The main source of vitamin D is exposure to ultra-violet radiation in sunlight, which produces vitamin D in the skin.  A small amount of vitamin D can also be obtained from the diet.

The researchers from the Northern Ireland Centre for Food and Health are to embark on a three year project in collaboration with colleagues from University College Cork to investigate what level of dietary vitamin D is needed to maintain good vitamin D status in the winter time.

Dr Julie Wallace, a senior research fellow at UU, said: "For six months of the year - roughly from the end of September until mid-March - we cannot make vitamin D through exposure to sunlight - even if we were out all day every day, because the sun is too low in the sky.  Our summertime levels will last for part of the year, but by the end of winter we know from our own research that those stores are low. "Traditionally the lack of Vitamin D was thought to be a problem for institutionalised elderly people who did not get exposure to sunlight, but more and more we are realising that it is a problem for the whole population."

She said that the use of sun creams and moisturisers with a sun protection factor also reduces UV absorption and so vitamin D production. "While we only need five or 10 minutes a day exposure of sun on our face and arms during the summer time to obtain the required daily amount of Vitamin D, the risk of skin cancer has meant that people are rightly cautious about sun exposure.  Also our lifestyles have changed, we spend more time in front of the TV or computer; traditionally we may have spent more time outdoors".

The three year project, which will also establish if there are differences in vitamin D levels between people living in Northern Ireland and those living in the sunnier southern climes of Cork.  To date the research team at UU has recruited over 120 adults aged 20-40 years who will be provided with vitamin D supplements from October to March to calculate how much vitamin D is needed from the diet to maintain our summer time levels.  Next year the study will move on to the 65-plus age group. The research is being funded by the Food Standards Agency.

Dr Wallace added: "While in the current study we are focusing on these two groups, we believe vitamin D research is also required in other groups including children, adolescents and pregnant women.  Unfortunately, there are very few foods that are good sources of vitamin D, and those that are, such as oily fish, are not widely consumed so it is actually quite difficult to get vitamin D from food alone.  For groups at risk of low vitamin D status it may be that supplements are needed."

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