VITAMIN D is a fat-soluble vitamin that is naturally present in very few foods, added to others, and available as a dietary supplement. It is also produced by the body when ultraviolet rays from sunlight strike the skin and trigger vitamin D synthesis.

However, vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation.

The first occurs in the liver and converts vitamin D to 25-hydroxy vitamin D, also known as calcidiol. The second occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D also known as calcitriol.

Vitamin D is essential for promoting calcium absorption in the intestines and maintaining adequate serum calcium and phosphate concentrations to enable normal mineralisation of bone and prevent hypocalcemic tetany.

It is also needed for bone growth and bone remodelling by osteoblasts and osteoclasts. Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults.

Together with calcium, vitamin D also helps protect older adults from osteoporosis.

We are now going beyond the skeletal system and going into the cardiovascular system. Vitamin D it appears, is good for the heart.

A new study presented on Nov 16 at the American Heart Association's Scientific Conference in Orlando, Florida, confirmed a strong association between the presence of reduced vitamin D levels and a greater risk of coronary artery disease, stroke, heart failure and dying among men and women 50 years of age and older.

This is both new and exciting. We seldom talk about vitamin D in association with heart disease.

Dr Brent Muhlestein and his colleagues at Intermountain Medical Centre in Salt Lake City followed 27,686 subjects with no history of heart disease for an average of 1.2 years. Serum 25-hydroxyvitamin D levels obtained during routine clinical care were classified as normal at over 30 nanogrammes per milliliter (ng/mL), low at between 15 to 30 ng/mL or very low at less than 15 ng/mL.

Over the follow-up period, 2,614 participants developed coronary artery disease, 1,742 developed heart failure, 314 experienced a stroke and 1,193 deaths occurred.

Those with very low vitamin D levels were 45 per cent likelier to develop heart disease, twice as likely to develop heart failure, 78 per cent more likely to experience a stroke, and 77 per cent likelier to die than those with normal levels.

It was notable that subjects whose vitamin D levels were classified as "low" as opposed to "very low" also had greater risks of these conditions, however, the increase compared to those with normal levels was not as great as the very low group.

"This was a unique study because the association between Vitamin D deficiency and cardiovascular disease has not been well-established," commented Dr Muhlestein, who is the director of cardiovascular research of Intermountain Medical Centre's Heart Institute.

"Utah's population gave us a unique pool of patients whose health histories are different than patients in previous studies," he said.

"For example, because of Utah's low use of tobacco and alcohol, we were able to narrow the focus of the study to the effects of vitamin D on the cardiovascular system.

Co-author Heidi May, PhD, who is an epidemiologist with the Intermountain Medical Centre research team noted: "We concluded that among patients 50 years of age or older, even a moderate deficiency of Vitamin D levels was associated with developing coronary artery disease, heart failure, stroke, and death."

"I think that this study is important because vitamin D deficiency is easily treated. Vitamin D is also somewhat inexpensive and widely available."

When you consider that cardiovascular disease is the leading cause of death in much of the industrialised world, you could understand how this research can help improve the length and quality of people's lives.

Very few foods in nature contain vitamin D. Fish meat (particularly salmon, tuna, and mackerel) and fish liver oils are among the best sources.

Small amounts of vitamin D are found in beef liver, cheese and egg yolks. Vitamin D in these foods is primarily in the form of vitamin D3 (cholecalciferol) and its metabolite 25(OH)D3.

Fortified foods provide most of the vitamin D in the American diet. For example, almost all of the US milk supply is fortified with 100 IU/cup of vitamin D.

In the 1930s, a milk fortification programme was implemented in the United States to combat rickets, then a major public health problem. This programme virtually eliminated the disorder at that time.

Other dairy products made from milk, such as cheese and ice cream, are generally not fortified.

Ready-to-eat breakfast cereals often contain added vitamin D, as do some brands of orange juice, yogurt, and margarine. In the US, foods allowed to be fortified with vitamin D include cereal flours and related products, milk and products made from milk, and calcium-fortified fruit juices and drinks.