Not unexpectedly sunscreen on the skin blocks this production. The remainder of our body's supply of this vital vitamin comes from a limited number of foods.
The main role of vitamin D is thought to promote intestinal absorption of calcium. Calcium is crucial for bone development. Children without enough vitamin D suffer from a disorder of abnormally soft bones known as rickets resulting in extreme bowing of the legs and dental problems. Adults with vitamin D deficiency can develop low blood calcium levels, osteoporosis, as well as softening of the bone known as osteomalacia.
Recent studies however, have shown that Vitamin D may serve many other important functions in the body.
Several investigators found that low Vitamin D plays a role in "seasonal affective disorder" which is a form of depression occurring during the winter when little sunlight is available.
Low levels of Vitamin D can be found in Alzheimer's patients, and some researchers believe low levels of this vitamin may contribute to loss of muscle tone and frequent falling in the elderly.
Recent studies link low Vitamin D levels in pregnant women to the development of preeclampsia (toxemia of pregnancy).
Vitamin D was also shown to help lung function and to delay a common but serious eye disorder known as macular degeneration.
Studies also show that Vitamin D may help benefit the immune system and help reduce the incidence of certain cancers.
Recent studies suggest vitamin D may help prevent or delay colon, breast and ovarian cancer. One study showed that 1000 IU of vitamin daily could cut the risk of colon cancer by 50% while another study found a similar risk reduction for pancreatic cancer in those who took vitamin D supplementation. Results of these studies are still inconclusive and require verification but point to an important role of vitamin D in cancer protection.
Until recently the recommended daily allowance of Vitamin D was 400 IU.
Some researchers now state that this amount is too low and advocate a daily intake of 1000 IU. Vitamin D can be found in Vitamin D supplemented milk, many multivitamins and calcium supplements. Yoghurt, margarine, cooking oils, breakfast cereal and bread may also be fortified with vitamin D. Oily types of fish such as salmon, tuna, and sardines are excellent sources of vitamin D. In particular the fish liver stores much of the vitamin, hence "cod liver oil" is an excellent vitamin D source.
The vitamin D we ingest in pills and food is an inactive form of the vitamin which then must be activated in the kidney. For this reason those with kidney disease frequently must receive medication with activated vitamin D to prevent bone disease or other conditions related to low calcium levels in the blood.
A note of caution about vitamin D is sounded by some experts. While there is still plenty of debate about the amount of vitamin D which could be toxic, most authorities believe that up to 2000IU daily should be safe.