Everything You Need to Know About Vitamin D, an Essential Nutrient That 35% of Americans Are Deficient In

If you keep an eye on your health, you likely eat a nutrient-rich diet—and may even be aware of the areas where you are falling short (and supplementing accordingly). Ultimately, there are 13 essential vitamins that our bodies need, and each one plays a unique role in our overall wellbeing, per the National Institutes of Aging (NIH). We hear a lot about vitamin C, an immune-boosting vitamin, as well as calcium—which helps keep our bones strong—but we don't often hear much about vitamin D. It's time to change that.

What Vitamin D Does for Your Body

Vitamin D plays a crucial role in a myriad of bodily functions, which we asked doctors to outline, below.

Bones

When we think about bone-bolstering nutrients, most of us go straight to the aforementioned calcium—but vitamin D impacts bone health, too. In fact, without vitamin D, our bodies cannot absorb calcium properly in the first place, which can lead to bone loss, and inevitably osteoporosis and fractures, says Dana G. Cohen, MD, an integrative medical doctor in New York, N.Y.

Immunity

Vitamin D plays a significant role in your body's immunity—your first line of defense against viral infection. "Vitamin D activates toll-like receptors, which are crucial for immune health," says Dr. Cohen. "Also, vitamin D is essential for production of cathelicidin, an important antimicrobial peptide that also protects immune function."

Mental Health

Low levels of vitamin D have been linked to several mental health conditions, including depression and schizophrenia, and ADD; this is likely due to the fact that deficiency causes symptoms—like anxiety, fatigue, mood changes, and loss of appetite—that exacerbate these conditions, notes Dr. Cohen.

Skin

Adequate amounts of vitamin D in the body promote skin health and reduce the risk of psoriasis and eczema, per research published in the Journal of Clinical Medicine. This nutrient also reduces inflammation, an important marker that often plays a role in disrupting the skin barrier.

The Main Sources of Vitamin D

Sunlight

The main way that we get vitamin D is through sunlight, which is why it is often referred to as the sunshine vitamin. "When uncovered skin is exposed to the sun in the form of UVB light, the rays' radiation penetrates the skin and converts a substance found in the skin called 7-dehydrocholesterol to previtamin D3, which eventually becomes vitamin D3," says Matthew L. Mintz, MD, an internal medicine specialist based in Bethesda, Md. "Unfortunately, environmental factors like cloudiness or smog—as well as being indoors (UVB radiation does not penetrate through materials, not even glass)—can affect the amount of vitamin D a person absorbs."

Food

Even though sunlight is our main source of vitamin D, Americans are spending less time outdoors than ever before, according to a 2019 Outdoor Foundation study. Luckily, we can still get vitamin D from certain food sources. Here are a few that are worth consuming if you're looking to up your intake.

• Salmon: This popular fish is loaded with good-for-you fats and contains a heaping amount of vitamin D. In fact, a single 3.5-ounce serving of farmed Atlantic salmon serves up 66% of your daily vitamin D value, per the United States Department of Agriculture (USDA).
• Sardines: These small, salty fish also contain a fair amount of vitamin D and can be purchased by the can, which helps extend their shelf life. A 3.5-ounce serving of sardines scores you 24% of your daily value of vitamin D, per the USDA.
• Cod liver oil: Although not quite as popular as other sea-derived items, cod liver oil can be consumed in supplement form and is a great way boost your vitamin D intake. In a single teaspoon, you get nearly 60% of your daily value. Another plus? It's high in other nutrients, such as omega-3 fatty acids.
• Egg yolks: That's right—the yellow part of eggs is another source of vitamin D, albeit a small one (it provides 5% of your daily value per yolk, per the USDA). Egg yolks also contain vitamin A, E, K and omega-3 fatty acids.
• Mushrooms: If you're a mushroom lover, you're in luck. All types, especially wild, provide a healthy amount of vitamin D. The reason? Mushrooms, like us humans, have the unique ability to synthesize vitamin D when they are exposed to direct sunlight, per research published in the journal Nutrients. One cup of mushrooms contains about 5 IU of vitamin D, per the USDA.

Vitamin D Deficiency

Unfortunately, too many of us are running low in vitamin D. In fact, an estimated 35% of American adults are deficient in the nutrient, per a study published in The Journal of Steroid Biochemistry and Molecular Biology—which means that a lot of us aren't meeting our recommended intake of 15 mcg or 600 IU each day.

Risk Factors for Vitamin D Deficiency

As it turns out, certain individuals are more at risk for a vitamin D deficiency than others, including those who don't spend much time outside; completely cover their bodies when exposed to sunlight; older folks; and those with chronic conditions, notes Dr. Mintz.

"Anything that prevents proper fat absorption in the body might also prevent proper absorption of vitamin D, so conditions such as liver disease, cystic fibrosis, celiac disease, Crohn's disease, and ulcerative colitis can lead to vitamin D deficiencies," Dr. Mintz says.

Vitamin D Deficiency Symptoms

While many people who have low levels of vitamin D may be completely without any symptoms at all, someone with a deficiency might experience fatigue, pain in their bones, muscle weakness, aches, and cramps, as well as mood swings, per the NIH. The current recommended daily amount of vitamin D one should get is 400 international units (IU) for children up to age 12 months, 600 IU for individuals ages 12 to 70 years, and 800 IU for adults over 70 years of age.

Vitamin D Deficiency and Heart Health

A vitamin D deficiency may also be associated with an increased risk of heart disease, as well as an overall increased mortality from cardiovascular disease, according to research published in the journal Clinical Hypertension. "Vitamin D acts to suppress many pathways related to heart disease progression, such as inhibiting vascular calcification and reducing inflammation," explains Sotiria Everett, Ed.D, R.D., a Clinical Assistant Professor in the Department of Family, Population, and Preventive Medicine's Nutrition Division at Stony Brook Medicine.

Vitamin D Supplements

If you're not a huge fan of those vitamin D-rich foods, you can consume this nutrient in supplement form, either by taking a multivitamin that contains it (most do) or consuming a separate vitamin altogether. "Multivitamin doses are usually much lower, so for those with true vitamin D deficiencies—or those who want to take it preventatively [to ward off] bone fractures—individual supplements are recommended," says Dr. Mintz.

Vitamin D2 vs. Vitamin D3

It's also important to note that vitamin D comes in two forms: Vitamin D2 (ergocalciferol) and D3 (cholecalciferol). While both are effective, Dr. Mintz notes that D3 seems to work better at higher doses. "There are several sources of vitamin D3 that manufacturers use and, based on patient preference and taste, one source may be better for some than others," he says. "For example, vitamin D can come from lanolin (a substance derived from sheep's wool), fish oils, and algae oil—the latter may be more suitable for people with vegan diets."

What to Look for in a Supplement

Other factors that Dr. Mintz recommends patients look for in a vitamin D supplement include dye- and gluten-free formulas; iterations that contain either may cause a slew of unpleasant symptoms (especially if you have sensitivities). "You also want to make sure that the vitamin D that you choose is certified, meaning that it actually contains what it says it contains," he adds. "USP and NSF are the two standards used in the United States. Both are good, but NSF is more rigorous."

Talk to Your Doctor

Before supplementing with vitamin D, it's a good idea to have your levels properly checked by your primary care provider. This can be done with a simple blood test at your yearly checkup. While uncommon, overdoing it on vitamin D can lead to negative health outcomes. One study published in the journal JAMA, for example, linked high doses of vitamin D to an increase in fractures and falls in older women. Dr. Mintz points out that too much vitamin D may actually counteract its benefits, so it's a good idea to discuss your interest in supplementation with your doctor to make sure that you do, in fact, need more—and that you're getting the proper dose.