Vitamin D: Everything You Need to Know

Last Update: May 16, 2023

It’s about that time of year when we can step outside and smile. Snowstorms, frigid air, and grey fog are starting to give way to blue skies and—at least for now—sunshine! Ahhh sunshine. It makes you happier not just because it’s nicer outside, but also because those warm rays boost your mood on a chemical level.

As the sun kisses your face, the body produces vitamin D. There’s been a groundswell of scientific studies that prove D might be one of the most essential nutrients for well-being. Case in point? It ups your mood instantly, and over time reduces the symptoms of clinical depression.

But vitamin D is sort of odd. First off, it’s not technically a vitamin—it’s a hormone. And unlike most of the other “letter vitamins,” it’s difficult to get enough just by eating D-rich foods, because there aren’t many out there. Because it’s found in very few foods, it’s estimated that nearly 75 percent of Americans are deficient.

Because it’s hard to find, our bodies have adapted to produce the most vitamin D when our skin is exposed to the sunlight; cholesterol in skin cells reacts with UVB rays to make it. Getting full exposure (that means sans sunscreen) on arms and legs a few times a week for about 15 minutes should do the trick, but if your hometown’s weather is a little more like Chicago than Honolulu, it can be a challenge to get enough rays for the body to create sufficient amounts of vitamin D year-round. What does vitamin D do?

Studies have shown that D increases muscle strength—but our cells and tissues also need it to function. Vitamin D receptors are found in nearly every cell in the body, and as soon as D binds to a receptor, it turns genes on or off, prompting changes within the cell. Studies completed over the last two decades have proven that this process turns off cancer-causing genes, turns on immunoprotective genes, and even tells cells which vitamins and minerals to absorb. More research is ongoing, but one thing is clear: vitamin D plays an essential role in preventing heart attacks and a wide range of other diseases.

Heart health

There’s no doubt that vitamin D and heart function are intrinsically linked. In two studies, low levels of vitamin D were correlated to an increased likelihood of a heart attack. Though there isn’t a clear reason why, some researchers believe that vitamin D acts as a “heart tranquilizer,” improving cardiovascular endurance and keeping heart muscle cells from growing too large. This in turn prevents thickening of the walls of the ventricles, which can block blood flow and cause a heart attack.

Strong bones

Without vitamin D, calcium can’t get absorbed into hard tissues like bones and teeth. But as we get older it’s challenging for us to get enough, as we lose our ability to effectively produce it from sunlight. In fact, vitamin D–deficient elderly women are 77 percent more likely to suffer a hip fracture. But supplementing with anywhere from 800 to 1,000 IUs (the unit of measurement for vitamin D) daily, can help lessen side effects associated with osteoporosis, like bone loss and brittleness.

Possible cancer prevention

It’s a little counterintuitive to think sun exposure could decrease your chances of getting cancer, but it seems that the positive effects of vitamin D outweigh the negatives of getting a little sun (But just a little—more on that below.) A four-year trial found that when postmenopausal women supplemented with both vitamin D and calcium, their likelihood of developing cancer dropped by a whopping 60 percent. D has also been frequently studied for its apparent relationship to breast cancer: In a study of 166 women undergoing treatment, nearly 70 percent had low levels of vitamin D. The good news is that, according to a pooled analysis, supplementing with 2,000 IUs a day (equivalent to about 12 minutes in the sun) could cut the incidence of breast cancer in half.

Improved fertility and prenatal health

Expectant moms probably need a lot more vitamin D than their prenatal vitamins deliver. The average prenatal vitamin contains anywhere from 400 to 1,000 IUs of D—but research suggests that women who take 4,000 IUs daily greatly reduced their risk of complications like gestational diabetes, preterm birth, and infection with zero negative side effects. Plus, women who supplement with it while undergoing in vitro fertilization (IVF) treatment are 6 percent more likely to successfully conceive.

Sources of Vitamin D

There are basically three sources of vitamin D: sunlight, food, and supplements. The easiest and most effective way for our bodies to produce it is through regular sun exposure. According to functional medicine practitioner Chris Kresser, exposure to the sun (on a bright day) for 30 minutes without clothing or sunscreen can produce between 10,000 and 20,000 IU of vitamin D. If the idea of going outside without sunscreen freaks you out—given the known skin cancer risk—doctors assure us that a small amount of sun is ok, as long as you slather on the SPF after your requisite 30 minutes.

Very few foods are natural sources of vitamin D—fatty fish, fish liver oil, beef liver, some cheeses, and egg yolks are the best options. Milk is often fortified with a small amount, as are some breads and cereals.

Because it’s so hard to get enough vitamin D through our food (unless you’re one of the rare people that swallows cod liver oil daily), there are tons of supplements available. They come in different formulations, so you can find the appropriate dosage for your needs.

Do you need to supplement with D?

Vitamin D deficiencies are very common—and unfortunately, they’re linked to a whole host of health issues. Rickets and osteoporosis are the two of the most common that are directly related to D deficiency, because both have to do with bone density and calcium absorption. But it’s interesting to note that 96 percent of heart attack patients are also low in vitamin D. In fact, “overall mortality” has been linked to low vitamin D—basically, the less you have, the more likely you are to croak from a wide array of causes. That’s a good enough reason to make sure you’re getting enough, right?

The daily recommended value varies depending on who you ask: National Institutes of Health recommends 600 IUs for adults and 800 for the elderly, but some experts recommend closer to 1,000 IUs for healthy adults. If you suspect you’re deficient in vitamin D, ask your doctor for a blood test to find out if you should supplement (on top of getting some good ol’ sun).

Vitamin D is in a league of its own—although it’s not exactly a vitamin, it’s massively important to overall wellness and healthy and certainly deserves to be considered an essential supplement.

Want to learn more? Check out our primers on:

Illustration by Foley Wu

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Michelle Pellizzon

Certified health coach and endorphin enthusiast, Michelle is an expert in healthy living and eating. When she's not writing you can find her running trails, reading about nutrition, and eating lots of guacamole.

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