Vitamin D: Effects, Benefits, and Safety

Vitamin D is a fat-soluble and essential vitamin your body needs to be able to absorb calcium and keep your bones healthy. You get vitamin D from sunlight, food, or from supplements. Only a handful of foods provide vitamin D in sufficient amounts, and deficiency is fairly common. This makes vitamin D one of the few cases where a supplement may be necessary. 

In this article, we will take an in-depth look at vitamin D. What it is, what it does, how to ensure you get enough of it, and whether or not vitamin D supplements can help you get stronger and perform better.

What Is Vitamin D and How Do You Get It?

Way back in 1645, Dr Daniel Whistler described vitamin D deficiency for the first time, in an early documentation of rickets.1 He obviously didn’t know the cause of what he observed, but rickets is a skeletal disorder caused by a lack of vitamin D. Nowadays the condition is uncommon in the west, but still prevalent in Africa, the Middle East, and certain parts of Asia.

Technically, vitamin D is actually a fat soluble prohormone, not a vitamin. It was classified as a vitamin by accident during the early part of the 20th century. The proper definition of a vitamin is a substance your body requires in small amounts on a regular basis but that it can’t produce on it’s own, making it necessary to provide it through food. However, unlike other vitamins, your body can make vitamin D on its own, as long as you get enough sunlight. For nutritional and public health purposes, vitamin D is still classified as a vitamin.

There are several types of vitamin D, the most common ones being vitamin D2 and vitamin D3. Vitamin D2 is called ergocalciferol, and vitamin D3 cholecalciferol. Vitamin D2 is only found in plant-based sources. It is also used to fortify foods with vitamin D, since it is cheaper than vitamin D3. You can only get dietary vitamin D3 from animal-based foods. Vitamin D2 and Vitamin D3 do not have equal nutritional value – Vitamin D3 is significantly more effective at increasing your vitamin D levels.2 3

In addition, your skin makes vitamin D3 when it is exposed to sunlight. You might know vitamin D under the name “the sunlight vitamin” for this very reason. In the epidermis of your skin, you have a sterol called 7-dehydrocholesterol. 7-dehydrocholesterol is a precursor to cholesterol and vitamin D. It is converted into vitamin D3 when it absorbs UV radiation from sunlight.

After this conversion, the D3 is metabolized in several stages, first in the liver and then in the kidneys, into biologically active molecules. Since you can make vitamin D from sunlight, you don’t actually need to get it from food at all. If you get enough sun exposure, you’re fine. In some countries, spending an hour or two in the sun every week provides all the vitamin D you need. In many countries, where the sun isn’t as strong, and where we spend our days mostly indoors or covered with clothes, sunlight can’t satisfy our vitamin D requirements.

If you don’t get enough vitamin D from the sun, you have to get it from your diet or from supplements. Unfortunately, there aren’t many good dietary sources of vitamin D. Fatty fish like salmon, mackerel and herring are good sources. The single best dietary source of vitamin D is cod liver oil. You can also get vitamin D from egg yolks, liver, and cheese. Fruits, nuts, and other plants are quite poor sources of vitamin D. Vitamin D-rich foods are actually pretty rare. Because of this, many common foods, like milk, cereals, and butter, are fortified with vitamin D to help meet the recommended intake. In fact, most of the vitamin D in a western diet comes from fortified foods.4

You can also get your vitamin D from supplements. This is fortunate, since vitamin D deficiency is one of the most common nutritional deficiencies. In the United States, more than 40% of the general population is vitamin D deficient.5

Dark skin prevents the synthesis of vitamin D from sunlight. The vitamin D deficiency rate in black Americans is a whopping 82%. Because of this, supplementation might not only be useful, but necessary, in many cases.

Vitamin D is stored in the body. If you get a lot of vitamin D at certain times, you might need less or none at all at other times. 

Intake Reference Values for Vitamin D

The Food and Nutrition Board at the Institute of Medicine of The National Academies has provided intake reference values for vitamin D.6 These include:

  • Recommended Dietary Allowance (RDA), the average daily intake which meets the requirements of 98% of the healthy population.
  • Adequate Intake (AI), the intake assumed to meet the requirements of a population, when there isn’t enough evidence to develop a proper RDA. In the case of vitamin D, this population consists of infants up to the age of 12 months.
  • Estimated Average Requirement (EAR), the average daily intake estimated to meet the requirements of 50% of the healthy population.
  • Tolerable Upper Intake Level (UL), the amount you can consume on a daily basis without expecting any adverse side effects.

Recommended intakes for vitamin D are usually listed both in International Units (IU) and micrograms (mcg). An International Unit is a measurement of mass or volume which varies depending on what is being measured. The variance is based upon the biological acitivity of a substance.

In the case of vitamin D, 1 IU equals 0.025 mcg of vitamin D, either cholecalciferol or ergocalciferol. This means that the biological activity of 40 IU vitamin D equals 1 microgram.

Here is a list of the current RDA, AI, and EAR values:

Dietary Reference Intakes for Calcium and Vitamin D. Washington (DC): National Academies Press (US); 2011.

The U.S. National Academy of Medicine has set an Upper Limit value of 4,000 IU of vitamin D per day for adults. However, there are no associations between vitamin D doses of 10,000 IU or less and harmful effects. Very high doses of 50,000 IU or more per day for several weeks or months can lead to hypercalcemia. That is when the calcium levels in your blood get too high. This can cause kidney stones, hurt your bones, and even cause heart problems.

You can’t overdose on vitamin D from sunlight. If you get more than you need to satisfy your vitamin D needs, you just produce less, maintaining optimal levels automatically.

Should You Use a Vitamin D Supplement?

Maybe. This depends on where you live, the color of your skin, how much time you spend in the sun, and whether or not you can test your vitamin D levels.

If you know you aren’t vitamin D deficient, you likely don’t have anything to gain from supplementation.

If you have the opportunity, you might want to consider taking a 25 hydroxyvitamin D test, just to be sure. It shows the level of vitamin D in your blood from both sunlight and from your diet.

The test is simple, and only requires a regular blood sample. You probably can’t do it at home, though, unless you have a lab in the basement. The results will come back something like this:7

  • Deficient: below 30 nmol/L (12 ng/mL)
  • Inadequate: 30 nmol/L (12 ng/mL) to 50 nmol/L (20 ng/mL)
  • Adequate: 50 nmol/L (20 ng/mL) to 125 nmol/L (50 ng/mL)
  • High: 125 nmol/L (50 ng/mL) and above

If your level of 25-hydroxyvitamin D is below 50 nmol/L, then a supplement would be a good idea. If it is at 50 or above, but not too much above, a supplement won’t hurt. Actually, some research indicates that 75 nmol/L should be considered the minimal desirable level.8 Test results close to 125 nmol/L mean you don’t need to supplement. If your level are above 125 nmol/L, you might want to drop the supplement you probably already are taking. You don’t usually get levels that high without supplementation. You can’t get there by being exposed to the sun. Diets providing the amount of vitamin D needed to go above 125 nmol/L would be quite extreme.

The decision tree below should help you decide if vitamin D in supplement form is right for you.

Sports Med. 2018; 48(Suppl 1): 3–16. Vitamin D and the Athlete: Current Perspectives and New Challenges.

Vitamin D and Health

Everywhere you look, vitamin D health-related claims pop up. Is there any merit to these claims, or are they largely marketing schemes? Let’s take a look at the evidence.

The major documented health effect of vitamin D is how it helps calcium improve and maintain bone health. However, vitamin D is also involved in regulating your immune and neuromuscular systems. In addition, it also helps regulate insulin production, myocardial contractility, and the secretion of thyroid-stimulating hormone, among many other important functions.

With the exception of bone health, there isn’t enough evidence to establish a cause and effect relationship between vitamin D and various health measures. The associations between vitamin D and health are numerous, however.

Bone Health

Children need vitamin D to build strong bones, and as adults, we need vitamin D to keep them strong. Vitamin D, calcium, and phosphate are all involved in complex mechanisms that form and strengthen your bone mass through direct and indirect mechanisms.9

Calcium and phosphate can compensate for low levels of vitamin D, to some extent, and vitamin D can compensate for low intakes and levels of calcium and phosphate. But only to some extent. For optimal, or even adequate, bone health, all are involved and necessary in proper amounts.

In general, maintaining a 25-hydroxyvitamin D level around 75 nmol/L likely provides the best of both worlds. It’s good for your bones, and generally regarded as safe. This level can be reached through sunlight exposure alone, although that might not be viable for everyone and all regions. A combination of sunlight and vitamin D supplementation might be the optimal approach for most people.

High Intakes of Vitamin D

More than 3% of US adults take more than 4,000 IU per day of vitamin D, reaching or exceeding the Acceptable Daily Intake level. A large, double-blind, randomized clinical trial with 311 healthy subjects recently examined if supplementing with more than 400 IU of vitamin D per day can improve bone health.10 Roughly a third of the participants received daily doses of 400 IU vitamin D, another third received 4,000 IU, and the last third received 10,000 IU. This was a long-term study, lasting 3 years.

The results did not provide any support for high or very high intakes of vitamin D in supplement form for improved bone health.

This suggests that you don’t need massive doses of vitamin D in order to keep your bones strong. You just need to focus on avoiding deficiency and on having adequate levels.

Cancer

In 1980, scientists hypothesized that vitamin D could lower the risk of cancer, both the risk of developing cancer and the risk of dying from it.11 During the following years, a number of animal studies demonstrated that vitamin D has the ability to inhibit the growth of cancer cells. Epidemiological data supports the association between vitamin D and cancer risk, but meta-analyses and reviews have not been conclusive. Just during the last two years, a number of large-scale meta-analyses appeared.

During the first half of 2018, one systematic review analyzed 30 randomized clinical studies with a total of more than 18,000 participants.12 It investigated the effects of vitamin D supplementation on cancer incidence and mortality. In other words, the risk of developing cancer and the risk of dying from cancer.

The median follow-up period in the examined studies were 1 to 6.2 years, and the researchers also included after-study events. The results showed that neither baseline vitamin D levels or vitamin D supplementation and dose reduced the risk of developing cancer or the risk of dying from cancer.

During early 2019, another meta-analysis added more recent randomized controlled trials to the data.13 In these trials, the participants received higher doses of vitamin D. The researchers included 10 trials with 6,537 participants and a follow-up period of 3–10 years when looking at cancer incidence, and 5 trials with similar follow-up periods and 1,591 deaths for the mortality analysis.

This didn’t result in any new discoveries when it comes to cancer incidence. Vitamin D supplementation still failed to reduce the risk of developing cancer. However, vitamin D supplementation significantly reduced cancer mortality, the risk of dying from cancer-related causes once the disease has been diagnosed. This was especially apparent in trials with daily doses of vitamin D instead of larger but more infrequent doses.

The third of the recent meta-analyses, published in May 2019, investigated the effects of vitamin D supplementation for primary prevention of cancer.14 This meta-analysis looked at randomized controlled studies using vitamin D supplements for the prevention of any type of disease. The researchers then specifically examined the role of vitamin D supplements in cancer incidence and mortality. For the final analysis, they included 10 trials with a total of more than 79,000 participants.

Just like in the previous two reviews, vitamin D didn’t reduce cancer incidence compared to placebo. Once diagnosed, however, vitamin D supplementation significantly reduced the risk of dying from cancer-related causes.

The most recent of the new meta-analyses appeared in August 2019.15 It investigated the association between vitamin D supplementation and all cause mortality. In addition, it looked at the risk of dying from cancer specifically. The analysis included 52 trials with more than 75,000 participants.

While the results didn’t show any effects of vitamin D supplementation on all cause mortality, it did suggest a significant association with cancer mortality. Compared to placebo or no treatment, vitamin D supplementation reduced the risk of dying from cancer-related causes by 16%.

In summary, several large-scale and recent meta-analyses have investigated if supplementing with vitamin D can help prevent cancer or reduce the risk of dying from cancer once it has been diagnosed. None of them show that vitamin D supplementation can reduce your risk of developing cancer. However, the majority of them suggest that, should the worst happen, vitamin D supplementation can reduce the risk that you will die from cancer-related causes.

It is important to remember that all these reviews and analyses show correlation, not causality. That is, they show an association between vitamin D and reduced cancer mortality. What they do not show is cause and effect. There is no way to tell if the vitamin D supplementation itself reduced the risk of dying from cancer, or if vitamin D was part of an effective lifestyle and treatment plan.

That being said, the association is there, and large-scale reviews show that it is significant. Vitamin D seem to at least play a part in preventing cancer-related death.

Depression

Depression is a leading cause of disability all over the world. The association between lack of exposure to sunlight and depression is well established. In fact, it was first noted two thousand years ago.16

Whether or not that association is related to vitamin D is controversial, as is the question if vitamin D supplementation can prevent or help treat depression.

Observational studies have found an association between low levels of vitamin D and depression. Unfortunately, the relationship between vitamin D and depression isn’t that simple. The same studies have not been able to establish if low levels of vitamin D contribute to depression, or if depressed people in general have low levels of vitamin D. Maybe people suffering from depression simply aren’t out and about in the sun enough.

A 2013 review and meta-analysis looked at the results of 14 studies with a total of more than 31,000 participants.17 It found a significant association between low vitamin D levels and depression.

A major problem with this analysis is the fact that the results are based upon case-control studies, cross-sectional studies, and cohort studies, not randomized controlled studies. Randomized controlled trials compare the results of a treatment with placebo. Observational studies don’t have the same level of evidence in medical research.

Three years later, in 2016, a large review analyzed the results of 20 observational studies and compared them to the results of 10 randomized trials.18

Most of the observational studies showed an association between vitamin D levels and mood, suggesting that vitamin D deficiency could be a risk factor for depression. The results from randomized controlled trials did not corroborate these findings. All but one of the placebo-controlled studies showed no statistically significant differences between placebo and vitamin D treatment. In other words, the people receiving vitamin D supplements had the same depression scores as those receiving sugar pills.

A 2018 meta-analysis of 4 randomized controlled trials found that vitamin D supplementation improved depressive symptom ratings in individuals with clinically diagnosed major depression. The researchers recommended caution when interpreting these results, however, since the trials were so few and filled with methodological bias.19

So, can vitamin D help prevent or alleviate depression? Hard to say. We certainly have observational evidence saying yes. On the other hand, randomized controlled trials, which generally provide a higher level of evidence, say no. Things aren’t as clear-cut as they might seem, though. The randomized controlled trials often feature low doses and short interventions, limitations that might make the results hard to interpret. Maybe high dose vitamin D supplementation over longer periods of time could be of use?

At this time, “maybe” is probably the answer we will have to make do with, until further research is available. Likely, the association between vitamin D and depression isn’t as tangible as some might think or want it to be. If you don’t have any vitamin D deficiency, chances are you won’t benefit from supplementation with the intent to treat or prevent depression. If you are deficient, correcting this deficiency through supplementation might reduce depressive symptoms. This conclusion is tentative due to the low quality of the available research.

Cardiovascular disease

While the links between vitamin D and cardiovascular disease are not as explored as the associations with, for example, cancer, a number of trials have investigated the effects of vitamin D supplementation on cardiovascular outcomes.

Back in 2011, an overview failed to conclude that vitamin D supplementation is beneficial for the prevention and treatment of cardiovascular disease, mainly because of the lack of available data at the time. The researchers did note that vitamin D supplementation is both cheap and safe, and might be considered even without concrete evidence, to be on the safe side. This could be a good idea, vitamin D deficiency being as common as it is, and since the cardiovascular system is a known target for vitamin D.20

Eight years later, in June 2019, a new, large-scale meta-analysis of randomized studies, including 21 randomized controlled trials with more than 83,000 participants re-examined if vitamin D supplementation is associated with cardiovascular disease risk.21

Unfortunately, 6 years of additional research revealed that there are no associations between vitamin D supplementation and reduced major cardiovascular events, myocardial infarctions, stroke, or cardiovascular mortality to be found.

In summary, vitamin D can exert effects on the cardiovascular system. However, randomized controlled trials do not support the use of vitamin D supplements for the prevention of cardiovascular disease. In other words: don’t expect any special effects on your heart health from vitamin D.

Diabetes

Low levels of vitamin D are associated with an increased risk of diabetes type 2.22 Does this mean that supplementing with vitamin D will help you avoid developing diabetes? Maybe. Studies have yielded inconclusive results.23

A recent double-blind, placebo-controlled trial gave 96 subjects with a high risk of diabetes or with newly diagnosed type 2 diabetes a daily dose of 5,000 IU vitamin D or placebo for 6 months. The researchers found that vitamin D increased insulin sensitivity and beta cell function. This suggests that vitamin D supplementation could help slow diabetes development.24

However, other large-scale studies have failed to find an association between vitamin D supplementation and the prevention of diabetes.25

Given that vitamin D mediates various cellular processes regulating normal pancreatic beta cell function, it would seem like a good idea to maintain adequate vitamin D levels. When vitamin D levels are inadequate, many of these processes and functions won’t work properly.26

Vitamin D and Muscular Strength

Since both our vitamin D levels and our muscle strength decline with age, there is clearly an association there. Not necessarily a causal connection, but definitely an association.

Recent evidence suggests that vitamin D is important for muscle strength and fitness. Not many studies have examined the relationship between vitamin D status and supplementation and strength, and even fewer with the focus on athletes. Fortunately, we do have enough of them to meta-analyze the results and get a better picture of the evidence. Can vitamin D help you get stronger?

Before we get to the meta-analyzed data of the available research, a quick summary of what is probably the largest and best study on the topic might be interesting.27

In a randomized controlled trial 20 young and 20 elderly men supplemented their diet with either 1,920 IU of vitamin D + 800 mg calcium or placebo (800 mg calcium only) over the course of 16 weeks. During the last 12 weeks of the study, they also performed progressive resistance training. Before and after the training period, the researchers tested the strength of the participants and took muscle biopsies to measure changes in muscle cross sectional area.

The results showed, not surprisingly, that the vitamin D levels in the group given vitamin D has increased significantly. However, this did not translate into any additive effects on strength or muscle hypertrophy. The placebo group had gained just as much strength and muscle mass.

So what does the collected evidence to date say? Let’s take a look.

A systematic review from 2017 gathered data from 6 randomized controlled trials looking at vitamin D levels and muscle strength in athletes between the ages of 18 and 45.28 These trials were of excellent quality, so while the evidence is limited, it should also be pretty reliable.

Looking at this data, the researchers found that 4 weeks to 6 months of vitamin D supplementation up to 5,000 IU per day improved muscle strength. As long as the type of administered vitamin D was D3. Vitamin D2 was ineffective in improving muscle strength. The increases in strength from vitamin D3 supplementation compared to placebo ranged from 1.37 to 18.75%.

Quite impressive results, but more research is needed for confirmation. We’re talking about a couple of studies, and we need more of them to be able to make any definitive conclusions.

In April 2019, another group of scientists quantified the above results by meta-analysing them.29 They also included a few more trials for a total of 8 randomized controlled trials featuring 284 athletes.

At first glance, their analysis wasn’t overly positive. They couldn’t find any effect of vitamin D supplementation of overall muscle strength outcomes. A subgroup analysis did reveal some interesting things, however. The researchers discovered that different muscle groups might respond differently to vitamin D supplementation.

The results after doing the subgroup analysis showed that vitamin D improved lower body strength, but not upper body strength or power. Take a look at the forest plot below. If the diamond is to the right of the line in the middle, that indicates a positive effect. Only lower limb strength consistently show a positive association with vitamin D supplementation. The effects on upper limb strength are all over the place, with no definitive conclusions possible.

PLoS One. 2019 Apr 30;14(4):e0215826. Effect of vitamin D supplementation on upper and lower limb muscle strength and muscle power in athletes: A meta-analysis.

Late November 2019 we got yet another meta-analysis, this time looking at the effects of vitamin D3 specifically.30 This time, the researchers analyzed 5 randomized controlled trials with a total of 149 athletes with documented results. Athletes with low levels of vitamin D normalized these by supplementing with 5000 IU of D3 for at least 4 weeks. Athletes who didn’t have a deficiency to begin with supplemented with 3750 IU of vitamin D3 during 12 weeks.

The results showed that neither the athletes who had low levels of vitamin D nor those who started off with adequate levels benefitted from supplementation. Regardless of initial vitamin D levels did not improve one repetition bench press max strength or increase quadriceps contraction.

We should also mention a fairly large-scale trial looking at the effects of vitamin D supplementation on muscle strength and muscle mass in untrained individuals with low plasma vitamin D levels.31

Eighty-one females with vitamin D deficiency were treated with either 2,800 IU of vitamin D3 or placebo over the course of 3 months during wintertime. The vitamin D treatment increased the vitamin D levels of the participants significantly, as expected, but the effects on their strength wasn’t favorable at all.

Compared with placebo, vitamin D actually decreased their handgrip strength by 9% and their knee flexion strength by 13%. Their lean body mass, well-being, or quality of life were no different after the treatment than before. Vitamin D didn’t improve any of these parameters compared to a sugar pill, despite the fact that the subjects started off with suboptimal vitamin D levels.

In summary, as an athlete, it seems like you can expect improvements in strength from vitamin D supplementation, but only in your lower limb muscles. Again, remember that these results are associations, not confirmed cause and effect results. Don’t expect any dramatic gains from vitamin D supplements, especially if you aren’t deficient to begin with.

In untrained subjects, vitamin D doesn’t seem to improve muscle strength, and might actually have an unfavorable effect.

Conclusion and Practical Recommendations

If you live in a region with plenty of sunlight, and you are out and about a lot, you might not need any supplemental vitamin D at all. However, many of us live in areas where adequate sunlight exposure is not viable, at least not during part of the year. In that case, 2-4,000 IU of vitamin D in supplement form can be a prudent safeguard to ensure that you get enough for optimal health and performance.


That’s it! You’ve reached the end of our guide on vitamin D!

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References

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  3. The American Journal of Clinical Nutrition, Volume 95, Issue 6, June 2012, Pages 1357–1364. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis.
  4. The American Journal of Clinical Nutrition, Volume 80, Issue 6, December 2004, Pages 1710S–1716S. Vitamin D fortification in the United States and Canada: current status and data needs.
  5. Nutrition Research, Volume 31, Issue 1, January 2011, Pages 48-54. Prevalence and correlates of vitamin D deficiency in US adults.
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  7. National Institute of Health, Office of Dietary Supplements, Vitamin D Fact Sheet For Professionals.
  8. Best Pract Res Clin Endocrinol Metab. 2011 Aug;25(4):681-91. Why the minimum desirable serum 25-hydroxyvitamin D level should be 75 nmol/L (30 ng/ml).
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  10. JAMA. 2019;322(8):736-745. Effect of High-Dose Vitamin D Supplementation on Volumetric Bone Density and Bone Strength.
  11. THE LANCET, VOLUME 334, ISSUE 8673, P1176-1178, NOVEMBER 18, 1989. SERUM 25-HYDROXYVITAMIN D AND COLON CANCER: EIGHT-YEAR PROSPECTIVE STUDY.
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  16. The Origin and Deeds of the Goths. Mierow C.C., editor. Princeton University Press; Princeton, NJ, USA: 2012.
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