Vitamin D deficiency which can lead to osteopenia, osteoporosis, muscle weakness, fractures, common cancers, autoimmune diseases, infectious diseases and cardiovascular diseases, has been found to be lacking particularly in inhabitants of colder climates.

*A recent study, for example, indicated that the median adult intake of vitamin D in the US is only 230 IU per day, versus the researchers' recommended 2000 IU per day.

Dutch researchers have also linked vitamin D deficiency to depression in older adults.

The researchers still had many unanswered questions, but noted that low levels of vitamin D lead to overproduction of a parathyroid hormone, which has been linked to signs of depression. The researchers are planning further tests to determine whether the low D levels are a cause or the affect of the depression.

In a separate study, a Canadian research team led by Dr. Pamela Goodwin studied the vitamin D levels in breast cancer patients.

The team found patients with sufficient levels of vitamin D were less likely to die of breast cancer or have the cancer spread.

This may be because vitamin D works in the body to block the growth of abnormal cells and also hinders the type of blood vessels that feed tumours.

With almost 50,000 deaths in the U.S. each year from breast cancer alone, these findings could have a huge impact on American health.

Weatherford's Dr. Nusrat (Ness) Khan, M.D. FAAP, added previous research suggests calcium and vitamin D could connect to receptors on some tumours causing the tumour cells to slow down production, and in some cases will cause the cells to die (a process called apoptosis).

Kahn also cited a 2006 study which found appropriate levels of calcium and vitamin D decreased polyps in the colon, and another study which indicated appropriate levels of calcium decreased the risks of breast cancer in pre-menopausal women.

"An appropriate amount of calcium and vitamin D - about 1,000 to 1,200 milligrams of calcium, and 400 International Units (IUs) of vitamin D - does have a protective effect on osteoporosis and maybe on pre-menopausal breast cancer and colon cancer," Khan summarised.

"But just because some is good, more is not necessarily better," he said.

Since there is no evidence showing increasing vitamin D through sun exposure outweighs the risks of developing skin cancer, using a vitamin D supplement is the safest way to be sure you're getting the required daily amount.

The key to maintaining healthy amounts of calcium and vitamin D lies in following recommended amounts and in consulting a physician for individual needs.

Khan also emphasised how people receive their vitamins and minerals is very important.

"We must emphasise natural sources," he said. "From a nutritional standpoint, we should rely first and foremost on our diet, and secondly on vitamin supplements."

For calcium and vitamin D, Khan recommends dairy products like milk and yoghurt. Seafood is also a good source of D.

Khan recommends using vitamin supplements to make up for what diet doesn't provide.

Vitamin D inadequacy can be easily and inexpensively treated using a prescription supplement, once or twice a week for four to six weeks, according to Hooten.

Deficiency in vitamin D affects inhabitants of colder climates, because sunlight induces synthesis of vitamin D in humans. As such, if a consumer is not getting adequate exposure to the sun for vitamin D and its subsequent impact on their serum 25 (OH) D levels, they should be supplementing with vitamin D.

Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive. The latter is derived from plants and only enters the body via the diet.