Vitamin D Benefits, Functions, Sources, Supplements & Deficiency Symptoms

Last Modified: Monday, September 24, 2018

Vitamin D is widely known as the sunshine vitamin, and you may even already know that one of the critical functions of this nutrient is supporting your bone health.

Both of these are true, but there is so much more to the story of vitamin D…

This is one of the vitamins with the greatest variety of roles in the body and the widest range of health benefits. It is necessary for bone health, but also for preventing many chronic disease.

What does vitamin D do for your body, and should you take vitamin D pills? Which vitamin D rich foods can you add to your diet? Are there any side effects of too much vitamin D? Keep reading for valuable information on the ‘sunshine vitamin’.


History of Vitamin D

Scientists have been able to describe macronutrients – or carbohydrates, fats, and proteins – for centuries, but the discovery of vitamins and the development of understanding how they work was much slower.

It took until 1922 to identify the presence of vitamin D, and until the 1930s to isolate it and describe its chemical structure. Not until the 1970s could scientists understand how the active form of vitamin D functions as a steroid hormone in the body, and the knowledge of vitamin D, its functions in the body, and its possible health benefits is still growing.

An article in the journal “Nature” describes the history of vitamin D. It was the fourth vitamin discovered after vitamins A, B (although “vitamin B” was later found to comprise many distinct vitamins), and C. So, it was named “vitamin D” after the fourth letter of the alphabet.


Vitamin D was identified in 1922 and isolated in the 1930’s. Its activity as a steroid hormone recognized in 1970’s.

It was the forth vitamin discovered after A, B & C.


What is Vitamin D?

vitamin-d2-molecule-with-chemical-formulaVitamin D is a fat soluble vitamin, which means it can be stored in the fat tissue in your body. The main active form of vitamin D is 1,25(OH)D. It comes from cholecalciferol, or vitamin D3, as you’ll see below.

As the Linus Pauling Institute explains, Vitamin D has a few different ways of acting in your body. For one thing, it can go directly to the nucleus, or center, of your cells. It acts on some vitamin D specific receptors, and actually influences how your genes are expressed.

Another way vitamin D works is to influence some of your endocrine hormones, such as parathyroid hormone to help maintain the balance of calcium and phosphorus, which are important bone minerals.


Types of Vitamin D

There are two main types of vitamin D that you can get from food. As the Linus Pauling Institute Micronutrient Information Center explains, vitamin D3, or cholecalciferol, is the form that is in animal-based foods. It is also the form that your skin synthesizes when it is exposed to UV radiation from the sun. Vitamin D in plant-based foods, yeasts, and mushrooms, is in the form of D2, or ergocalciferol.

Vitamin D2 and D3 are not the active forms of vitamin D in your body. Your body needs to convert them to their active forms so they can carry out their functions. In your liver, vitamin D3 receives a chemical –OH (hydroxy) group to convert it to calcidiol, or 25(OH)D, which is read as “25 hydroxyvitamin D.”

Next, calcidiol goes to your kidneys, where it receives another hydroxyl group to become 1,25(OH)D (“1, 25 dihydroxyvitamin D”), or calcitriol. Calcitriol is the major active form of vitamin D in your body.

Most D supplements come in the form of vitamin D3, but you can also find D2 supplements. It appears that supplements with the D2 version can be as effective as D3 at boosting your body’s level of vitamin D.

Vitamin D pills come in capsule and softgel forms. Supplements are also available in liquid forms, chewable tablets, and drops. Many multivitamins contain vitamin D, as do a lot of calcium supplements. For skin conditions, supplements can come in the form of topical creams and vitamin D oil.


Cholecalciferol (Vitamin D3) comes from:

  • Animal-based foods
  • UV exposure

Ergocalciferol (Vitamin D2) comes from:

  • Plant-based foods
  • Yeasts & mushrooms

D2 and D3 are not active forms and need to be converted in the body. Calcitriol is the major active form of Vitamin D.

Most supplements of the vitamin are D3.

Diagram depicting Foods containing Vitamin D. skin absorbs solar uvb radiation and synthesis of vitamin d. calcium homeostasis and metabolism.


Why is Vitamin D Important? Benefits and Functions.

Vitamin D is important for so many reasons. Many of them go well beyond bone health, although strong bones are not to be minimized! This nutrient is necessary for healthy blood pressure and blood sugar, a strong immune system, and preventing certain disease.


Vitamin D for Strong Bones

Calcium is the major mineral in your bones, but vitamin D plays a major role.

When necessary, it helps increase your body’s calcium levels by increasing calcium absorption from the diet and decreasing calcium excretion.

Vitamin D does the same for phosphorus, which is another important bone mineral.

Finally, this nutrient works closely with parathyroid hormone (PTH) to maintain healthy blood and bone levels of calcium.

The benefit is lower risk of osteoporosis.


Vitamin D:

  • Increases calcium and phosphorus absorption
  • Helps maintain healthy bone and blood levels of calcium


Vitamin D for Cancer Risk Reduction

The National Cancer Institute explains that researchers began studying a possible protective effect of vitamin D against cancer development because of observations that people from southern latitudes, where ultraviolet radiation from the sun is more intense, had lower rates of certain cancers than people living in northern latitudes.

In mice and in tumor cells, vitamin D appears to have the potential to stop or slow cancer by inducing cell death, preventing blood vessels from forming, and slowing cell growth.

While there is still no definitive evidence, it appears that higher intakes may be protective against colorectal cancer.


Vitamin D may stop or slow cancer development.

It may also be protective against certain cancers (eg. colorectal).


Vitamin D Antioxidant Capacity

Vitamin D has antioxidant capacities; that is, it can fight oxidation by potentially harmful chemicals in your body called free radicals. The Vitamin D Council cites research finding that higher levels of the vitamin were associated with lower levels of oxidative damage in cells.

In other studies, pregnant women with higher vitamin D levels had higher antioxidant capacity, and liver cells exposed to the antioxidant vitamin were less likely to show oxidative damage.

High levels of vitamin D are associated with lower levels of oxidative damage in cells.


Autoimmune Disorder Prevention & Management

Vitamin D promotes a healthy immune system, and low levels can lead to your immune system acting abnormally. The result can be an autoimmune disorder, in which your body attacks itself in an immune response.

These are some autoimmune diseases linked to low vitamin D levels, along with the cells your body attacks.

  • Multiple sclerosis, or MS: the cells that produce fatty myelin tissue insulating your nerves.
  • Type 1 diabetes: your pancreatic beta cells that produce insulin.
  • Rheumatoid arthritis: the cells that produce collagen to cushion your joints.
  • Systemic lupus erythematosus: cells in various tissues, including skin, heart, lungs, and kidneys.

By promoting a healthy immune system, Vitamin D can prevent and manage certain auto-immune diseases.

Additionally, some auto-immune conditions are linked to low levels of the nutrient.


Better Heart Health

Vitamin D can help protect against cardiovascular disease, which is the leading cause of death in the United States.

For one thing, it helps keep blood pressure to healthy levels by regulating the kidney’s production of renin, which is a hormone that increases blood pressure.

Vitamin D also helps reduce vascular endothelial dysfunction, or problems with the way your blood vessels function.

The result can be a lower risk for cardiovascular events such as heart attacks and strokes.


Vitamin D:

  • Assists healthy blood pressure
  • Reduces problems with the blood vessels
  • Lowers risk of heart attacks and strokes


Healthier Pregnancy Outcomes

Every parent wants a healthy pregnancy and infant, and getting enough vitamin D can improve your chances.

Lower vitamin D levels are linked to low birth weight, not to mention a higher risk for the child later developing autoimmune diseases.

Vitamin D can lower your risk for gestational diabetes and problems such as obesity and diabetes risk in the child later on.

Having optimal Vitamin D levels improves the chances of a healthy pregnancy and infant.


Better Brain Maintenance

Vitamin D can protect against disorders that harm your brain and nervous system.

For example, having adequate vitamin D lowers your risk for cognitive decline and dementia, including resulting from Alzheimer’s.

Parkinson’s disease, which includes low dopamine levels and degeneration of the basal ganglia of the brain, is also linked to poor vitamin D status.


Optimal Vitamin D levels reduces risk of:

  • Alzheimer’s Disease
  • Parkinson’s Disease


Promotion of Skin Health

Topical application of the vitamin is useful as a treatment for psoriasis.

Vitamin D is not just made in the skin. It also promotes healthier skin, as the Linus Pauling Institute describes. As one of the effects of this vitamin’s role in gene expression, vitamin D allows skin cells to regenerate to maintain healthy skin. Skin that has enough vitamin D heals faster and provides a better protective barrier to the environment as the first layer to your body’s complex immune system.

Properly functioning skin cells lower your risk of developing skin cancer, as does the photo-protective effect of vitamin D. This vitamin reduces UV radiation’s effects such as skin aging, DNA damage, and harmful inflammatory responses.

Topical vitamin D has been used to treat psoriasis, which is a condition where your skin produces too many cells called keratinocytes. Psoriasis can lead to red rashes in many areas of your body and cause joint pain known as psoriatic arthritis.


Optimal Vitamin D levels :

  • Speeds up skin healing
  • Lowers skin cancer risk

Which are the Foods High in Vitamin D?

There are not many foods that are naturally high in vitamin D, but you can get good amounts from your diet if you make a special effort.

Fatty fish are the vitamin D rich foods with the greatest amounts of this vitamin. Examples include salmon, mackerel, sardines, trout, and tuna. A 3-ounce serving of canned pink salmon provides over 400 International Units (IU), or over 100 percent of the daily value.

Another natural source of vitamin D in the cholecalciferol form is eggs. The vitamin is only in the yolk, so you are not getting any if you stick to egg whites or liquid egg whites.

Margarine and cod liver oil also provide vitamin D.

If you buy mushrooms that have been irradiated, you will be getting vitamin D in the ergocalciferol form, or vitamin D2.

  • Fatty Fish
  • Egg Yolk
  • Cod Liver Oil
  • Irradiated Mushrooms
  • Fortified Milk
  • Other Fortified Products

After the discovery of vitamin D and some of its roles in the early 1900s, researchers found that the population of the U.S. was largely deficient in vitamin D, with consequences such as rickets. To combat this, public health officials pushed for fortification of common foods with vitamin D. An article in the “American Journal of Clinical Nutrition” explores fortification in the United States.

Since 1945, vitamin D has been required to be added to fortified milk in the U.S. This includes fluid milk products, but dairy products such as cheese and yogurt are not required to contain vitamin D, although they may. Fluid milk contains about 40 International Units (IU) per 100 mL, or about 25% of the daily value for vitamin D per cup.


Some other fortified foods contain vitamin D, although they are not required to. To find out whether they do, check the nutrition label and read the list of ingredients. Orange juice, breakfast cereals, almond and cashew milk, soy milk, yogurt, and cheese are among the more common vitamin D enriched foods.

Some ways to increase your intake of vitamin D rich foods are to drink more milk, have tuna instead of luncheon meat in a sandwich, eat the whole egg instead of just the white, and specifically look for vitamin D fortified foods.

Because there are not many foods naturally high in vitamin D, it is good that the majority of your vitamin D can come from exposure to the sunlight. However, depending on the sun is not fail proof, and there are many people who should consider vitamin D pills because they are not getting enough. Consult your doctor if you think you may need vitamin D supplements.


Not many foods are naturally high in Vitamin D and due to high deficiency rates certain foods such as milk, orange juice and breakfast foods have been enriched with Vitamin D.

Sun exposure is the main source of vitamin D for many people. Those not receiving enough exposure to the sunlight may need to supplement with the nutrient.


How Much Vitamin D Do You Need?

The recommended dietary allowance, or RDA, for vitamin D is 600 IU for adults up to 70 years old. The value is set at 800 IU for adults 70 and older to account for a decreased ability for older adults to synthesize this vitamin in their skin. The RDA is set based on the estimated amount to help you maintain bone health, but higher amounts may be necessary for optimal health in areas such as promoting thyroid health and lowering risk of chronic diseases.

The daily value for a nutrient is the amount that nutrition labels are based on. It is set at 400 IU vitamin D per day, according to the Food and Drug Administration (FDA). So, if you are going by the label, you would need to consume 150% of the daily value (DV) to get to the 600 IU mark that is the RDA. And, hitting this amount may not mean you are getting “enough” vitamin D, even if you are staving off severe deficiency. You might want to talk to your doctor about the possibility of vitamin D pills to get enough.

The amount of vitamin D you need depends on your status, why you are taking the vitamin, and how much you are getting from the sun. A multivitamin often has 400 to 600 IU to help you be sure you are not going to be deficient. Mayo Clinic lists some common conditions that people use vitamin D for, and the vitamin D dosage that has been used for them. These are just a few examples.

  • Preventing fractures: 20-1,370 IU daily, or 100,000 IU three times a year.
  • Cancer prevention: 400 to 1,100 IU daily, or up to 100,000 IU three times a year.
  • Cystic fibrosis: 800 to 2,000 IU daily or even more.
  • Inflammatory bowel disease: 1,000-1,200 IU daily.
  • High blood pressure: 400-8,571 IU daily.
  • Heart disease: 200-2,000 IU daily.

As you can see, the vitamin D dosage varies widely even within a certain condition, so it is necessary to ask your doctor. The proper dosage also depends on the form of the vitamin D you are taking and how often you are taking it.


The Recommended Daily Allowance (RDA) of Vitamin D is:

  • 600 IU for adults up to 70 years of age
  • 800 IU for adults over 70 years of age

The RDA is the amount estimated to help maintain bone health. Certain health conditions may require higher levels of intake.


Who is at Risk of Vitamin D Deficiency?

You may be at risk of vitamin D deficiency even if you are basically healthy and you eat a good diet. For many vitamins and minerals, deficiency or suboptimal status is generally uncommon among otherwise healthy people when it comes to the United States and other wealthy countries. However, vitamin D deficiency is a concern all over the world, including the U.S.

In fact, deficiency in the U.S. actually increased between 1988-1992 and 2001-2002. This conclusion was based on analysis of data from a national survey called NHANES, or the National Health and Nutrition Examination Survey. It is conducted by the Centers for Disease Control and Prevention, or CDC. A third of the population was found to be insufficient, which is shocking.

Why would vitamin D deficiency be increasing when we are clearly getting enough to eat?

Some of the major reasons for increased deficiency are somewhat ironic, since they are the result of efforts to improve health. As public health officials warned people about the dangers of sun exposure and an increased risk for skin cancer, people listened. They used sunscreen more often, and put on long sleeved shirts to block the sun. That may be great for reducing the risk of skin cancer, but an unintended consequence of less sun exposure is – you guessed it – lower vitamin D levels and greater rates of deficiency.

According to “Scientific American,” even lower strength sunscreens can dramatically reduce your skin’s ability to produce vitamin D. Your ability drops by 99 percent with the use of a sunscreen with a sun protectant factor (SPF) of 15. Many people use products with an SPF of 30, 45, or even 100 on all areas of their exposed skin, leaving themselves without a chance to produce sufficient vitamin D based on sun exposure alone.

Another reason for vitamin D deficiency increasing is a lower intake in diets. For example, replacing vitamin D fortified milk with sugar drinks such as soft drinks, sports drinks, and coffee beverages can lead to a lower intake. Of course, it does not help that there are few natural sources. Are salmon, anchovies, and mackerel really part of your daily diet? They are for a few of us, but not for the average American.

Another factor that on the surface seems unrelated to nutrition but that contributes to increases in low vitamin D status is urbanization, or the shift in populations to living in cities. This leads to more indoor time rather than spending time outdoors working, such as on farms. In the U.S., 80% of the population lived in urban settings by 2000, up from two thirds of the population in the 1960s and only half in 1920. The trend is similar worldwide as people move from rural to urban environments.

woman outside tent applying sunscreen to her face

Besides the general population and specific groups of otherwise healthy people, there are some groups at higher risk for vitamin D deficiency. The CDC points out that NHANES showed lower vitamin D levels in African Americans and Mexican Americans as compared to non-Hispanic whites. Men and women had similar rates and patterns of vitamin D deficiency prevalence. People with dark skin are at greater risk for deficiency because their skin does not produce as much vitamin D, and the same is true for older adults.

Since vitamin D is a fat soluble vitamin that gets absorbed in the presence of fat, it makes sense that people with fat malabsorption disorders such as cystic fibrosis and liver diseases are at risk for vitamin D deficiency. Crohn’s disease and other inflammatory bowel diseases also reduce absorption. People with gastric bypass weight loss surgery can also be low in vitamin D because of impaired nutrient absorption.


Vitamin D deficiency is a concern, even in health individuals who eat a good diet.

Those most at risk are individuals who:

  • Have limited sun exposure
  • Eat few naturally Vitamin D rich or fortified foods
  • Are African American or Mexican American
  • Are elderly
  • Have certain fat malabsorption disorders such as Crohn’s disease, IBD, cystic fibrosis or liver disease
  • Have had gastric bypass surgery


Signs of Vitamin D Deficiency

The best known signs of severe vitamin D deficiency are related to bone health. In children, vitamin D deficiency leads to rickets. In this disease, bone mineralization is incomplete in growing bones such as the leg and arm bones, the skull, and the ribs. You may see bow-legged children and deformities in the ribs. Children with vitamin D deficiency can also have seizures due to low calcium levels in the blood which result from lack of sufficient vitamin D to regulate your body’s use of calcium.

In adults, bone-related symptoms of vitamin D deficiency include osteomalacia, or softening of the bones as they lose bone mineral. If you have osteomalacia, you may have bone pain. You are also at higher risk for osteoporosis, or porous bones that put you at risk for bone fractures.

Vitamin D deficiency can be painful. It may not only cause pain in your bones, but also in your muscles. It can even weaken your muscles, which makes your bone condition worse because weaker muscles make you more likely to fall and break a weak bone.

Low vitamin D levels, but not outright deficiency, can increase your risk for diseases such as certain cancers, autoimmune disorders, and heart disease, among others. You probably will not be able to see signs of slightly low vitamin D status, but a simple blood test can give you the information. If you have concerns, ask your doctor to order a test so you can decide whether you need vitamin D capsules.


You are not likely to see signs of a slightly low vitamin D deficiency but it can contribute to diseases such as:

  • Certain cancers
  • Autoimmune disease
  • Heart disease
  • Osteoporosis

A more severe deficiency may be noted by:

  • Rickets in children
  • Seizures in children
  • Bone and muscle pain and weakness in adults
  • Bone fractures due to osteoporosis


Why Use Vitamin D Supplements?

The first reason to use vitamin D supplements is to make up for what you are not getting from the sun or your diet, or for what you are not absorbing. You want to prevent or treat a vitamin D deficiency. Taking supplements can help get rid of the need to worry about whether you are eating enough vitamin D rich foods or getting out in the sun for long enough each day.

Another reason to use vitamin D supplements is to make sure you have a healthy baby if you are pregnant, nursing, or formula feeding. Vitamin D in breast milk depends on vitamin D in the mother, so you want to be absolutely sure you have plenty in your body. And, some breast-fed babies still could use a little extra. If your infant is formula feeding, get a formula with vitamin D or ask your doctor about adding vitamin D drops to the formula.

The next reason to use vitamin D pills is to gain extra health benefits beyond the bone strength that you are likely to maintain by hitting the RDA. That is because higher levels are linked to better outcomes for a variety of chronic diseases, even if you weren’t deficient before.

For example, higher levels are even linked to healthier livers, according to according to a study in “PloS One.” The study included nearly 3,000 men and women over 45 years old. Researchers in Shanghai, China, looked at their vitamin D levels as well as markers of liver health. They found that lower vitamin D levels were linked to unhealthy conditions such as higher fat content in the liver and the presence of non-alcoholic fatty liver disease.


Vitamin D supplements are useful for many reasons:

  • Remove the worry of not getting enough through diet or sun exposure
  • For healthy pregnancy and breast milk
  • Improved liver health
  • Lower risk of many diseases


Who Shouldn’t Take Vitamin D Supplements?

Most people can safely take vitamin D supplements under the supervision of a doctor.

However, this vitamin can affect your blood sugar, so be careful if you have diabetes or take medications to control blood sugar.

The same is true if you have high blood pressure or take blood pressure medications, since vitamin D can also lower blood pressure.

Pregnant and breastfeeding women should take vitamin D supplements with caution, just as they should approach all supplements.

Other people who need to be especially careful with vitamin D include those with calcium excretion problems, autoimmune disorders, and allergies to the supplement.


Many people can safely take vitamin D but certain people should seek professional advice first, they are:

  • Those on diabetic or blood pressure medication
  • Pregnant/breastfeeding women
  • Individuals with certain pre-existing conditions


How Much Vitamin D is Safe?

Vitamin D supplements are generally safe, especially when you choose a brand of vitamin D pills that is reputable and use them as directed by a doctor.

The tolerable upper level of intake, or UL, is 4,000 IU per day, which is 10 times the daily value and nearly 7 times the RDA.

The UL is set by the Food and Nutrition Board of the Institute of Medicine, and it is considered a level that you can take every day for the long term without having long-term effects.

Some people believe a level of up to 10,000 IU per day is safe.


Vitamin D supplements are generally safe when used as directed.

Some people believe up to 10,000IU per day is safe but you should be guided by your doctor.


Vitamin D Toxicity Symptoms/Side Effects


You cannot get vitamin D toxicity just from sun exposure, since your skin stops producing it when you have enough. Signs of too much supplemental vitamin D over time include high blood calcium levels causing hardening of the heart, kidney stones, and bone loss.


Side Effects

Vitamin D3 side effects can include gastrointestinal distress such as stomach pain, diarrhea, constipation, and vomiting. Your risk for urinary tract and respiratory tract infections can increase with too much vitamin D, and you could be excessively tired or have weak muscles.



Staying on top of your vitamin D levels is one of the best things you can do to take charge of your health. Vitamin D can help keep your bones strong, promote a healthy immune system, and lower your risk for the leading causes of death in the U.S., namely, heart disease and cancer. Ask your doctor if you should take supplements and which forms of vitamin D are best for you.