Twenty minutes' lying in the sun this weekend could provide your best chance of avoiding colds and flu, according to new research which demonstrates that vitamin D, not vitamin C, provides the most efficient protection against cold viruses.
The exceptional spring weather, which is forecast to continue into next week with a high of 24C today, will offer the best opportunity so far this year to top up D-levels that have become depleted over the winter, scientists say.
Vitamin D is created by the action of sunlight on the skin and levels in all UK residents are at their lowest at this time of year, after the long winter. Short days and cloudy skies mean 60 per cent of the British population are deficient by the start of spring.
The malign consequences have been revealed by in a study from the United States which shows that boosting vitamin D may be the most effective way of warding off infections that cause winter colds.
The authors, from Winthrop University Hospital, Mineola, New York, who publish their findings in the journal Epidemiology and Infection, say vitamin D stimulates "innate immunity" by activating peptides in the body that attack bacteria, fungi and viruses.
"Vitamin D supplementation, particularly with higher doses, may protect against the typical winter cold and flu ... Since there is an epidemic of vitamin D insufficiency in the US, the public health impact of this observation could be great," they write.
Traditional advice has been to swallow large doses of vitamin C at the first sign of a sniffle. But the latest findings suggest we may have been turning to the wrong vitamin. The revelation is the latest addition to a long list of scientific studies highlighting the beneficent qualities of a previously underrated health resource.
Vitamin D has been described as "the wonder vitamin" after a 40-year review of research found a daily dose could halve the risk of breast and colon cancer. It has also been shown to play a vital role in heart disease, cancer, diabetes, high blood pressure, schizophrenia and multiple sclerosis as well as being essential of bone health.
Widespread deficiency in the vitamin among populations in the northern hemisphere suggests it may account for several thousand premature deaths from cancers alone, American scientists from the University of San Diego said in the American Journal of Public Health in December 2005. They reviewed 63 scientific papers and concluded "public health action" to boost vitamin D levels was needed.
High rates of heart disease in Scotland have been blamed on the weak sunlight and short summers in the north. Some experts believe the benefits of the Mediterranean diet have as much to do with the sun the as with the regional food.
The evidence of its role in fending off two of the commonest infections in the western world - colds and flu - has come from research by US scientists who gave supplements of the vitamin to 208 women over three years in a randomised controlled trial. Half the women were given a 20-microgram dose of vitamin D, increased to 50 micrograms after two years, and the other half were given a placebo. Those who took the genuine vitamin reported a 70 per cent reduction in colds and flu, from 30 episodes to nine over the three-year study.
All the women were Afro-Caribbeans who were being tested with vitamin D supplements to see if it prevented bone loss, which is a common problem following the menopause. People with dark skins make less vitamin D when exposed to the sun and consequently tend to have lower levels. The startling results offer a new motive to seek, in moderation, the sunlight.
Twenty minutes in the sun, with the hands, arms and face exposed, is all that is needed to get an adequate dose, but it needs to be regularly topped up through the summer. In winter the only way of maintaining levels is by taking cod liver oil or supplements.
Many countries have modified their warnings about the dangers of sunbathing in the light of the growing evidence for the benefits of vitamin D. Australia preached avoidance of the sun with its "slip, slap, slop" campaign to cover up and use sunscreen. But the Association of Cancer Councils of Australia acknowledged two years ago that some exposure to the sun was necessary to achieve adequate vitamin D levels.
There are also reports that vitamin D boosts sports potential. If proved, the findings could have important repercussions for Olympic training schedules.
The decline in vitamin D levels during the winter, which mirrors the incidence of colds and flu, could be the "seasonal stimulus" that accounts for the peak in infections between November and March, according to a study in the American Journal of Public Health by researchers at Harvard School of Public Health published last year. "Vitamin D deficiency predisposes children to respiratory infections. Ultraviolet light ... reduces the incidence of viral respiratory infections, as does cod liver oil," they write.
* It takes 20 minutes in full sun for a person with white skin to make the maximum amount of vitamin D that can be generated in one day, but begin cautiously with just a few minutes' exposure.
* Cover-up, apply high-factor sunscreen or seek shade if you feel any unpleasant burning or baking sensations.
* Babies can be taken into weak sunlight (avoiding 11am to 3pm) in only their nappies (no suncream) for five to 10 minutes a day, according to Professor Nick Bishop of Sheffield University.
* Some experts suggest allowing children to play in weak sunlight, but to avoid exposure around midday. Others believe that children should be exposed in a swim suit to full midsummer sunlight for 15 to 30 minutes a day, provided they don't burn.
The lowdown on vitamins
* VITAMIN A
Essential for: Healthy skin and mucous membranes.
Sources: Cheese, eggs, oily fish, milk, margarine, yogurt.
How much you will need: 0.7mg a day for men and 0.6mg for women. Can be stored in the body.
* VITAMIN B6
Essential for: Formation of haemoglobin, the constituent of red blood cells.
Sources: Pork, chicken, bread, eggs, vegetables, milk.
How much you need: 1.4mg a day for men and 1.2mg for women. Water soluble, so needed every day.
* VITAMIN B12
Essential for: Making red blood cells, keeping the nervous system healthy, releasing energy from food.
Sources: Most meat, cod, milk, salmon, cheese and eggs.
How much you need: 0.0015mg a day. Water soluble, so needed every day.
* VITAMIN C
Essential for: Protecting cells, and helping the body to absorb iron from food.
Sources: Fruit and vegetables, such as broccoli, peppers, Brussels sprouts, oranges and kiwi fruit.
How much you need: 40mg a day for adults.
* VITAMIN E
Essential for: Protecting cell membranes; acts as an antioxidant.
Sources: Avocado, corn, olive and soya oils, nuts and seeds.
How much you need: 4mg a day for men, 3mg a day for women.
* VITAMIN K
Essential for: Blood clotting and healing.
Sources: Green leafy vegetables, vegetable oils; cereals.
How much you need: 0.001mg a day per kg of body weight for adults.
* FOLIC ACID
Essential for: Forming red blood cells and reproduction.
Sources: Broccoli and Brussels sprouts, peas, Marmite, brown rice and some fruit.
How much you need:0.2mg a day. It is water-soluble.
Essential for: Breaking down fat, protein and carbohydrate in the diet.
Sources: Beef, pork, chicken, wheatflour, eggs and milk.
How much you need: 17mg a day for men, 13mg for women.
Essential for: Keeping nerves and muscles healthy.
Sources: Pork, vegetables, milk, cheese, fruit, wholegrain bread and fortified cereals.
How much you need: 1mg a day for men; 0.8mg for women.
Essential for: Healthy skin, eyes, nervous system and mucous membranes.
Sources: Milk, eggs, rice, mushrooms; fortified cereals.
How much you need: 1.3mg a day for men, and 1.1mg a day for women. It is water-soluble so is needed every day.