When the Chief Medical Officer in England, Dame Sally Davies, announced last week that she is worried that young children and some adults are not getting enough vitamin D, she gave official recognition to growing concern among health professionals that deficiency of this vitamin is a very significant issue in the UK.
Perhaps most worrying is the resurgence of rickets, a bone disease caused by serious vitamin D deficiency which is now being diagnosed in hundreds of young children every year. But vitamin D deficiency is also increasingly being identified in the elderly, where its myriad effects, from bone and muscle weakness to disruption of the immune defences, are contributing to falls, fractures, infections and many other life threatening conditions.
Cardiovascular disease, cancer, eye disease, multiple sclerosis and autoimmune disease have all been linked to low levels, and research continues to identify new problems. Scientists at the University of Copenhagen recently discovered that vitamin D is vital for the immune system where it activates T-lymphocyte 'killer' cells to fight off serious infections. Although the economic impact is unknown, there's no doubt that vitamin D deficiency is making its mark on the health of the nation.
The problem lies partly in the mantra taught from school-age onwards - that we can get most of the vitamin D we need from the action of sunlight on our skin. A regular dose of sunshine is all you need to keep levels topped up. But recent evidence shows that in northern Europe we can't rely on the sun to meet our needs.
Vitamin D is measured either in international units (IU) or micrograms (μg) and you might see either measure on a label (40 IU is equal to 1μg). The recommended daily intake (RDA) in the UK is 400 IU/day (10μg) for an adult and 280 IU/day or 7μg for children aged six months to three years.
In the summer, about 30 minutes of sunlight on the face and forearms at around noon, two-three times a week, generates about 2000 IU of the vitamin in fair-skinned people, which is an effective dose.
But many people, such as those with darker skin and the elderly may need up to 10 times this exposure. Even then, geography plays against us, with 90% of the UK lying above the latitude that permits adequate exposure to the ultraviolet B wavelengths necessary for vitamin D synthesis in the skin.
So try measuring vitamin D levels in the low light of winter and the picture is very different. For at least six months of the year as many as 50% of the UK population have insufficient levels and 16% have a more serious deficiency.
Multiethnic populations may show much higher rates - in one London study, more than 90% of people of South Asian origin had insufficient or deficient levels. And cloudy summer weather, a mostly indoors lifestyle (especially for older people) and the emphasis on using sunblock to avoid skin damage, means that insufficiency may occur year-round.
So we need to look more closely at the other main source of vitamin D - our diet. But not many foods naturally contain significant amounts of vitamin D. The main dietary source is oily fish, with up to 400 IU per 100gm, but few people have more than one serving a week. Egg yolk, liver, and wild mushrooms all contain a little but there is negligible amounts in most vegetables. Surveys confirm that dietary intake is only around 80-160 IU/day. As a result, a growing number of experts believe that most people may need vitamin D supplements to guarantee that they get the recommended daily intake. And some question whether the RDA levels are set too low - perhaps the bare minimum needed to stave off bone disease, but insufficient for optimal health in other body systems. In the USA, the RDA has been increased in recent years to 600IU/day from the age of one and 800IU/day for those over 70.
A government committee is currently addressing the issue here to clarify exactly how much vitamin D we need for health, and how we can get it but it is not due to report until 2014. Meanwhile the Department of Health recommends that vulnerable groups such as those aged under five and over 65, or pregnant and breast feeding women should take vitamin D supplements.
Some believe that the best way forward is to supplement basic foods - in Europe fortified milk is widely available but in the UK only infant formula milk and margarine have statutory vitamin D supplementation.
Manufacturers are beginning to realise the health benefits of vitamin D - for example Actimel are now adding vitamin D to their drinking yoghurts while an increasing number of breakfast cereals contain it. But while cereal and yoghurts are popular in the UK, a wider range of foods may need to be supplemented to reach all those at risk especially, for example, ethnic minority groups with very different diets. The only other alternative may be more widespread use of daily vitamin D supplements.