What Does It Do?
The most important thing vitamin D does is to maintain blood levels of calcium within normal limits. It does this by regulating the way the intestines absorb calcium and phosphorus from food. Without vitamin D, a deficiency of dietary calcium will cause the body to rob the bones of calcium to maintain appropriate calcium blood levels.
Vitamin D also may reduce the risk of colon cancer, breast cancer and arthritis. Studies to investigate these possible benefits of vitamin D are under way.
Where Do You Get It?
Most foods have little or no vitamin D, and few foods besides milk are fortified with it. So where do we get the vitamin D that is necessary to maintain blood levels of calcium? Actually, our bodies can manufacture vitamin D for us when the skin is exposed to adequate sunlight. Here's how it works:
- High-energy ultraviolet light penetrates the skin.
- The UV-B converts a precursor in the body into a form of vitamin D.
- The liver and kidneys act on this substance to turn it into the active form of vitamin D.
To ensure that this important process occurs, you need to expose your hands, face and arms to the sun for 10-15 minutes, two or three times a week, between 8 a.m. and 4 p.m. (The exact amount of time you need depends on the sensitivity of your skin to sunburn and on your latitude.) Using sunscreen during this brief period may interfere with the process.
Although not many foods are fortified with vitamin D, those that are can provide some of (and in the case of milk, a great deal of) the vitamin D you need.
- Milk leads the list, with 100 IU (international units) per cup.
- Some cereals and breads are fortified with small amounts of vitamin D, up to 15 IU.
- Margarine is fortified with 20 IU per teaspoon.
Other foods that contain vitamin D:
- Cod liver oil contains vitamin D, but in excess it leads to toxic levels of Vitamin A.
- Some fatty fish are sources, but unless you eat them often you will not get enough.
- Eggs provide a little vitamin D (27 IU).
People sometimes mistakenly think that some foods that contain milk have vitamin D. Remember that these dairy products do not contain vitamin D:
Multivitamins or other supplements are a source of vitamin D (200400 IU). Many calcium supplements have extra vitamin D added (100125 IU per tablet). Note: If you are taking a supplement blend, read the label carefully. If the ingredient list includes "core level products" or "glandular tissue," do not take it. It may contain excessive amounts of vitamin D.
How Much Do We Need?
- If you are under 50 years of age, 200 IU of vitamin D a day are enough.
- If you are 5170, you need 400 IU daily.
- If you are over 70, you need 600 IU of vitamin D every day.
Not getting enough vitamin D can cause serious health problems, including:
- rickets in children
- osteomalacia, a mineralization defect in adults
- the acceleration of bone loss and risk of fracture in adults, especially in post-menopausal women
- arteriosclerosis, or hardening of the arteries
Is It Safe?
Excess vitamin D is toxic. You should get no more than 1,000 IU per day from milk, food and supplements. The very upper limit of safe intake from these sources is 2,000 IU daily. Too much vitamin D causes birth defects. It also can lead to excessively high blood calcium levels, which could cause calcium deposits in the kidneys and arteries.