Vitamin D Forms, Requirements, and Dietary Sources

by OneGoodFoodBlog
Vitamin D Forms, Requirements, and Dietary Sources

Your body needs vitamin D for the proper absorption and utilization of calcium—an essential mineral that helps keep your bones and teeth strong and is necessary for normal blood clotting, muscle and nerve function. Vitamin D is a fat-soluble vitamin made through exposure to sunlight and can also be obtained through diet or supplements.

Forms of Vitamin D

Vitamin D can take one of four different forms: cholecalciferol, calcifediol, calcitriol, and ergocalciferol.


This form is also called vitamin D3, and it’s made from cholesterol in your body when your skin is exposed to ultraviolet light. Cholecalciferol is not biologically active; it has to travel through your bloodstream to the liver, where it is converted into another form of vitamin D called calcifediol.

Most people need to get 5 to 30 minutes of sun exposure at least two days a week for their body to make sufficient amounts of vitamin D. The amount of time needed also depends on the weather conditions and the time of year.


The storage form of vitamin D is called 25-hydroxyvitamin D or calcidiol. It’s also the form of vitamin D measured in blood tests when your healthcare provider wants to test for vitamin D deficiency.

If your calcidiol levels are low, you may not be making enough vitamin D, which can lead to a softening and weakening of your bones. In children, this is called rickets, and in adults, it’s called osteomalacia. Osteomalacia can result in osteoporosis.


Your kidneys take calcifediol and convert it to the biologically active form of vitamin D called 1,25-hydroxyvitamin D, or calcitriol. This kind of vitamin D promotes calcium absorption and helps balance blood levels. It also has a role in normal cell growth and nerve and muscle function.

Calcitriol is also necessary for a healthy immune system and may help to reduce inflammation.

Your body regulates your blood levels of calcitriol very carefully, so it isn’t a proper form for testing or monitoring vitamin D deficiency. In fact, calcitriol levels may remain normal while calcifediol levels begin to drop.


Vitamin D2, or ergocalciferol, is similar to cholecalciferol, but it’s the form of vitamin D synthesized in plants. Your liver can convert ergocalciferol to calcifediol. It isn’t converted to calcidiol as efficiently as cholecalciferol, but it appears sufficient for use as a dietary supplement.

Where Does Vitamin D Come From?

Vitamin D isn’t found in many foods unless they have been fortified, such as milk, yogurt, soy milk and breakfast cereals. Small amounts of vitamin D are found in sources like oily fish, beef liver, and mushrooms.

Your body makes vitamin D when your skin is exposed to the ultraviolet B (UVB) rays in sunlight. About 5 to 30 minutes of exposure to the skin on your face, arms, back or legs (without sunscreen) twice a week is enough for most people.

Too much exposure to sunlight is a risk for skin cancer. You should use sunscreen after a few minutes in the sun, even on hazy or cloudy days.

The amount of UVB exposure required for adequate vitamin D production also depends on the time of the year. The UVB rays are more intense in the northern hemisphere during the summer months and less intense during the winter months.

In fact, if you live north of the 42-degree latitude, you’ll have a difficult time getting enough sun exposure from November through February.

Picture a map of North America. If you live north of a line drawn on a map from the northern border of California to Boston, Massachusetts, you will probably need to get more vitamin D from the foods you eat or from supplements you take during the winter months, even if you do go outside every day.

Clouds and pollution also reduce the intensity of UVB rays. The UVB rays don’t travel through glass, so sitting next to a window will not give you enough sunlight to make the vitamin D your body needs.

Why Do You Need Vitamin D?

Your body needs vitamin D to absorb and utilize calcium, which keeps your bones and teeth strong and is essential for normal blood clotting and muscle and nerve function.

A vitamin D deficiency can happen if you don’t get enough sun exposure, if your kidneys cannot convert the storage form to the active form, or if you can’t absorb vitamin D due to problems with your digestive system.

A chronic lack of vitamin D can lead to weakened bones and disease (rickets in children and osteomalacia in adults).

The National Academies of Sciences, Engineering, and Medicine, Health and Medicine Division sets daily dietary reference intakes (DRIs) for vitamins and minerals.

Although sunlight exposure is the primary source, the IOM has set a daily requirement for dietary vitamin D based on age. It’s the same for both males and females.

These DRIs represent an amount needed by a healthy person. If you have health issues, consult your healthcare provider about your vitamin D needs.

Observational studies indicate that having insufficient vitamin D levels may be correlated with an increased risk of cancer, multiple sclerosis, diabetes, and high blood pressure.

Taking supplements to treat these conditions hasn’t been shown to be beneficial. However, vitamin D supplements can be useful for some people, especially if they typically avoid sun exposure or get less during the winter months.

Supplemental vitamin D comes in two forms:

  • Vitamin D2 (ergocalciferol), which is found in plants
  • Vitamin D3 (cholecalciferol), which is found in animals

Your body may absorb vitamin D3 a little better than D2, but most research studies indicate that either type will give you enough vitamin D as long as you follow the directions on the label or advice from your doctor.

A 2017 study that examined the use of juice or cookies fortified with the two forms of vitamin D found that women who were given the D3 form for 12 weeks tended to have higher blood levels of vitamin D than women who had taken D2.2 Still, more research is needed to make any changes in current recommendations.

Taking large doses of vitamin D supplements for extended periods of time may result in vitamin D toxicity, so the Institute of Medicine determined tolerable upper levels.

Vitamin D: Tolerable Upper Limits

Children ages 1-3: 2500 IUs per day

Children ages 4-8: 3000 IUs per day

Children ages 9 and above, teens, and adults: 4,000 IUs per day

Long-term use of dosages above these amounts might cause tissue calcifications that may result in damage to the heart, blood vessels, and kidneys. Don’t use large doses of vitamin D supplements without speaking with your doctor first.

It’s important to note that vitamin D toxicity does not occur from the vitamin D that your body makes when your skin is exposed to the sun.

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