What Is Vitamin D? A Detailed Scientific Guide

You probably already know that vitamin D helps build and maintain strong bones. You may even know that it's produced in your body when the sun’s ultraviolet B (UVB) rays shine on your skin, as the Harvard T.H. Chan School of Public Health notes.

But you may be less familiar with what vitamin D is good for when it comes your body's daily function. You may also be curious how much vitamin D you need per day and whether you actually need to take a supplement.

Here’s what beginners need to know.

For starters, the so-called sunshine vitamin is different from other nutrients because it acts as a hormone — a switch that happens when your body absorbs the vitamin.

According to the National Academy of Sciences, that means vitamin D acts as a messenger in your metabolism, potentially affecting everything from weight to organ function.

The nutrient also helps the body process sufficient calcium and phosphorus, as the National Institutes of Health notes. These minerals are critical for building and maintaining strong bones. An ample amount of vitamin D is key for warding off conditions like rickets in children, osteomalacia in adults, and osteoporosis in the elderly.

“Vitamin D may also be helpful in reducing inflammation and boosting immune function and cell growth,” says Kerry Hackworth, RD, who is based in Chicago.

There are two main forms of vitamin D — vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) — and the difference between them is in their chemical structure and the way they’re made, research notes.

Vitamin D3 is the form of vitamin D that the body makes from sunshine hitting your skin. You can also get it from animal-derived sources. Of the two types, vitamin D3 is most effective at raising your blood serum levels to where they need to be, research suggests.

Vitamin D2, on the other hand, is plant based. If you are following a vegan diet, you may opt for D2 supplements.

Because these types of vitamin D are so different, their dosages may not be interchangeable. An integrative doctor or a registered dietitian-nutritionist can help you identify the proper dose, as official guidelines don't make a distinction by type.

Vitamin D dosage is not one-size-fits-all.

Here are the age-based recommended daily allowance (RDA) amounts for vitamin D.

Infants 

Growing bodies need vitamin D. It’s important to note that breast milk does not provide infants with adequate vitamin D, even in mothers who take supplements. Indeed, research suggests that breast milk, on average, contains less than 20 percent of a baby’s daily vitamin D needs. For this reason, the Centers for Disease Control and Prevention (CDC) recommends that breastfed or partially breastfed infants should take 400 IU, or 10 mcg, of vitamin D each day via supplementation, beginning within a baby’s first few days of life.

Children and Teenagers 

Youngsters need 600 IU, or 15 mcg, of vitamin D per day.

Adults 

When you reach adulthood, 600 IU per day (15 mcg) is recommended. This also includes pregnant and breastfeeding mothers.

Elderly (Over Age 70) 

Adults older than 70 need a bit more vitamin D: 800 IU (20 mcg) per day.

Pregnant or Breastfeeding Women

The NIH recommends that pregnant and lactating women ages 14 to 50 get 600 IU (15 mcg) of vitamin D daily: the same amount as children and adults through age 70.

There are a handful of top ways to ensure you’re getting adequate vitamin D.

Sunlight

When you step into the sun, your body synthesizes vitamin D3 in your skin, absorbing it through cholesterol and changing it into a hormone, where it performs its various functions in the body. About 10 to 15 minutes of direct sunlight (through a window or screen doesn’t count!) should do the trick if you’re light-skinned; more time may be needed for those with darker skin, per U.S. News & World Report.

Any longer and you’ll need to apply sunscreen, which may interfere with vitamin D being synthesized in the skin but can reduce your risk of skin cancer, according to the Skin Cancer Foundation.

Foods

Foods high in vitamin D, combined with enough sunlight and a supplement, when needed, can help you reach sufficient levels.

Because food sources of vitamin D are limited, even a healthy diet may be lacking in this nutrient. Peruse the list below and keep an eye out for foods fortified with vitamin D.

Foods That Contain Vitamin D

Vitamin D occurs naturally in the following foods:

• Salmon
• Tuna
• Sardines
• Eggs
• Cod liver oil
• Beef liver
• Mackerel
• Oysters
• Mushrooms

Foods That May Be Fortified With Vitamin D or Calcium

During manufacturing, these eats may be fortified with the sunshine vitamin and calcium:

• Milk
• Packaged cheese, such as Swiss
• Egg yolks
• Yogurt
• Kefir
• Orange juice
• Cereal
• Margarine
• Protein shakes
• Snack bars
• Packaged fruit chews or snacks
• Oatmeal
• Soy milk
• Tofu

Supplements

Vitamin D2 and D3 supplements are readily available at the pharmacy or health food store, and come in liquid and capsule form, or may be mixed into products like multivitamins and health shake powders.

“The best recommended form of vitamin D supplementation is vitamin D3 or cholecalciferol,” says Zanini. Previous research suggests that vitamin D3 is better absorbed when taken with dietary fats.

Because the U.S. Food and Drug Administration (FDA) regulates supplements differently from conventional drugs and food products, check with your doctor to pick a responsible brand and proper dosage.

Also, keep in mind supplements have the potential to cause drug interactions. If you have an underlying health condition, and especially if you’re on medication, consult your healthcare team before taking any supplement.

Possible Drug Interactions 

Vitamin D supplements may interact with the following drugs, according to the NIH:

• Statins
• Thiazide diuretics
• Orlistat
• Steroids

It’s clear your body needs vitamin D. But you may have also heard that getting enough of the vitamin or increasing your intake, usually by taking a supplement, is associated with certain health perks.

Here’s a closer look at the claims and related research on vitamin D and your health.

Strengthened Bones

According to the Mayo Clinic, vitamin D helps your body absorb calcium, a key mineral for bone health. In turn, vitamin D deficiency can lead to bone-related ailments, especially in women as they age. Women who are past menopause are at the highest risk of osteoporosis, an age-related disease that results when bone loss exceeds bone creation in the body. Osteoporosis is one of the top causes of bone fractures and breaks in seniors.

Other bone diseases that can result from inadequate vitamin D include rickets, which causes soft and weak bones in children, and osteomalacia, which is a similar ailment in adults.

While adequate vitamin D in your diet may support overall bone health, research on supplementation and the possible prevention of bone diseases is mixed.

For example, a meta-analysis published in Food & Function in 2020 found that both dietary and supplemental forms of calcium and vitamin D increased bone mineral density in post-menopausal women, thereby possibly reducing the risk of hip fractures.

Yet other more recent research suggests that supplementing with vitamin D won’t help prevent common bone health problems, such as supplements preventing falls or bone fractures, nor do supplements have any clinically meaningful effect on bone mineral density. A study published in July 2022 in the New England Journal of Medicine found that vitamin D3 supplementation was not helpful in reducing bone fractures in middle-aged and older adults. If you are concerned about your own bone health, consider talking with a doctor before supplementing with vitamin D or calcium.

Better Parathyroid Health

Vitamin D may help prevent hyperparathyroidism, according to the American Association of Endocrine Surgeons.

The condition involves an excess of parathyroid hormone in the bloodstream, which can cause calcium levels in the blood to rise and can lead to osteoporosis, joint pain, and other problems, the Cleveland Clinic notes.

Improved Cancer Survival 

Vitamin D supplements may prevent death from cancer and reduce cancer risk in Black Americans. Findings published in January 2019 in the New England Journal of Medicine — from the VITAL study, a large, randomized clinical trial involving more than 25,000 participants — suggested that people who developed cancer had a 25 percent lower death rate when they were taking vitamin D supplements. The researchers also reported a possible reduction in cancer risk for Black Americans, though they noted that further study is needed.

Reduced Seasonal Affective Disorder

Vitamin D is an established therapy for seasonal depression, also called seasonal affective disorder (SAD). Yet research on the effectiveness of supplements for addressing the disorder is mixed, notes the National Institute of Mental Health.

Greater Protection Against Heart Disease and Type 2 Diabetes

Prior claims about how vitamin D supplements can protect against cardiovascular events or type 2 diabetes haven’t held up in the face of more rigorous randomized, controlled clinical trials. Vitamin D supplements did not reduce the risk of heart attack, stroke, or cardiovascular death in VITAL study participants.

Nor did vitamin D supplements affect the risk of type 2 diabetes in another recent randomized, controlled trial. Here, researchers compared type 2 diabetes rates in people taking 4,000 IU of vitamin D3 per day versus those taking a placebo, and didn’t observe a significant difference in the groups’ risk.

Stronger Immune Function

According to the Harvard T.H. Chan School of Public Health, vitamin D has been a key area of research in immune function, with scientists looking into the possible benefits this micronutrient may have in reducing the risk of autoimmune diseases, as well as protection against more acute illnesses. Some preliminary research even suggests that people with more severe COVID-19 infections may also have low vitamin D levels.

Still, much more research needs to be done to confirm whether vitamin D supplements could help reduce your immediate risk of infections related to COVID-19 and similar diseases, or if supplementation could decrease your risk of developing more chronic immune-related diseases. If you’re concerned about recurring illnesses, you may consider asking your doctor if you might be vitamin D deficient.

Up to 42 percent of Americans may have inadequate vitamin D, according to data from a study that defined this as levels under 20 nanograms per milliliter (ng/mL).

The number of Americans who have vitamin D deficiency, on the other hand, is unknown. The NIH defines deficiency as a measurement of less than 12 to 20 ng/mL.

An optimal level of vitamin D is 29 ng/mL, according to the NIH.

Causes and Risk Factors

Numerous factors can affect how much vitamin D you’re able to take in, including:

• The amount of clothing you’re wearing
• Whether you have sunscreen on
• The season of the year

Certain individuals are known to be at an elevated risk for low vitamin D, including those who are:

• Elderly
• A person of color
• Overweight or obese
• A breastfed infant
• Diagnosed with a bone or gastrointestinal disease

While research is ongoing, there’s currently no evidence to show that pregnancy alone is a risk factor for vitamin D deficiency, meaning there’s no need to supplement unless your doctor advises it.

Diseases Associated With Vitamin D Deficiency

People who have trouble absorbing fat may be among those predisposed to vitamin D deficiency. Medical conditions that may interfere with fat absorption include:

• Celiac disease
• Crohn’s disease
• Cystic fibrosis
• Liver disease
• Ulcerative colitis

Signs and Symptoms

The symptoms of vitamin D deficiency are often hard to detect because they’re so subtle.

According to MedlinePlus, the following are potential signs of vitamin D deficiency:

• Bone pain
• Muscle weakness
• Bone fractures

Diagnosis

According to Harvard Health, it’s important to check vitamin D levels, particularly for those who are considered at high risk of deficiency.

Request a blood test to check your vitamin D levels if you:

• Have a risk factor for vitamin D deficiency
• Live in an area with poor sun exposure during the winter
• Are noticing symptoms that may point to a vitamin D deficiency

Treatment

If a blood test reveals you have a vitamin D deficiency, your doctor may recommend supplementation.

“I would consider vitamin D supplementation if [you’re] unable to get most of your daily needs from food,” says Lori Zanini, RD, CDCES, of Los Angeles. The Endocrine Society recommends 1,500–2,000 IU (37.5–50 mcg) per day, from supplements and foods, for adults and 600–1,000 IU (25 mcg maximum) for children. That’s in order to get blood serum levels of the vitamin at 30 ng/mL (75 nmol/liter) or higher, which is what they recommend for maximum health benefits.

Vitamin D toxicity, clinically called hypervitaminosis, happens when people take in too much vitamin D, according to the Mayo Clinic.

Causes

It’s rare to get too much vitamin D from food and sunlight, but the overuse of vitamin D supplements — especially taking them without a doctor’s recommendation — could lead to toxicity.

Vitamin D toxicity may be becoming more common, as supplements are widely available over the counter, and individuals may be under the impression that taking them might heal certain health ailments. According to Harvard Health, there was a nearly 3 percent increase in the number of people taking unsafe levels — classified as those above 4,000 IU a day — between 1999 and 2014.

Signs and Symptoms 

Vitamin D toxicity is marked by a buildup of calcium in the body, which is called hypercalcemia. Signs that you have had too much vitamin D include:

• Weakness
• Loss of appetite
• Excessive thirst
• Dehydration
• Nausea and vomiting
• Pain and muscle weakness

Consequences 

Too much vitamin D may increase calcium absorption in the body, thereby causing hypercalcemia. Aside from the aforementioned symptoms, hypercalcemia is a serious side effect of vitamin D toxicity, which could cause kidney stones and other kidney problems.

In the most serious cases, the NIH says that vitamin D toxicity carries the risk of:

• Calcification in soft tissues throughout the body, such as your heart valves
• Heart arrhythmias
• Renal failure
• Death

Prevention

Again, it’s always best to err on the side of caution and discuss any supplements with your doctor before adding them to your routine — including vitamin D. If your doctor suspects a deficiency, they can order the proper testing and advise you of the right dosage.

The NIH recommends that people ages 9 and up do not exceed 4,000 IU per day in supplements, and that those ages 8 and under take no more than 1,000–3,000 IU, depending on their age. Anything beyond that may lead to hypervitaminosis.

According to the American College of Obstetricians and Gynecologists, pregnant women who are at risk of vitamin D deficiency may be instructed by their providers to take up to 1,000 to 2,000 IU per day. Again, given the potential risks to growing babies, pregnant women should not take vitamin D supplements without prior testing and diagnosis by their doctor.

Treatment

To treat vitamin D toxicity, you will first need to stop taking any supplements that contain this nutrient. Additionally, a doctor may recommend restricting both supplementary and dietary forms of calcium.

According to the National Library of Medicine, other treatment options for vitamin D toxicity include treating hypercalcemia with intravenous medications, as well as the possible administration of isotonic saline to help reverse dehydration. In the case of related kidney damage, you may need to undergo a process called hemodialysis to help remove calcium buildup.

Vitamin D is an important nutrient that aids a variety of functions in people of all ages, including bone health and metabolism. While some individuals may need vitamin D in supplemental form, these may be dangerous for others, particularly in high doses. If you’re concerned about your vitamin D intake, talk with a doctor so they can help you figure out how to proceed.