Vitamin D is a powerful nutrient that has far-reaching impacts on everything from immune function to thyroid health to bone strength. In truth, vitamin D is more of a hormone than a vitamin and as a consequence, it plays a huge role in many different biological functions inside your body.
Since it can’t be synthesized inside your body, there are really only two ways to get the benefits of vitamin D: either by exposure to sunlight (which generates vitamin D in your skin), or by taking a supplement.
Since many people don’t get enough sun exposure (especially in wintertime), a supplement is the way to go if you want to reap the following health benefits of vitamin D.
Vitamin D benefits
1. Vitamin D helps your body absorb calcium
For that reason alone, it’s crucial that we get enough of this important vitamin in our diets. After all, calcium (along with phosphate) is the essential building block of bones.
If you don’t get enough calcium in your diet, or if you aren’t absorbing the calcium you do consume, bone production and bone tissue are severely impacted.
2. The best natural source of vitamin D is sunlight
It’s estimated that worldwide, around a billion people don’t get enough vitamin D (1). There are very few food sources of vitamin D, but beef liver, cheese, mushrooms, and egg yolks provide tiny amounts.
Fatty fish like salmon provide even more, but the best way to get your vitamin D is to drink fortified milk, get at least 15 minutes of full sunshine on a large part of your body every day, or take supplements.
In fact, it’s very difficult to get all the vitamin D from food sources. Likewise, if you live in a northern part of the world it can be hard to get your vitamin D from sunshine, too.
Not only that, but certain segments of the population have trouble getting enough of the vitamin from the sun, too: people with dark skin.
Finally, the elderly need more vitamin D than the rest of us. For all these reasons, vitamin D supplements are among the most widely necessary supplements on the planet.
3. High vitamin D levels might protect your body against cancer
Thirty years ago, it was discovered that people living in northern areas had higher rates of colon cancer (2).
That led to the theory that lower vitamin D levels might increase colon cancer risk. Dozens of studies have been performed since then that strongly support the theory, but that doesn’t necessarily mean vitamin D supplements will lower the risk.
4. High levels of vitamin D might also help prevent heart attacks
A 20-year study (3) found that men with vitamin D deficiencies had double the risk of having a heart attack.
While more research is needed, this association may be related to the fact that vitamin D has been linked to your overall levels of inflammation.
5. Vitamin D can help retain muscle strength in older adults
Many older adults get weaker as they age–a condition known as sarcopenia. Vitamin D might be able to augment the effects of protein supplementation in retaining muscle strength and aging healthier.
That’s according to a 2015 study on elderly patients found that adding leucine (as well as whey protein) to the vitamin D supplementation regimen of people at risk for sarcopenia found that this supplementation regimen helped prevent muscle loss (4).
Vitamin D side effects
Vitamin D has been established as safe at doses of at least 4000 IU per day. However, there is some risk of adverse effects if you take extremely high doses for a long time.
What’s ‘excessive’? Even doses of 10,000 IU per day is not known to cause significant side effects in the general population (5). These side effects at very high doses (many tens of thousands of IUs of vitamin D) can be serious, and include dysregulated blood levels of calcium.
Vitamin D can interact with thyroid hormones. People with thyroid conditions seem to be at a higher risk for vitamin D toxicity, because of the relationship between vitamin D and thyroid hormones.
For people who do not have thyroid problems, though, typical doses of vitamin D found in over the counter supplements pose little or no risk.
Vitamin D dosage
Official recommendations are 600-800 IUs per day. Current recommendations from medical governing bodies recommend that adults take 600 IUs of vitamin D per day and the elderly take 800 IUs of vitamin D per day (6).
Some researchers think these recommendations are too low. However, the current recommendations have generated substantial controversy.
One scathing review of the most recent recommendations pointed out that with 600 IUs of vitamin D per day as your only input of vitamin D, your blood levels of the metabolically active form of vitamin D would only be one-third of the minimum biologically healthy level (7).
Second, the recommendations for vitamin D intake as it stands are based solely on evidence for its role in bone health, not its role in preventing chronic diseases or improving physical performance.
Many nutritionists recommend 1000-2000 IUs per day. What’s a typical dosage among adherents to the ‘more vitamin D is necessary’ hypothesis? Usually a few thousand IUs per day, though researchers note that people who are overweight and obese need higher doses (as with most supplements) to get the same effect as a lean person, simply because of their greater body mass.
In terms of dosage safety, up to 4000 IUs of vitamin D is known to be safe, based on experimental research. Higher doses are not known to be dangerous, but have not had their safety firmly established in controlled research settings.
Vitamin D benefits FAQs
Q: Does vitamin D supplementation work?
A: Yes, as long as the dosage is high enough, vitamin D supplementation is a very successful strategy for raising levels of the biologically active form of vitamin D, which is the 25-hydroxy form (technically, 25-hydroxycholecalciferol).
Doctors have noted that it is difficult to effectively raise vitamin D levels with the moderate doses (600-800 IUs per day) recommended for healthy adults, so in cases of clinical vitamin D deficiency, doctors often use substantially higher doses, especially initially, to kick-start an increase in vitamin D levels in the blood.
Q: How is a vitamin D deficiency treated?
A: The treatment plan for a vitamin D deficiency depends on how low your blood levels of vitamin D are. For severely low blood levels, direct medical supervision is usually necessary.
This is diagnosed with a blood test for the 25-hydroxy form of vitamin D, which is the biologically active form. If your levels of 25-hydroxy vitamin D are extremely low, you might be given a massive initial dose to boost your levels of vitamin D rapidly, then sustain these levels with regular high-dose supplementation going forward.
Moderate to mild vitamin D deficiency is usually treated with a daily vitamin D supplement, typically that provides a few thousand IUs per day of vitamin D. If you are deficient in vitamin D, just taking the recommended 600-800 IUs per day for healthy adults is often not enough to actually raise your vitamin D levels.
Q: Why is vitamin D important for athletes?
A: Athletes put huge loads on their body, and bone strength is a huge concern. Vitamin D helps improve bone strength and may help prevent stress fractures, but on top of that, it may benefit performance as well.
A 2018 study showed that impaired vitamin D levels hurts endurance exercise performance, and these impairments are ameliorated when supplementation or sun exposure is used to bring vitamin D back up to a healthy level (8).
Q: Why do you need more vitamin D in the winter?
A: The main pathway that your body typically uses to synthesize vitamin D is through exposure to sunlight.
In the winter, not only is there less light during the day, but you typically have most of your skin covered up because of the cold. As a result, vitamin D levels drop precipitously in the winter, especially in cold climates.
A vitamin D supplement is the best way to keep your vitamin D levels high during the winter if you live in a cold climate.
Related: Our best vitamin D picks
Vitamin D is an essential nutrient that you can only get in high amounts from two places: sunlight exposure or a vitamin D supplement.
Research suggests that vitamin D plays a role in everything from bone strength to immune function to cardiovascular health, so it’s clearly plays a critical role in health.
Add to that the fact that a huge swath of the population is deficient in vitamin D (especially people who live in northern climates), and you’ve got great justification for making sure you have optimal vitamin D levels.