In the ABCs of vitamins, D is one of the most important. It helps the body absorb calcium, a mineral that's crucial to build and maintain healthy bones. Vitamin D also has anti-inflammatory and antioxidant properties, and it's essential for healthy immune function and early childhood development.
A new study suggests getting enough vitamin D may even help slow down biological aging and lower the risk of age-related diseases.
Often called the “sunshine vitamin,” vitamin D is synthesized in the skin when exposed to sunlight. It's also found naturally in some foods and added to others. Additionally, vitamin D is a popular supplement and present in most multivitamins.
In fact, vitamin D is one of the most widely consumed dietary supplements in the United States, according to data from the U.S. Centers for Disease Control and Prevention. However, not everyone needs to take extra vitamin D.
So, how much vitamin D do you need and who should be taking a a supplement? When is best time of day to take it? Experts break down the benefits and risks, and how to take vitamin D safely.
What is vitamin D?
Vitamin D, also known as calciferol, is a fat-soluble vitamin that the body needs to absorb calcium from the intestines.
"Vitamin D has two main responsibilities: helping calcium reach your bones, which keeps them strong, and supporting your immune system so it can fight off infections," Julia Zumpano, a registered dietitian at the Cleveland Clinic Center for Human Nutrition, tells TODAY.com.
Vitamin D also helps support a healthy nervous system, as well as muscle and brain function. "It's very important in cell growth," NBC News medical contributor Dr. Natalie Azar said on TODAY.
There are two main forms, vitamin D3 and vitamin D2. Vitamin D3 is produced when the skin is exposed to sunlight and found in animal-based foods. Vitamin D2 is found in some plant sources, or human-made and added to foods, according to the National Institutes of Health’s Office of Dietary Supplements.
These inactive forms of vitamin D get stored in fat cells until they are converted by the liver and kidneys into the active form of vitamin D, which the body can use.
Vitamin D benefits
“Vitamin D is very important in the body because it contributes to the bone health, musculoskeletal health, immune function and many functions,” Dr. Julia Adamian, an internist at NYU Langone Health, tells TODAY.com.
During childhood, vitamin D is crucial to prevent rickets, or softening of the bones, per the Mayo Clinic.
In adults, vitamin D, together with calcium, can help prevent osteoporosis, which causes bones to become weak and brittle, increasing the risk of fractures, Azar noted.
Additionally, vitamin D can fight inflammation, protect heart and brain health, and help regulate glucose metabolism, according to the NIH.
Research suggests that getting more vitamin D could even shave years off your biological age, which is the natural wear and tear our cells accumulate over time, said Azar.
The findings, published May 22, 2025, in The American Journal of Clinical Nutrition, suggest vitamin D may help maintain telomeres, the caps on the ends of chromosomes that shorten during aging, which is linked to the development of age-related diseases.
How much vitamin D do you need per day?
The recommended daily intake of vitamin D for healthy people varies by age, per the NIH.
- Children (1–18 years): 15 mcg (600 IU)
- Adults (18–70 years): 15 mcg (600 IU)
- Older adults over 70: 20 mcg (800 IU)
Sources of vitamin D
Vitamin D can be obtained from several different sources.
Sunlight
Humans get a lot of their vitamin D from exposure to ultraviolet (UV) light from the sun. "The production of the vitamin D is activated in the skin through contact with UVB rays," says Adamian. The body produces vitamin D3, which is converted into active vitamin D.
Getting five to 30 minutes of sunlight midday at least a few times a week is thought to be enough to maintain sufficient vitamin D levels, per the NIH.
Sunscreen does protect against UVB rays, which can also damage and age the skin, says Adamian. However, research shows people who wear sunscreen can still synthesize vitamin D from exposure to sunlight.
Food sources of vitamin D
Few foods are naturally rich in vitamin D. These include cod liver oil, salmon, trout and sardines, per the NIH. Raw mushrooms that are exposed to UV light are a good source of vitamin D2. Eggs, beef liver and cheese contain small amounts of vitamin D3.
Vitamin D is also added to foods. Vitamin D-fortified foods provide most of the vitamin D in American diets, per the NIH. These include fortified cow's milk, plant-based milk (soy, almond or oat), orange juice and cereals, nutritionist Joy Bauer tells TODAY.
Vitamin D supplements
Vitamin D dietary supplements contain vitamin D3 or vitamin D2. Research shows that both forms are well-absorbed, but vitamin D3 supplements may elevate blood levels of vitamin D to a greater extent, per the NIH.
Most multivitamins contain vitamin D but usually a lower amount than vitamin D supplements.
Most healthy adults can get enough vitamin D from sunlight, foods or a multivitamins. “It is tough to get enough from diet alone, so it’s usually a combination of all these,” says Adamian. However, some people need a little help to meet their intake.
Who should take a vitamin D supplement?
People who do not get enough vitamin D may need a supplement. Over time, inadequate vitamin D intake can lead to a vitamin D deficiency, which can cause bone pain, muscle weakness and fatigue, says Adamian.
A deficiency occurs if a person doesn't get enough vitamin D from sunlight or food, or if their body can't synthesize or absorb vitamin D properly due to an underlying condition or medication.
A common risk factor is limited UV exposure, says Adamian, because the skin stops producing vitamin D. This can happen in colder climates or during the winter, when the UVB rays are less strong and more skin is covered.
Melanin can also affect vitamin D levels. People with darker skin are less able to produce vitamin D from sunlight and may be at risk of a deficiency, per the NIH.
“Vitamin D stores also decline with the age,” says Adamian, adding that she often recommends vitamin D supplements for older adults.
Certain conditions can affect the body's ability absorb vitamin D, such as liver or kidney disease, celiac disease, and inflammatory bowel disease, Adamian adds. Medications that can lower vitamin D levels include laxatives, steroids and cholesterol drugs, per the Cleveland Clinic.
A vitamin D deficiency is diagnosed through a blood test, says Zumpano.
Vitamin D supplement dosage
Always talk to your doctor to find out which dosage is appropriate for you. The recommended daily intake for vitamin D is 600 IU for healthy adults, but people who are deficient may need to take more.
“You want to have a ‘Goldilocks’ amount. You don’t want to be too deficient, and you don’t want to take too much,” said Azar.
Most vitamin D supplements are between 1,000 and 5,000 IU, but some are as high as 10,000 IU, says Adamian. "There's so many different supplements, so it's very important to consult a health care provider," Adamian adds.
While vitamin D supplements can fill nutrient gaps, there isn’t much evidence that taking extra has added health benefits if you don't have a deficiency, TODAY.com previously reported.
The Endocrine Society does not recommend vitamin D supplements for healthy adults ages 19 to 74.
When is the best time to take vitamin D supplements?
The best time to take vitamin D supplements depends on the person. Ideally, you should choose a time of day that's easy to remember, says Adamian.
Many people prefer the morning for convenience, Adamian adds, but it's okay to take vitamin D at any time of day, as long as you stay consistent. "If you choose one time to take it, then you are more likely to take it on a daily basis,” says Zumpano.
If you take other medications, talk to your doctor about timing. “Vitamin D can interact with statins, steroids and thiazide diuretics,” Zumpano says.
The time of day matters less than whether you take it with food, which affects the absorption of vitamin D, the experts note.
Should you take vitamin D with or without food?
“It's best to take a vitamin D supplement with food, and at least a source of fat,” says Zumpano.
Vitamin D is fat-soluble, so taking it with a fatty food can help improve the absorption of it in the body, Adamian explains.
The experts suggest taking vitamin D supplements during or after a meal rich in healthy fats, such as avocado, nut butter, salmon, or whole-fat yogurt.
When is the best time of year to take vitamin D supplements?
It's safe to take vitamin D supplements year-round, which may be necessary if you're deficient, but always follow your doctor's guidance.
Some people only take vitamin D during the fall and winter, when their body doesn't produce enough vitamin D due to decreased UV exposure, Adamian notes. In this case, the best months to take vitamin D will depend on your location and the climate.
Side effects of vitamin D supplements
When taken in the appropriate dosage as directed by a doctor, vitamin D supplements are generally safe. However, taking too much vitamin D can cause side effects or health issues.
“Vitamin D is a fat-soluble vitamin, therefore it is not excreted in your urine and will accumulate in your body,” says Zumpano.
In rare cases, vitamin D can reach toxic levels. This can increase calcium absorption, leading to abnormally high levels of calcium in the blood, says Zumpano.
Vitamin D toxicity is typically caused by taking high doses of the vitamin in supplement form, not from food or sunlight, Zumpano adds.
Symptoms of vitamin D toxicity include:
- Nausea
- Vomiting
- Constipation
- Loss of appetite
- Dehydration
- Confusion
- Kidney stones
"In extreme cases, vitamin D toxicity can cause kidney failure, cardiac arrhythmias and even death," says Zumpano.
If you experience side effects, stop taking vitamin D and talk to your doctor.
