You can't get adequate UVB exposure sitting indoors or in a car.
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Q. Does sunlight through window glass provide vitamin D?
A. Sunlight doesn't actually "provide" you with vitamin D. Rather, your body produces vitamin D when skin is exposed to the sun's ultraviolet rays, which trigger vitamin D synthesis. The liver and kidneys convert this biologically inert form of vitamin D into biologically active forms the body can use to promote calcium absorption and bone health.
But sunlight consists of both ultraviolet A, or UVA, which penetrates deep within the skin layers and can cause premature aging; and ultraviolet B, or UVB, which causes the redness of sunburn. It's the UVB rays that trigger the synthesis of vitamin D.
Many people can derive the vitamin D that their bodies need through direct exposure to sunlight during the summer months. As little as 10 minutes a day of sun exposure is typically adequate. But for many, particularly those living in northern climes, production of vitamin D will be inadequate during the winter months.
And you can't get adequate UVB exposure sitting indoors or in a car. Virtually all commercial and automobile glass blocks UVB rays. As a result, you will not be able to increase your vitamin D levels by sitting in front of a sunny window, though much of the UVA radiation will penetrate the glass and may be harmful.
"It doesn't matter if it's winter or summer, you will make no vitamin D sitting in front of a window — zip," said Dr. Michael Holick, a professor of medicine, physiology and biophysics at Boston University School of Medicine.
Those concerned about low vitamin D levels can get more of the vitamin through foods. The best dietary source for vitamin D is old-fashioned cod liver oil. Other dietary sources include swordfish and salmon and, to a lesser extent, fortified milk, orange juice and yogurt, as well as sardines canned in oil, egg yolks and fortified cereals. Dietary supplements are also available.
Vitamin D deficiency means that you do not have enough vitamin D in your body. Vitamin D is unique because your skin actually produces it by using sunlight. Fair-skinned individuals and those who are younger convert sunshine into vitamin D far better than those who are darker-skinned and over age 50.
Why is vitamin D so important?
Vitamin D is one of many vitamins our bodies need to stay healthy. This vitamin has many functions, including:
Keeping bones strong: Having healthy bones protects you from various conditions, including rickets. Rickets is a disorder that causes children to have bones that are weak and soft. It is caused by a lack of vitamin D in the body. You need vitamin D so that calcium and phosphorus can be used to build bones. In adults, having soft bones is a condition called osteomalacia.
Absorbing calcium: Vitamin D, along with calcium, helps build bones and keep bones strong and healthy. Weak bones can lead to osteoporosis, the loss of bone density, which can lead to fractures. Vitamin D, once either taken orally or from sunshine exposure is then converted to an active form of the vitamin. It is that active form that promotes optimal absorption of calcium from your diet.
Working with parathyroid glands: The parathyroid glands work minute to minute to balance the calcium in the blood by communicating with the kidneys, gut and skeleton. When there is sufficient calcium in the diet and sufficient active Vitamin D, dietary calcium is absorbed and put to good use throughout the body. If calcium intake is insufficient, or vitamin D is low, the parathyroid glands will 'borrow' calcium from the skeleton in order to keep the blood calcium in the normal range.
What are the health effects of vitamin D deficiency?
Getting enough vitamin D may also play a role in helping to keep you healthy by protecting against the following conditions and possibly helping to treat them. These conditions can include:
Heart disease and high blood pressure.
Diabetes.
Infections and immune system disorders.
Falls in older people.
Some types of cancer, such as colon, prostate and breast cancers.
Multiple sclerosis.
What are the sources of vitamin D?
You can get vitamin D in a variety of ways. These can include:
Being exposed to the sun. About 15-20 minutes three days per week is usually sufficient.
Through the foods you eat.
Through nutritional supplements.
What does sunlight have to do with getting enough vitamin D?
There are health benefits of sunlight. Vitamin D is produced when your skin is exposed to sunshine, or rather, the ultraviolet B (UV-B) radiation that the sun emits. The amount of vitamin D that your skin makes depends on such factors as:
The season: This factor depends a bit on where you live. In areas such as Cleveland, OH, the UV-B light does not reach the earth for six months out of the year due to the ozone layer and the zenith of the sun.
The time of day: The sun's rays are most powerful between 10 a.m. and 3 p.m.
The amount of cloud cover and air pollution.
Where you live: Cities near the equator have higher ultraviolet (UV) light levels. It is the UV-B light in sunlight that causes your skin to make vitamin D.
The melanin content of your skin: Melanin is a brown-black pigment in the eyes, hair and skin. Melanin causes skin to tan. The darker your skin, the more sun exposure is needed in order to get sufficient vitamin D from the sun.
What does your diet have to do with getting enough vitamin D?
Vitamin D doesn't occur naturally in many foods. That's why certain foods have added vitamin D. In fact, newer food nutrition labels show the amount of vitamin D contained in a particular food item.
It may be difficult, especially for vegans or people who are lactose-intolerant, to get enough vitamin D from their diets, which is why some people may choose to take supplements. It is always important to eat a variety of healthy foods from all food groups. The vitamin content of various foods is shown in the following table.
Vitamin D content of various foods
Food
Vitamin D content in International Units (IUs) per serving
Cod liver oil, 1 tablespoon
1360
Swordfish, cooked, 3 ounces
566
Salmon (sockeye) cooked, 3 ounces
447
Tuna, canned in water, drained, 3 ounces
154
Orange juice fortified with vitamin D, 1 cup
137
Milk, vitamin-fortified, 1 cup
115-124
Yogurt, fortified with 20% of the daily value of vitamin D, 6 ounces
80
Sardines, canned in oil, drained, 2 sardines
46
Liver, beef, cooked, 3 ounces
42
Egg yolk, 1 large
41
Cereal, fortified with 10% of the daily value of vitamin D, 1 cup
40
Cheese, Swiss, 1 ounce
6
Source: Vitamin D. Health Professionals. Dietary Supplement Fact Sheet. National Institutes of Health. Office of Dietary Supplements. August 7, 2019.
It is important to check product labels, as the amount of added vitamin D varies when it is artificially added to products such as orange juice, yogurt and margarine.
How much vitamin D do you need?
In healthy people, the amount of vitamin D needed per day varies by age. The chart below shows the often-cited recommendations of the Institute of Medicine, now the Health and Medicine Division of the National Academies of Sciences, Engineering, and Medicine. It is important to know that these are general recommendations. If your doctor is checking your blood levels, he or she might recommend higher or lower doses based on your individual needs.
If you have osteoporosis, your doctor might suggest a blood test of your vitamin D levels. The amount of vitamin D supplement can be customized for each person, based on the results. For many older patients, a vitamin D supplement containing anywhere between 800 to 2000 IUs daily, which can be obtained without a prescription, can be both safe and beneficial. It is important to speak with your doctor about your individual needs.
People by age
Recommended dietary allowance (IU/day)
Upper level intake (IU/day)
Infants 0-6 months*
400
1,000
Infants 6-12 months*
400
1,500
Children 1-3 years old
600
2,500
Children 4-8 years old
600
3,000
People 9-70 years old
600
4,000
People over 70 years old
800
4,000
Females 14-50 years old, pregnant/lactating
600
4,000
*refers to adequate intake vs recommended dietary allowance of the other age groups.
Symptoms and Causes
What causes vitamin D deficiency?
Vitamin D deficiency can be caused by specific medical conditions, such as:
Cystic fibrosis, Crohn's disease, and celiac disease: These diseases do not allow the intestines to absorb enough vitamin D through supplements.
Weight loss surgeries. Weight loss surgeries that reduce the size of the stomach and/or bypasses part of the small intestines make it very difficult to consume sufficient quantities of certain nutrients, vitamins, and minerals. These individuals need to be carefully monitored by their doctors and need to continue to take vitamin D and other supplements throughout their lives.
Obesity: A body mass index greater than 30 is associated with lower vitamin D levels. Fat cells keep vitamin D isolated so that it is not released. Vitamin D deficiency is more likely in obese people. Obesity often makes it necessary to take larger doses of vitamin D supplements in order to reach and maintain normal D levels.
Kidney and liver diseases: These diseases reduce the amount of an enzyme needed to change vitamin D to a form that is used in the body. Lack of this enzyme leads to an inadequate level of active vitamin D in the body.
What other factors can lead to vitamin D deficiency?
Age: The skin's ability to make vitamin D lessens with age.
Mobility: People who are homebound or are rarely outside (for example, people in nursing homes and other facilities) are not able to use sun exposure as a source of vitamin D.
Skin color: Dark-colored skin is less able to make vitamin D than fair-colored skin.
Human breast milk: A woman's breast milk only contains a small amount of vitamin D. Often infant formulas also only include a small amount of D also. Therefore infants are at risk of not receiving enough vitamin D. This is especially true for infants who are only fed breast milk.
Can medications cause a vitamin D deficiency?
Yes. Vitamin D levels can be lowered by certain medications. These include:
Laxatives.
Steroids (such as prednisone).
Cholesterol-lowering drugs (such as cholestyramine and colestipol).
Seizure-control drugs (such as phenobarbital and phenytoin).
A tuberculosis drug (rifampin).
A weight-loss drug (orlistat).
Always tell your doctor about the drugs you take and any vitamin D supplements or other supplements or herbs/alternative health products that you take.
What are the signs and symptoms of vitamin D deficiency?
Severe lack of vitamin D causes rickets, which shows up in children as incorrect growth patterns, weakness in muscles, pain in bones and deformities in joints. This is very rare. However, children who are deficient in vitamin D can also have muscle weakness or sore and painful muscles.
Lack of vitamin D is not quite as obvious in adults. Signs and symptoms might include:
Fatigue.
Bone pain.
Muscle weakness, muscle aches, or muscle cramps.
Mood changes, like depression.
Diagnosis and Tests
How is a vitamin D deficiency diagnosed?
Your doctor can order a blood test to measure your levels of vitamin D. There are two types of tests that might be ordered, but the most common is the 25-hydroxyvitamin D, known as 25(OH)D for short. For the blood test, a technician will use a needle to take blood from a vein. You do not need to fast or otherwise prepare for this type of test.
What do vitamin D test results mean?
There are some differing opinions about what levels of vitamin D work the best for each person. Laboratories might use different numbers for reference. Please discuss your results with your doctor.
How often do you need to get your vitamin D levels checked?
Doctors do not usually order routine checks of vitamin D levels, but they might need to check your levels if you have certain medical conditions or risk factors for vitamin D deficiency. Sometimes vitamin D levels can be checked as a cause of symptoms such as long-lasting body aches, a history of falls or bone fractures without significant trauma.
Management and Treatment
How is vitamin D deficiency treated?
The goals of treatment and prevention are the same—to reach, and then maintain, an adequate level of vitamin D in the body. While you might consider eating more foods that contain vitamin D and getting a little bit of sunlight, you will likely be told to take vitamin D supplements.
Vitamin D comes in two forms: D2 and D3. D2, also called ergocalciferol, comes from plants. D3, also called cholecalciferol, comes from animals. You need a prescription to get D2. D3, however, is available over the counter. It is more easily absorbed than D2 and lasts longer in the body dose-for-dose. Work with your doctor to find out if you need to take a vitamin supplement and how much to take if it is needed.
Can you ever have too much vitamin D?
Yes. You can get too much vitamin D if you overdo the supplements. Interestingly, you cannot get too much vitamin D from the sun. Vitamin D toxicity is, thankfully, quite rare but can lead to hypercalcemia and together the symptoms can include:
Nausea.
Increased thirst and urination.
Poor appetite.
Constipation.
Weakness.
Confusion.
Ataxia (a neurological condition that may cause slurring of words and stumbling).
Do not take higher-than-recommended doses of vitamin D without first discussing it with your doctor. However, your doctor might recommend higher doses of vitamin D if he or she is checking your blood levels and adjusting your dose accordingly. Also, be cautious about getting large doses of vitamin A along with the D in some fish oils. Vitamin A can also reach toxic levels and can cause serious problems.
Prevention
How can I help prevent vitamin D deficiency?
The goals of treating and preventing the lack of vitamin D of treatment and prevention are the same—to reach and keep an adequate level of vitamin D in the body. Your healthcare provider will let you know if you need to take or keep taking vitamin D supplements. If so, they will also let you know how much you should take. You might also want to consider:
Eating more foods that contain vitamin D: See the vitamin D food sources table included in this article. Keep in mind that foods alone usually don't meet the daily recommended levels of vitamin D.
Getting some exposure to sunshine—but not too much: Exactly how much sun exposure is needed isn't clear. 10 to 15 minutes of sun exposure two to three times a week to the face, arms, legs or back may be all that is needed to absorb a suitable amount of vitamin D. You might need more sun exposure (especially in early spring and late fall) if:
You are older.
You have a darker skin color.
You live in northern climates.
The use of sunscreen, and standing behind a window, prevents vitamin D from being produced in the skin. However, you should remember that too much sunshine increases the risk of skin cancer and ages the skin. That is why taking an appropriately dosed D supplement is far safer than intentionally getting routine sun exposure.
Share on PinterestExperts are researching if vitamin D deficiency is linked to severe COVID-19 symptoms. Jasmin Merdan / Getty Images
Several studies have linked vitamin D deficiency to coronavirus infection and more severe COVID-19.
Experts are now investigating if taking vitamin D may help protect against severe COVID-19 symptoms.
Even before these studies are finished, there are good reasons to consider taking a vitamin D supplement.
All data and statistics are based on publicly available data at the time of publication. Some information may be out of date. Visit our coronavirus hub and follow our live updates page for the most recent information on the COVID-19 pandemic.
With no coronavirus vaccines approved for full use, many people are turning to vitamins and other supplements to bolster their immune system and help them fight off COVID-19.
While plenty of posts on social media push these supplements as personal coronavirus shields, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, cautioned last month that most "so-called immune boosting supplements" actually do "nothing."
However, there is one supplement that may be of use to the immune system — vitamin D, also known as the "sunshine vitamin."
"If you are deficient in vitamin D, that does have an impact on your susceptibility to infection," Fauci said in an Instagram Live interview with actress Jennifer Garner. "So I would not mind recommending — and I do it myself — taking vitamin D supplements."
Fauci was not speaking about COVID-19 specifically, but about vitamin D's general role in supporting the immune system — in particular, about the benefits of this nutrient for people who are deficient.
Right now, the National Institutes of Health says there's not enough evidence to show that vitamin D can prevent or treat COVID-19.
But there is enough evidence that scientists are exploring the benefits of this and other supplements against the coronavirus.
Earlier research showed that vitamin D reduces the risk of respiratory tract infections other than COVID-19. The benefits varied quite a bit among these studies, but vitamin D supplementation seemed to be more helpful for people who were deficient in this nutrient.
More recent research suggests that vitamin D deficiency may increase the risk of coronavirus infection and severity of COVID-19.
For example, some studies found that COVID-19 deaths tend to be higher in countries farther away from the equator, where many people are deficient in vitamin D due to the reduced amount of sunlight, especially during winter.
"We make vitamin D by being exposed to sunlight, and in the colder months people can become deficient," said Nicole Avena, PhD, an assistant professor of neuroscience at the Icahn School of Medicine at Mount Sinai in New York.
Many factors could account for the greater number of COVID-19 cases in more northerly latitudes. But a study published last month in JAMA Network Open reinforces the importance of vitamin D for a healthy immune system.
"In our study, patients who have lower vitamin D levels were more likely to test positive for COVID-19," said study author Dr. David Meltzer, a professor of medicine at The University of Chicago Medicine.
This trend still held when they tried to control for other factors — age, obesity, other medical conditions — that might worsen COVID-19 and are linked to lower vitamin D levels.
Other studies show that patients with lower vitamin D levels who are diagnosed with COVID-19 tend to have worse outcomes.
One of these was published last month in PLoS ONE. Researchers found that people hospitalized for COVID-19 who were deficient in vitamin D were more likely to have severe symptoms or die from COVID-19.
Not all research, though, supports the link between vitamin D and COVID-19.
A UK biobank study published earlier this year in Diabetes & Metabolic Syndrome: Clinical Research & Reviews found no link between vitamin D levels and coronavirus infection.
However, Meltzer and his colleagues pointed out in their paper that the UK researchers measured vitamin D levels up to 14 years before COVID-19 testing and didn't look at whether people had been treated with vitamin D since then.
This could have weakened the observed link between vitamin D levels and coronavirus infection, they wrote.
Scientists are also eyeing other nutrients as potential treatments for COVID-19, including zinc and vitamin C, both touted for their cold-fighting benefits.
In one study, researchers from Spain reported that patients with low blood levels of zinc were more likely to die from COVID-19.
Their results were presented last week at the European Society of Clinical Microbiology and Infectious Disease (ESCMID) Conference on Coronavirus Disease. This research has not been peer-reviewed, so it should be viewed with some caution.
All of these studies are observational, meaning researchers collected data on people over time, rather than assigning people to different treatments. That makes it impossible to show a cause-and-effect relationship between vitamin D and COVID-19.
What's needed are randomized controlled trials (RCTs), the "gold standard" of clinical research. Some of these are already underway.
In one study carried out in Spain, researchers gave calcifediol, an active form of vitamin D, to 50 people hospitalized with COVID-19. Another group of 26 patients didn't receive vitamin D.
All patients were treated equally with hydroxychloroquine and azithromycin.
Only one of the patients treated with vitamin D ended up in the intensive care unit, while half of the untreated patients did.
"There are some imperfections [to the study] — it's not a very large study, there are some slight imbalances between the groups — but it's pretty impressive," said Meltzer.
While this study used vitamin D to treat people who already had COVID-19, Meltzer says the nutrient might work equally well at keeping people out of the hospital.
"One possibility is that vitamin D is not preventing COVID-19," he said, "but is causing people who are infected to have such minimal symptoms that they never get tested, never get found."
More research is needed to know if this is true.
Two randomized controlled trials starting at the University of Chicago should provide some answers. In both, people will be treated with vitamin D and then followed to see if they contract the coronavirus and how severe their symptoms are.
One trial focuses on healthcare workers. The other will recruit people from the community, with an emphasis on enrolling Black and Hispanic people, who are at high risk for both vitamin D deficiency and COVID-19.
At the Cleveland Clinic, researchers are enrolling people into a study to see if vitamin C or zinc — or a combination of the two — can reduce the duration of COVID-19 symptoms. Patients will be given the supplements after they have tested positive for COVID-19.
How soon results are available from these studies depends on how quickly researchers can enroll people into the studies. Meltzer says study volunteers are essential for the success of COVID-19 research.
"People who can enter clinical trials do a tremendous service to society," he said.
Although Meltzer is enthusiastic about the potential benefits of vitamin D, he cautions that we won't really know until the randomized controlled trials are finished.
"This doesn't mean we should let down our guard or not do any of the other things we're doing," he said. "I certainly wouldn't stop wearing a mask and go to a big party thinking everything was going to be OK because I was taking vitamin D."
Even before these studies are finished, there are good reasons to consider taking a vitamin D supplement.
"The vast majority of Americans are vitamin D deficient and could probably benefit from some modest level of vitamin D supplementation, at very little risk," said Meltzer.
In the United States, an estimated 41 percent of adults are deficient in vitamin D. People with darker skin are at greater risk — 63 percent of Hispanic people and 82 percent of Black people have low vitamin D levels.
Other factors increase the risk of vitamin D deficiency, including age, limited sun exposure, obesity, and certain medical conditions.
Avena says it is difficult to get enough vitamin D from foods. "So taking a supplement may be advised," she says, "but always check with your doctor first."
Meltzer says he takes 4,000 international units (IU) per day. The National Institutes of Health says this is the upper limit of what is safe for the vast majority of people.
"Don't consume more than 4,000 IU per day of vitamin D without your doctor's permission," said Avena. "It exceeds the safe upper limits of intake."
Larger doses can be toxic and should only be taken under medical supervision.
Avena says zinc may also be helpful once you feel symptoms of a cold coming on.
She recommends taking between 13.3 milligrams and 23 milligrams of zinc every 2 hours while you have cold symptoms, but no longer than 1 week. This is the dose found in over-the-counter gummies and lozenges.
As with vitamin D, we don't yet know if zinc will reduce symptoms of COVID-19.
While most people get enough zinc from food sources, some groups — vegetarians, people with gastrointestinal diseases, people who misuse alcohol — are at higher risk of zinc deficiency and might want to consider taking a supplement.
