Vitamin D Requirements and Dietary Sources

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Vitamin D is a fat-soluble vitamin. It exists in four different forms: cholecalciferol, calcifediol, calcitriol, and ergocalciferol.

Cholecalciferol: This form is also called vitamin D3, and it's made from cholesterol in your body when your skin is exposed to ultraviolet light. For most of us, it takes about 5 to 30 minutes of sun exposure two days each week to make sufficient amounts of vitamin D though it varies based on weather conditions and time of year.

Cholecalciferol is not biologically active; it has to travel through your bloodstream to the liver where is it converted into another form of vitamin D called calcifediol.

Calcifediol: The storage form of vitamin D is called 25-hydroxyvitamin D or calcifediol. It's also the form of vitamin D that's measured in blood tests when your health care provider wants to determine when you have symptoms of 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.

Calcitriol: 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 the 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.

Ergocalciferol: 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 the calcifediol. It isn't converted to calcidiol as efficiently as cholecalciferol, but it appears to be 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 oily fish and beef liver. 

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) two times every week is enough.

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

The amount of exposure also depends on the time of the year. In the northern hemisphere, the UVB rays are more intense 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) during the winter months, even if you do go outside every day.

The intensity of UVB rays is also reduced by clouds and pollution. The UVB rays will not travel through glass so sitting next to a window will not give you enough sunlight to make vitamin D.

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 from due to problems with your digestive system. A chronic lack of vitamin D leads to weakened bones and diseases called rickets in children and osteomalacia in adults.

The National Academies of Sciences, Engineering and Medicine, Health and Medicine Division sets daily dietary reference intakes for vitamins and minerals. Although sunlight exposure is the primary source, the IOM has set a daily requirement for dietary vitamin D that's based on age. It's the same for both males and females.

These DRIs represent an amount needed by a healthy person, so if you have any health issues, you should consult your health care provider.

Dietary Reference Intakes

  • Up to age 70: 600 International Units (IUs) per day
  • Ages 71 and older: 800 IUs per day

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

But so far, taking supplements with the idea of treating these conditions hasn't been shown to be beneficial.

Vitamin D supplements may be useful for some people, especially during the winter or for those who typically avoid sun exposure. You can choose from two forms, either vitamin D2 (ergocalciferol, the form found in plants) or vitamin D3 (cholecalciferol, the type found in animals). Your body may absorb vitamin D3 a little better that 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.

One 2017 study 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 compared to women who had taken D2, however 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. 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.

Tolerable Upper Limits

  • Ages 1-3: 2500 IUs per day
  • Ages 4-8: 3000 IUs per day
  • Ages 9 and above: 4,000 IUs per day

Vitamin D toxicity does not occur from the vitamin D that your body makes when your skin is exposed to the sun.

Sources:

Tripkovic L, Wilson LR, Hart K, Johnsen S, de Lusignan S, et.al. "Daily supplementation with 15 μg vitamin D2 compared with vitamin D3 to increase wintertime 25-hydroxyvitamin D status in healthy South Asian and white European women: a 12-wk randomized, placebo-controlled food-fortification trial." Am J Clin Nutr. 2017 Jul 5. pii: ajcn138693. doi: 10.3945/ajcn.116.138693.

National Academies of Sciences, Engineering and Medicine, Health and Medicine Division. "Dietary Reference Intakes Tables and Application."

National Institutes of Health, Office of Dietary Supplements. "Vitamin D - Health Professional Fact Sheet." 

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