Vitamin D is actually a group of related hormonelike substances, one of which is cholecalciferol, or vitamin D3. It’s often called the sunshine vitamin since your body starts producing this important nutrient every time you take a stroll in the bright light of day. Human skin contains a vitamin precursor, known as a provitamin, which converts to vitamin D when exposed to the sun’s ultraviolet rays. Vitamin D plays several roles in the body; one is to make calcium more easily absorbed into the bloodstream from the intestine.

You can also get vitamin D from your diet, although achieving the levels some experts recommend through food and drink alone isn’t easy if you’re trying to eat sensibly. Buy food or drink that is fortified with vitamin D, as are many breakfast cereal. Fish—particularly oily ones, such as mackerel, sardines, and tuna—have modest vitamin D levels, too. But if you shun the sun, plan on eating a lot of liver, butter, and eggs to meet the minimum daily requirement. Unless that is, you take a vitamin D supplement.


how vitamin d works in the body


How it Works in the Body

Several studies have suggested that taking vitamin D supplements may reduce bone loss and the risk of broken bones in the elderly. Vitamin D deficiency is surprisingly common in the United Kingdom, particularly in the winter—even among apparently healthy people. One survey in the UK showed that about 1 in 5 adults and about 1 in 5 children in the UK have low vitamin D levels.

Without sunshine or a source of ultraviolet light—or in people with very dark skin—vitamin D production is significantly impaired. Vitamin D3 (cholecalciferol) is actually manufactured in the skin when ultraviolet light, either from the sun or a tanning bed, interacts with the enzyme 7-dehydrocholesterol to form it. Then the liver and kidneys take over, converting vitamin D3 into the major circulating, active forms of vitamin D called 25-hydroxy cholecalciferol and 1,25-dihydroxy cholecalciferol.

If vitamin D is ingested from either an animal or plant source (it exists in only minute amounts in the plant kingdom), it is absorbed through the walls of the small intestine with the aid of bile coming from the gallbladder (or the liver, in those people without gallbladders). Some conditions such as food allergies can bind vitamin D and prevent its absorption from the gut.

When vitamin D is ready to use, your kidney ensures calcium and Vitamin D are used efficiently.

Looking into how your body absorbs Vitamin D through food is a tad bit more complex. But we’ll give it a shot. Your body soaks up dietary Vitamin D like it would cholesterol. What happens is your bile breaks down the lipids, which deliver the vitamin to your fatty tissue. Your liver steps in next to clear it and have it set to be metabolized. It then ensures you have enough in your system by reading the amount in your blood, what amount is utilized by your intestine, followed by the amount reabsorbed back into your kidney when urine is being made.

Most people would rather not venture too far into the technical aspect of things. That in itself is a whole separate language. What is important is that you have an idea of what your body does with Vitamin D, because this will help you better recognize if you are getting the amount you need each day to function.

Why Do We Take Vitamin D Supplements?

To prevent osteoporosis. You probably already know that calcium is essential for strong bones. What you may not realize is that eating a calcium-rich diet and popping calcium supplements won’t do you much good if your body is deficient in vitamin D.

What Else Should I Know About Vitamin D Supplements?

Very high doses of vitamin D—in the range of 50,000 IU daily—can be toxic. The body absorbs too much calcium and phosphorus, causing excessive thirst, stomach upset, and depression. Over time, getting too much vitamin D could cause kidney stones. A typical multivitamin provides 400 IU of vitamin D. If you’re under fifty and get outside every day for at least fifteen minutes (without sunscreen, which blocks UV rays), you may not need vitamin D supplements. A complete bone-health regimen should include adequate calcium; unless you consume a lot of dairy products, you should consider adding a calcium supplement.

Bottom line

Vitamin D may have an important role in preventing osteoporosis. Talk to your physician about how, and if, these supplements should be included in a regimen to promote bone health. While you’re at it, be sure to discuss other important interventions that may prevent osteoporosis, including calcium and—for women who have reached menopause—estrogen replacement therapy.