What Does Vitamin D Do?

As Jerry Seinfeld might say, “What’s the deal with vitamin D?!” It’s known as the “sunshine vitamin” because your body absorbs it during time spent outdoors — but then again, it’s also in some of the foods you eat.

How can both be true? What is the deal with vitamin D?

This important vitamin has an important role to play in keeping your bones strong and your immune system healthy. Registered dietitian Devon Peart, RD, MHSc, BASc, helps make sense of vitamin D, including what it does, how much you need and how to get enough of it.

What is vitamin D?

Vitamin D, formally known as calciferol, is a fat-soluble nutrient that has two main responsibilities: helping calcium reach your bones (which keeps them strong) and supporting your immune system so it can fight off infections.

But an estimated 35% of adults have a vitamin D deficiency, and around the world, that number is much higher. Why? Because vitamin D is a little bit tricky — and harder to get enough of than you might expect.

“Our bodies make vitamin D from exposure to sun,” Peart explains. “It’s also naturally present in some foods, like salmon, and added to others, like milk. Depending on a few factors, like how sunny it is where you live, you might need a vitamin D supplement.”

What does vitamin D do for your body?

Vitamin D plays a key role in bone health and helps your immune system stay strong. Peart delves deeper into both of these benefits.

Supports bone health

Calcium is an essential mineral that’s critical to healthy bones and teeth. And vitamin D is like calcium’s trusty sidekick, lending it a helping hand so it can get the job done.

“Vitamin D helps calcium get into your bones,” Peart further explains. “When your body has enough vitamin D, it’s better able to absorb calcium from your small intestine.”

Studies show that when your body has enough vitamin D, its overall calcium absorption is 30% to 40%. But when vitamin D levels are low, calcium absorption from food is only 10% to 15%.

An untreated vitamin D deficiency can lead to a condition called hypocalcemia, which means there’s not enough calcium in your blood. This can cause issues like soft bones (osteomalacia in adults and rickets in children), brittle bones (osteoporosis) and an increased risk of bone fractures. In one study, more than half of postmenopausal women with osteoporosis were found to have a vitamin D deficiency.

Supports a healthy immune system

When you think about taking vitamins to ward off sickness, you probably think about vitamin C, which may help make a cold less severe. But vitamin D also plays an important role in promoting healing and helping your body fight off illness.

“Vitamin D has been shown to help regulate the immune system,” Peart shares, “and having a good level of vitamin D helps prevent infections.”

Studies show that having low vitamin D levels is associated with a higher risk of infections, especially those that affect the respiratory system. Vitamin D deficiency has even been associated with a higher risk of developing autoimmune diseases, including:

• Inflammatory bowel disease.
• Lupus.
• Multiple sclerosis.
• Rheumatoid arthritis.
• Type 1 diabetes.

How to get enough vitamin D

There are three ways to get your fill of vitamin D: sun exposure, food and supplements. Let’s take a closer look at all three.

From the sun

Your body naturally produces vitamin D when you’re in the sun. “When you’re exposed to ultraviolet (UVB) rays from the sun, a compound in the skin (7-dehydrocholesterol) is converted to pre-vitamin D3, and then vitamin D,” Peart explains, “but the amount you make, and how readily you make it, depends on factors like age, skin tone and time spent outdoors.” (We’ll talk about all those factors a little later.)

From food

Though sunlight is your body’s main source of vitamin D, it’s also found in some foods. The thing is, not many whole foods are good sources of this critical vitamin, which can make it difficult to get enough through diet alone. You can find vitamin D in:

• Fatty fish, like trout, salmon, halibut and mackerel.
• Canned or tinned fish, like herring, sardines and tuna.
• Cod liver oil.
• Egg yolks.
• Mushrooms.

Some foods are fortified with vitamin D, meaning that vitamin D has been added to improve their nutritional value. These foods include milk, orange juice, yogurt and some types of cereal.

From supplements

If you can’t get enough vitamin D from sun exposure and diet, your healthcare provider may recommend that you start taking a vitamin D supplement. But you should never start taking supplements without your provider’s go-ahead.

How much vitamin D do you need?

The National Institutes of Health lays out the following recommended dietary allowances for vitamin D:

• Infants under 1 year: 10 micrograms (mcg) (400 IU).
• Adults and children ages 1 to 70, including those who are breastfeeding/chestfeeding: 15 mcg (600 IU).
• Adults over age 70: 20 mcg (800 IU).

While those recommendations seem straightforward, there’s more to consider. Unlike many other vitamins and nutrients, diet isn’t the only way — or even the primary way — to get vitamin D.

“Sunlight is our main source of vitamin D,” Peart points out, “so depending on where you’re located and how much sun you get, you may not be getting enough of it.” If you spend most of your time in Alaska or Antarctica, for example, you won’t get as much vitamin D as people who live in Florida or Fiji.

In addition to your location, other sun-related factors that play a role in your vitamin D level include:

• Season: “Our vitamin D levels are often much lower in winter, when the sun’s rays aren’t as strong and we’re covered up more often,” Peart explains. This is especially likely to be true if you live someplace that experiences all four seasons, including a cold winter.
• Time spent outdoors: If you live in a sunny locale but rarely go outdoors, your body won’t make enough vitamin D.
• Time of day outdoors: Your body makes less vitamin D when the sun’s rays are less direct.
• Sun protection: Here’s a tricky one: Wearing sunscreen lowers the amount of vitamin D your body naturally makes. But of course, wearing sunscreen is always recommended, as sun exposure comes with serious health risks like skin damage, eye damage and skin cancer.

There are a lot of reasons you might need more vitamin D and a lot of reasons why you might not be getting enough. A few of them have to do with your own body:

• Age: As you age, your body doesn’t make vitamin D as efficiently.
• Skin tone: “Melanin is a natural protection from the sun,” Peart says, “so people with darker skin usually make less vitamin D.”
• Body weight: Having a body mass index (BMI) greater than 30 is associated with lower vitamin D levels.
• Certain health conditions and medications: People with conditions like kidney disease, liver disease, cystic fibrosis, Crohn’s disease or celiac disease may be prone to lower levels of vitamin D. Some medications can also lower your levels.

Do you need a vitamin D supplement?

Not getting enough vitamin D can wreak havoc on your body, starting with symptoms like moodiness, muscle cramps and fatigue. If it’s not treated, it can lead to serious issues with your bones, including an increased risk of bone fractures.

In other words, it’s important to get enough vitamin D. But as celebrity cook, Ina Garten is fond of saying, “If you can’t make your own, store-bought is fine” — which, in this case, means you may need a vitamin D supplement.

“If you live in a cold climate like Cleveland or Canada, where there’s not as much sun throughout the year, you will likely need a supplement, especially in winter,” Peart says. “People who spend a lot of time outdoors in the summer may only need to supplement in the winter.”

You may also need a supplement if:

• You don’t spend much time outdoors, even in the summer.
• You’re very diligent with sunscreen (which is a good thing!).
• You’ve entered menopause.
• You’re over age 65.
• You have darker skin.
• You have overweight or obesity.

If your healthcare provider recommends that you take a vitamin D supplement, there are a few important things to know:

• Take it with food: “Vitamin D is a fat-soluble nutrient, which means that your body absorbs it better with food,” Peart advises.
• Don’t take too much: If you take too much of a water-soluble nutrient, you’ll pee out the excess — but that’s not the case with fat-soluble nutrients like vitamin D. “Excess vitamin D accumulates in the bloodstream,” she warns, “so in theory, it can build to toxic levels. That means you should never take more vitamin D than you need to maintain adequate blood levels.”
• But if you forget, it’s OK to double up sometimes: If you miss a day of your vitamin D supplement, Peart says you can take two the next day to get back on track. Just be sure not to take more than 4,000 IU on a given day.

What’s your vitamin D level?

The best way to determine how much vitamin D you need to take is by having your levels tested. Vitamin D testing isn’t typically included in routine bloodwork unless you have specific risk factors for a deficiency, but Peart says all you have to do is ask.

“When you have an annual physical, just request that it be added to your bloodwork,” she advises, “and keep in mind that your levels are often much lower in winter.”

If you don’t have access to bloodwork, she typically recommends taking 2,000 IU in the winter and 1,000 IU in the summer. But the best-case scenario, Peart says, is to base your dosage on your individual blood levels of vitamin D.