Experts are concerned by the high rates of vitamin D deficiency in the UK.

Vitamin D has fallen under the spotlight recently, with the nutrient playing many important roles in the body, such as supporting the development and maintenance of healthy bones and regulating cell growth.

Increasingly, scientists are linking vitamin D to other roles as well, including the reduction of inflammation, regulation of the immune system and even the prevention of diseases such as cancer, heart disease and multiple sclerosis.

While a small number of foods contain the nutrient, most of our vitamin D is manufactured by the body in response to sunlight; but herein lies a major problem.

With public awareness of the dangers of excessive sun exposure growing, many of us are avoiding sunlight by covering up with long sleeves and high-SPF sunscreens.

As a result, several studies have uncovered high rates of vitamin D deficiency, such as 2011's Feeding for Life Foundation report which claimed that a quarter of all toddlers are deficient.

An independent advisory committee is currently reviewing the government's recommendations on vitamin D supplementation, with the results expected in 2014.

Current advice states that pregnant and breastfeeding women, over-65s and people with little exposure to sunlight should take a daily vitamin D supplement containing 10ug (400 IU).

But is this really enough?

There seems to be little consensus on 'how much is too much' when it comes to vitamin D.

The Department of Health warns that taking too much vitamin D over long periods can cause excessive calcium absorption, potentially leading to kidney damage and harming the bones.

It does not state a maximum tolerable limit, but suggests that taking 25ug (1,000 IU) in supplement form each day "is unlikely to cause any harm".

Other countries have different recommendations, with the European Food Safety Authority advising a maximum tolerable intake of 250ug (10,000 IU) for over-17s.

Health Canada and the US Institute of Medicine advise a tolerable upper intake of 100ug (4,000 IU), although the latter emphasises that the long-term effects of this level of supplementation are unknown and these doses are therefore not recommended.

However, it is quite possible that far higher doses are safe, with published cases of vitamin D toxicity all involving intakes of at least 1,000ug (40,000 IU) per day - well over the government's guideline amount.

And it may well be that as evidence for the importance of vitamin D continues to grow, guidelines will be altered to recommend a far higher level of supplementation for a much greater proportion of the public.