Vitamin D is unlike other vitamins in that our bodies synthesize it as a result of the action of sunlight on our skin. Ultraviolet rays initiate the production of vitamin D from cholesterol which is stored in the skin's oil glands.

Recent studies indicate that vitamin D plays an important role in keeping our eyesight sharp. Vitamin D is also needed for the growth and development of bones and teeth. It is involved in cell growth, regulating the immune system and blood sugar levels, as well as maintaining a healthy nervous system. Vitamin D is necessary for your body's effective absorption of calcium, magnesium and phosphorous.

Vitamin D deficiency causes rickets (malformed bones) in children and osteomalacia (softening of bone) in adults.

Since few foods naturally contain vitamin D, most milk is fortified with the vitamin. Also, liver, egg yolks, cheese, cold-water fish and fish liver oils are good sources of vitamin D. Caution should be exercised when taking vitamin D supplements -- excess vitamin D can be toxic.

The sun is our main source of vitamin D but we must include some in our diet, too -- especially if your nutritional score indicates a Vitamin D imbalance. You can get it from vitamin D fortified milk, eggs, cheese, yogurt, sardines, mackerel, herring, tuna (and other cold water fish) and beef or lamb liver. If these foods aren't in your diet or if you live in a cold climate, or seldom go out in the sun, you would benefit from a supplement. A good multivitamin/mineral tablet with 400 I. U.'s of vitamin D is a good choice.

Alcohol interferes with the conversion of vitamin D into its usable form in our body. As we age we absorb and synthesize less vitamin D so a vitamin D supplement is a good idea for the elderly. Cod liver oil capsules can be a good source, but they can become rancid if stored improperly or for too long. Don't take them if they have a strong, fishy smell. Vitamin D is stored in the body and you can overdose on supplements -- even cod liver oil, so don't take high doses of this vitamin.