Vitamin D is actually a term for a group of hormones that are stored mainly in the liver, as well as fat and muscle tissue. It is one of three vitamins naturally manufactured by the body, and it is produced by a chemical reaction to the ultraviolet radiation contained in sunlight.

Why do we need it?

Vitamin D increases the body's absorption and metabolism of calcium and phosphorus. This makes it essential to maintaining strong, healthy bones and teeth.

How much vitamin D should I take?

According to the National Academy of Sciences, the recommended daily allowance (RDA) for vitamin D is as follows:

• Adult men: 200 international units (5 micrograms)/day
• Adult women: 200 international units (5 micrograms)/day
• Adults age 51-70: 400 international units (10 micrograms)/day
• Adults 71 and over: 600 international units (15 micrograms)/day
• Children aged 7-10: 200 international units (5 micrograms)/day
• Infants: 200 international units (5 micrograms)/day
• Pregnant/lactating women: 200 international units (5 micrograms)/day

What are some good sources of vitamin D?

Exposure to sunlight is the easiest way to build up stores of vitamin D. By exposing the face, hands and forearms for between 15-20 minutes two or three times per week, most people can manufacture all the vitamin D they need.

Vitamin D is also found in a number of food products, most notably vitamin D-fortified milk. Other sources include egg yolks, fish, cheese, fortified cereals and liver.

What can happen if I don't get enough vitamin D?

Vitamin D deficiency can result in bone-related disorders such as rickets in children and osteomalacia in adults. Vitamin D deficiency also increases the risk of hip fractures in postmenopausal women, and has been linked to higher incidences of prostate cancer and breast cancer.

What can happen if I take too much vitamin D?

High doses of vitamin D can be very toxic. In children, large doses can cause mental retardation, stunted growth and kidney failure. In older children and adults, too much vitamin D can result in weakness, anorexia, nausea, diarrhea and changes in a person's mental state. With the exception of kidney failure, low-calcium diets and withdrawal of vitamin D from a person's diet can usually reverse these side-effects.