Leading article: An old tonic vindicated anew

The teaspoonful of foul-tasting cod liver oil was a daily ritual for millions of children growing up in post-war Britain in the 1950s. After wartime rationing, the priority was to build up the nation's youth with a diet high in protein and boosted with vitamin supplements to ward off diseases such as rickets, which softened the bones and led to the tell-tale bow legs that had been common in Victorian times.

Half a century later, middle-class, health- conscious families are once again turning to cod liver oil - a rich source of vitamins A and D and omega-3 fish oils - as a protective measure against many of the degenerative conditions of modern life. The translucent amber capsules are now among the most widely consumed vitamin supplements in Britain.

Heart disease, cancer, diabetes, high blood pressure, schizophrenia, multiple sclerosis, osteoporosis and rickets - all have been linked with a lack of vitamin D. Research interest in the vitamin has been fuelled by the recognition that, as the main source is the sun - through its action on the skin - levels drop during the winter months and are generally lower in northern climes.

Although some vitamin D can be obtained from the diet, an estimated 60 per cent of people in Britain are deficient by the end of winter. Some experts have called for the fortification of staple foods such as bread and milk with the vitamin. Supporters of the vitamin D hypothesis include some of the most notable names in medicine. Sir Donald Acheson, the former chief medical officer, published a paper last year showing that people who spent more time in the sun were less likely to get multiple sclerosis, based on a review of 430,000 people treated in Oxford for neurological and immune related diseases over almost 50 years. Sir Donald was one of the first epidemiologists to note in the 1960s that MS was less common among white populations living close to the equator. Interest in vitamin D was also taken up by Sir Richard Doll, discoverer of the link between smoking and lung cancer, who was considering research studies into its role in a range of conditions at the time of his death earlier this year.

And the growing evidence of the vitamin's role in protecting against disease has led cancer specialists around the world to rethink their advice on covering up in the sun. Australia led the way with a statement earlier this year from the Cancer Councils of Australia that "a balance is required" between avoiding skin cancer and obtaining enough vitamin D. Coming from a nation where the sun is notoriously fierce and skin cancer rates are among the highest in the world, this was powerful testimony of the newly recognised importance of vitamin D.