Ask the Doctors by Eve Glazier, M.D. and Elizabeth Ko, M.D

Adequate Amount of Vitamin D Is Essential to Good Health

Dear Doctor: Why is it that vitamin D is so important? I know you can get it from food and supplements, but I'd like to get it the old-fashioned way -- from the sun. Is that too dangerous?

Dear Reader: It can't be overstated: Vitamin D is critical to good health. It's needed for calcium absorption from the intestinal tract, and for the regulation of blood phosphorus levels. Both of these are essential to developing and maintaining strong and healthy bones. Even when you get enough calcium and phosphorus in your diet, you can't absorb it unless you're also getting enough vitamin D.

Without adequate vitamin D, bones become thin and brittle and can easily bend or break. Rickets was once a widespread disorder in which children's bones were so thin and spongy from a lack of the vitamin that their skeletons failed to develop properly. In some cases, their legs became visibly bowed. Since the addition of D to milk, yogurt, breakfast cereals and orange juice, rickets has largely been wiped out.

Vitamin D also plays a vital role in brain development, muscle function, maintaining a healthy respiratory and immune system, and in optimal cardiac function. Recent research has also uncovered a potential link between a deficiency of vitamin D and depression. In fact, we now know that vitamin D receptors are present in virtually every tissue, so we fully expect research to continue to discover even more ways in which the vitamin is essential.

As you point out in your question, our bodies manufacture vitamin D in response to sunlight. That's why it's also known as the "sunshine vitamin." Specifically -- and this is a bit of a deep dive, but we think it's fascinating -- the process of vitamin D synthesis begins when ultraviolet rays strike the skin. This triggers a chemical process known as hydroxylation, which involves the liver, kidneys and certain cellular structures, and ultimately creates the chemical compound that we refer to as vitamin D. This process of synthesis also takes place with the vitamin D that we get from food and supplements.

The challenge is that getting enough D from sun exposure can take vigilance. Variables include air pollution, cloud cover, latitude, time of year, time of day, clothing, the use of sunscreen, and an individual's skin type, age and lifestyle. Each of these can make it more difficult to get enough D.

To trigger vitamin D synthesis in the body, the general recommendation is to spend between 10 to 30 minutes with arms, legs and/or torso exposed to sunlight during peak hours -- without sunscreen -- two to three times per week. Melanin protects the skin from sun damage, so darker skin requires longer exposure. For those who prefer not to deal with the potential risks of sun exposure, good dietary sources of D include the fortified foods we just mentioned, as well as fatty fish such as salmon, tuna and mackerel, and fish liver oils. The vitamin is also present in beef liver and egg yolks, but in small amounts. And if you decide to take supplements, please be sure to follow the dosage guidelines. For all its many benefits, it's possible to get too much vitamin D.

(Send your questions to, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)