Vitamin D supplements — evidence about safety and effectiveness


The claims about the usefulness of vitamin D supplements are all over the internet. And they seem trendy, especially with COVID-19, even though much of the evidence is extraordinarily weak.

The purpose of this article is to review some of the old and new information about vitamin D supplements. It’s about scientific articles that either support or refute a claim, that’s it. It’s not about what we believe or we do not believe, it’s about evidence.

I don’t think vitamin D is worthless. It is an important micronutrient for human health, and if someone has a chronic deficiency, supplementation may be medically necessary.

On the other hand, proponents of megadoses of vitamins, called megavitamin therapy or orthomolecular medicine (pseudoscientific terms to sound like they are based on real science), seem to work on the unscientific belief that if a little dose helps, a whole boatload dose will help a lot more. Most of these ideas have been debunked and are considered quackery and fads.

I wanted to take a look at the science of vitamin D supplements while examining their actual benefits to health compared to the possible dangers of excess supplementation.

vitamin d supplements
Chemical structure of cholecalciferol. By SbroolsOwn work, CC BY-SA 3.0, Link

What is vitamin D

Vitamin D is a group of fat-soluble secosteroids (it’s a scientific name for steroids with a “broken” ring). The most important chemicals in this group are vitamin D3 (known as cholecalciferol) and D2 (known as ergocalciferol).

Very few foods contain either of the important types of vitamin D. However, there are some foods that can be good sources for the vitamin:

  • Fatty fish, like tuna, mackerel, and salmon
  • Foods fortified with vitamin D, like some dairy products, orange juice, soy milk, and cereals
  • Beef liver
  • Cheese
  • Egg yolks

Many people with broad diets that include a lot of fish, eggs, and other foods can get sufficient vitamin D without supplementation. As I’ve repeated often, short of chronic malnutrition, we get plenty of vitamin D.

Moreover, vitamin D is produced by a process called dermal synthesis. That is, sunlight, specifically UV-B radiation, causes the synthesis of vitamin D in the skin. Technically, vitamin D isn’t a vitamin, because we can manufacture it, it is really a hormone. For this article, we’ll just call it a vitamin, even though scientifically it is not — I hope that makes some sense.

Although we can manufacture sufficient vitamin D by sunbathing every day, the body has a feedback loop that shuts down production to prevent toxicity. Yes, excess vitamin D is quite dangerous leading to many conditions such as over absorption of calcium to hypertension to fatigue. But it also can lead to some dangerous chronic conditions that we’ll discuss later.

Although humans can manufacture vitamin D by sitting in bright sun, there’s one major problem – the risk of skin cancer. As I’ve written before, there are very few ways to actually prevent cancer, but staying out of the sun is one of them.

Finally, the vitamin D we consume or produce in sunlight is not biologically active. It is generally activated by enzymatic conversion (in a process called hydroxylation) in the kidneys and liver so that the body can use it.

Photo by Leohoho on Unsplash

What does vitamin D do or don’t do?

Based on real scientific evidence, vitamin D has a very narrow, but important, set of effects.

First, it is responsible for enhancing intestinal absorption of calcium, iron, magnesium, phosphate, and zinc, important minerals for the continued health of any human being. Many of these minerals (and vitamin D itself) are necessary for good bone health.

Second, since vitamin D is important to calcium homeostasis and metabolism, a deficiency of the vitamin results in rickets, or the adult form of the disease, osteomalacia. Rickets, because it happens in immature bones, leads to frequent fractures and skeletal deformities. Osteomalacia, because it occurs in adults with fully formed bones, usually results in numerous fractures.

At this point, that’s it. Robust evidence only supports those two physiological effects.

Here are a few claims made by the supplement-pushing crowd:

Smoked salmon and eggs benedict on a bagel. Perfect vitamin D breakfast without supplements. Photo by John Baker on Unsplash

New research

Over the past few months, several new articles have been published that reviewed vitamin D supplements and various health conditions. Let’s take a look.

  • A June 2019 meta-analysis of 21 randomized clinical trials (n=83,291 participants) showed that vitamin D supplements offer no benefit for cardiovascular disease (CVD) prevention. These results represent fairly conclusive evidence that vitamin D is useless for the prevention of CVD.
  • In the accompanying editorial to the article above, the authors concluded that “The popularity of vitamin D supplementation is at least partly owing to the misinterpretation of impressive epidemiologic associations between vitamin D status and a breadth of health metrics, leading to a potentially flawed assumption of causality.” This goes to the old adage that correlation does not equal causation
  • An August 2019 randomized clinical trial showed that vitamin D supplements might not be much help for strengthening bones among healthy adults without osteoporosis, even at doses far higher than recommended daily allowances. Moreover, patients on the highest dose of vitamin D experienced the most instances of hypercalciuria and hypercalcemia, both conditions could become serious health issues.

Vitamin D and cancer

One of the big claims in some miraculous way, vitamin D can reduce the risk of cancer. There isn’t a lot of good data for a few reasons:

  • There are hundreds of cancers, each with a different etiology and pathophysiology — and there are thousands of different ways to treat them.
  • Cancer can take decades to appear, so correlation, let alone causation, may be difficult to establish when vitamin D levels need to be measured over many years.
  • Over 67% of cancers are random mutations. You could have perfect vitamin D levels, and you still have a significant risk from cancers that just happen.
  • Although there are some studies that show a small reduction in risk for some cancers by keeping a normal blood level of vitamin D, most studies show no effect. And there are certainly no studies that show taking huge doses of vitamin D prevent any cancer.

Vitamin D supplements don’t harm, right?

That’s the appropriate question. All evidence-based medicine should weigh the evidence that a particular treatment has a benefit against any potential harm.

Right now, we know that vitamin D does two things – helps the absorption of minerals and prevents calcium homeostasis which causes bone damage. Most people in developed countries get sufficient vitamin D from sunlight or foods.

And even if a person doesn’t, there are blood tests that can confirm a vitamin D deficiency, and an appropriate supplementation level should be prescribed. In other words, if an issue is discovered through a real diagnosis, then a healthcare plan can be implemented to prevent any consequences of low vitamin D.

But let’s say you refuse to accept my evidence that vitamin D is more or less useless, except for those very few who have low levels. You just believe in the charlatans who are pushing megadoses of vitamin D, even if you have not been diagnosed with a chronic deficiency. I would almost say, no harm, no foul.

Except for a small little issue. There is a lot of evidence that vitamin D supplementation not only has no benefit (except again in two physiological situations) but it might actually cause harm. Here are just some of the risks of vitamin D supplements:

And I barely touched the surface of the body of research that establishes the potential dangers of excess vitamin D. And unless you have had blood tests to determine the level of vitamin D in your blood, even a small supplement may push you over into the toxic range.

I want to reiterate that people may need vitamin D supplements to get their blood levels up and to benefit from it. But just because a small dose works well, it does not mean that over-supplementation is a good idea. It isn’t.

Vitamin D supplements
Photo by Kindel Media on Pexels.com

A summary of vitamin D supplements

Without any doubt, vitamin D is necessary for human health. It is critical for the absorption of some minerals and maintaining proper calcium homeostasis, necessary for bone health.

In developed countries, vitamin D insufficiency is relatively rare. We either produce sufficient amounts of the vitamin while exposed to sunlight (not a really safe way to do it)  or through a varied diet. But if one has a low level of vitamin D, it is very easy to diagnose and treat through a properly measured dose of the vitamin.

There is very little evidence that vitamin D supplements have any effect on any other medical condition. It doesn’t prevent cancers or cardiovascular disease. It does not reduce your risk of death. It doesn’t do much of anything, except in the two, very narrow, physiological aspects.

And no, if a little vitamin D helps maintain some aspects of your health, a lot will not make it better. It might make it worse.

Key citations

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