Roger Hanson: The vital role of vitamin D
Like most organs of the human body, the skeleton comprises billions of cells that undergo a continuous repair and replacement programme. New bone is deposited by osteoblast cells and and old bone is broken down by osteoclast cells.
The main chemical element present in bone is calcium; vitamin D promotes calcium absorption in the intestines.
It also promotes resorption of damaged bone material, allows the body to maintain calcium and phosphate levels and and it enables the parathyroid hormone to maintain the required calcium levels in the blood.
The vital role of vitamin D in the complex biochemistry of bone maintenance and remodelling is the reason that deficiency in this vitamin produces bone related diseases such as rickets and osteoporosis.
* New research suggests most people gain nothing from vitamin D supplements
* Dr Tom Mulholland: Low vitamin D? I prescribe a sunny holiday
* Most vitamins are useless, but these are the ones worth taking
There are several types of vitamin D, but it is D2 and D3 which are most important in humans. D2 is mainly ingested from diet; the major source of D3 is by synthesis (building of its chemical structure) in the skin from sun exposure.
Surprisingly, vitamin D isn't actually a vitamin, strictly speaking it is a hormone because its synthesis and activity occur at different sites.
Before it can be used by the body, the vitamin D molecule has to be activated by enzymes present in the liver and kidney.
Vitamin D is synthesised biologically using a photochemical process. We and other land animals acquire vitamin D either by sun exposure or by ingesting vitamin D present in some foods.
A specific wavelength of ultra-violet light called UVB radiation is required for the chemicals in skin to synthesise vitamin D. Exposure to sunlight through windows doesn't allow skin to synthesise the vitamin because UVB wavelengths are absorbed by glass.
Vitamin D was discovered in the early 20 th century when American researchers noticed that dogs fed with cod liver oil didn't develop rickets. Cod liver oil was known to be a source of vitamin A.
The conclusion was that vitamin A prevented rickets, however in 1922 Elmer McCollum removed vitamin A from cod liver oil and was surprised to see that dogs were still cured of rickets. He had discovered a new vitamin which he called vitamin, D, since it was the fourth to be named.
Sunscreen is designed to block the sun's rays and although very important as a means of preventing skin cancer, some sun exposure is healthy.
Melanin present in the skin acts as a natural sunscreen; people with darker skin have more melanin and there is evidence that dark skinned people need to be exposed to the sun longer than fair skinned people to make the same amount of vitamin D.
According to Mayo Clinic researcher Dr Michael Holik, between 5 and 30 minutes sun exposure on your arms and legs is needed twice per week, depending on your location, to top up your vitamin D.
People living in latitudes south of 40 degrees should be aware that in the winter months the reduced sun exposure may be a problem.
The body has mechanisms to deal with vitamin D produced by extended exposure to sunlight, this means that toxicity from vitamin D produced by sunlight is extremely rare. The elderly are more vulnerable to bone disease and bone fractures, partly because the body becomes less efficient with age at synthesising vitamin D.
Vitamin D is not present in many foods but is present in mushrooms, cod liver oil, beef liver, salmon, canned sardines and mackerel. 100gms of cooked pink salmon contains over 200 micrograms of vitamin D.
Researchers at Stanford and Shanghai universities have shown evidence for a link between colon cancer and vitamin D deficiency. The association with other cancers has also been shown.
There is no international agreement on the recommended dietary allowance for vitamin D, however the New Zealand government recommendation is between 5 and 15 micrograms per day depending on age, 5 micrograms for babies and 15 for anyone over 70.
Although there is little risk of having too much vitamin D as a result of exposure to the solar radiation, it must always be remembered that excessive exposure to sunlight can result in DNA damage to skin, cancer, and premature ageing of the skin.