The body carefully regulates its production of vitamin D. You can’t overdose on vitamin D through sun exposure (Institute of Medicine, 2011). Moreover, not all the evidence points to sunscreen blocking vitamin D synthesis. One study found that people applying enough sunscreen to prevent sunburn still made vitamin D (British Journal of Dermatology, Nov. 2019).

New technology makes it possible for people to monitor their UV exposure. A pilot study in Brazil found that people using the Sun4Health phone app with a wearable UV detector mostly avoided sunburn (Photochemical & Photobiological Sciences, Nov. 2022). Nevertheless, they increased their blood levels of 25-hydroxyvitamin D.

Should You Take Vitamin D2 or Vitamin D3?

Q. Can you tell me about vitamin D? I am confused about the difference between vitamin D2 and vitamin D3.

Here in the Pacific Northwest, I don’t get a lot of sunlight on my skin. I have been wondering if I should take a supplement, but how much and what kind is a puzzle. Can you overdose?

A. The good questions you have asked are not as simple as they seem. Our skin does make vitamin D when exposed to sunlight. In your area, you probably would benefit from taking vitamin D during the winter, when you don’t get enough sunlight on your skin. Once you can spend 20 or 30 minutes a day in sunshine, you might not need the supplement any longer.

Vitamin D2 vs. Vitamin D3:

Vitamin D2 is derived from plant sources like mushrooms. Vitamin D3, on the other hand, comes from animal sources like wool. Although D3 is considered more potent, both forms will raise vitamin D blood levels (measured as 25-hydroxyvitamin D).

The optimum dose depends on weight and skin pigmentation as well as geographical location. For those details, we suggest you consult our eGuide to Vitamin D and Optimal Health, found under the Health eGuides tab.

The RDA of vitamin D for people between 14 and 70 years of age is 600 IU (15 mcg). Those over 70 should get at least 800 IU daily. Some people need more to maintain appropriate blood levels. Blood tests are the best way to determine this.

It is possible to get too much, though. Adults taking more than 4,000 IU a day for an extended time risk vitamin D toxicity. That could result in elevated blood levels of calcium, nausea, vomiting, kidney stones, loss of appetite and muscle weakness, among other problems.

Vitamin D Supplements and Disappointing Results:

Researchers have certainly noticed health problems associated with low levels of vitamin D. They have conducted many experiments to see how well supplements of vitamin D can treat the disorders connected to vitamin D deficiency.

A study in the European Heart Journal, May 12, 2017 reported that vitamin D supplements “did not reduce mortality in patients with advanced HF [heart failure].”

Research published in BMJ Open (online, Oct. 23, 2013) found that vitamin D supplements did not help patients with a severe autoimmune condition called sarcoidosis.

High use of vitamin D supplements did not lower the risk of early menopause in Nurses’ Health Study II. This was published in the American Journal of Clinical Nutrition (May 10, 2017). When these women got their vitamin D from food, however, there was a modest reduction in early symptoms of menopause.

High-dose vitamin D supplements were not beneficial for overweight, vitamin D-deficient patients who were diagnosed with type 2 diabetes (American Journal of Clinical Nutrition, May 10, 2017).

The authors concluded that it would be unlikely that vitamin D supplements

“would be an effective strategy for reducing diabetes risk even in vitamin D-deficient populations.”

More Research:

Other studies have also yielded disappointing results. A recent example is a study published in The Lancet Diabetes & Endocrinology (April 28, 2017).

The investigators recruited more than 5,000 volunteers in Auckland, New Zealand, for a placebo-controlled trial. Half of them were given cholecalciferol (vitamin D3) at a dose of 100,000 IU once a month. The other half got a placebo once a month. All of the participants kept track of any falls or fractures they experienced during the three-plus years the study continued.

The investigators didn’t find any differences between the two groups, whether in falls, fractures or death. They concluded that this form of supplementation was ineffective.

To give them credit, however, they noted:

“Further research is needed to ascertain the effects of daily vitamin D dosing, with or without calcium.”

Daily Dosing: How Important?

We too would like to see more studies with daily vitamin D dosing. We suspect that one reason so many studies that use enormous doses once a month or even once every few months haven’t shown benefit is that the scientists are taking the wrong view of this vitamin.

They are treating it as if it were a drug. But a hormone that is intimately involved in so many processes in the body might not behave like a drug at all. The body may not respond normally to such a high dose. In nature, the body gets a smaller amount every day. And the body regulates that natural vitamin D quite effectively. It may also be that the vitamin D (and its derivatives) made by the body from sun exposure work differently in the body than the oral vitamin D supplements so many people take.

A recent study found that people taking 2,000 IU of vitamin D daily had better bone mineral density in the hip region after three years (Journal of Bone and Mineral Research, April 13, 2024). Unfortunately, the abstract doesn’t specify if the volunteers took D2 or D3 supplements.

Learn More:

To learn more about this crucial hormone, you may wish to read our Guide to Vitamin D Deficiency. It can be downloaded from our website.

Share your own experience with vitamin D supplements in the comment section below. If you found this information of interest, please vote at the top of the article.

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About the Author
Joe Graedon is a pharmacologist who has dedicated his career to making drug information understandable to consumers. His best-selling book, The People’s Pharmacy, was published in 1976 and led to a syndicated newspaper column, syndicated public radio show and web site. In 2006, Long Island University awarded him an honorary doctorate as “one of the country's leading drug experts for the consumer.”.
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  • Tobias DK et al, "Association of body weight with response to vitamin D supplementation and metabolism." JAMA Network Open, Jan. 17, 2023. doi:10.1001/jamanetworkopen.2022.50681
  • Umehara K et al, "Association between serum vitamin D and all-cause and cause-specific death in a general Japanese population - The Hisayama Study." Circulation Journal, April 20, 2017. DOI: 10.1253/circj.CJ-16-0954
  • Kockott D et al, "New approach to develop optimized sunscreens that enable cutaneous vitamin D formation with minimal erythema risk." PLoS One, Jan. 29, 2016. doi: 10.1371/journal.pone.0145509
  • Young AR et al, "Optimal sunscreen use, during a sun holiday with a very high ultraviolet index, allows vitamin D synthesis without sunburn." British Journal of Dermatology, Nov. 2019. DOI: 10.1111/bjd.17888
  • Young AR et al, "Innovative digital solution supporting sun protection and vitamin D synthesis by using satellite-based monitoring of solar radiation." Photochemical & Photobiological Sciences, Nov. 2022. DOI: 10.1007/s43630-022-00263-7
  • Kistler-Fischbacher M et al, "Effects of vitamin D3, omega-3 s and a simple strength training exercise program on bone health: the DO-HEALTH randomized controlled trial." Journal of Bone and Mineral Research, April 13, 2024. DOI: 10.1093/jbmr/zjae054
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