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AVOIDING sunlight may damage the health, especially in the elderly, according to a recent research project covering 19 towns in 11 countries. Old people who shunned the sun had low blood levels of vitamin D, increasing the risk of the bone diseases osteoporosis and osteomalacia.

Advice from doctors about sunlight is one of the clearest examples of the ways that medical opinion can change within a few years. In northern Europe for most of this century exposure of the skin to sunlight was seen as health-promoting. People were encouraged to sunbathe: sunlight was classed with fresh air and exercise as naturally protective against diseases, especially those associated with overcrowded, smoke- darkened cities: tuberculosis and rickets.

This belief in the life-enhancing qualities of sunlight was not shared in southern Europe, where people built their houses to provide shade and took a siesta when the sun was at its strongest.

Warnings of the dangers of sunlight began long before scientists began to worry about the ozone layer. Research in Aus-tralia in the 1950s and 60s showed that Europeans with pale skins and blue eyes were especially prone to develop skin cancers if they spent a lot of time in the sun, and the Queensland Melanoma Project led the way in teaching people how to prevent skin cancer and recognise early cancers at a curable stage. These lessons have been extended to countries where sunlight is less intense as ozone depletion has increased the penetration of the atmo-sphere by dangerous ultraviolet radiation.

So is sunlight dangerous or health-promoting? The research project described in the Lancet (22 July, p207) - which studied diet, lifestyle and vitamin D concentrations in 824 elderly people - may put the issue into perspective. Vitamin D - essential for healthy bones - is present in foods such as oily fish but the body can manufacture it in the skin when exposed to the sun.

The research showed that 36 percent of men and 47 percent of women had blood levels of vitamin D below the safe level. The lowest levels were found in Greece, Spain and Italy - where the elderly shun the sun. Higher, safer levels were found in northern Europe, where the tradition, especially in Scandinavia, is to shed most of the clothing the moment the sun appears. More detailed analysis showed clear associations between high vitamin D levels and going outside during periods of sunshine.

This research does not contradict warnings about the dangers of overexposing a naturally pale skin to strong sunlight. What it does show is that the northern European tradition of soaking up what sunlight was available was soundly based. Now that ozone depletion has made prolonged exposure to even moderate sunlight potentially dangerous, however, there is an alternative. Several northern countries such as Norway, Denmark and the Netherlands require vitamin D to be added to margarine and many cooking oils have the vitamin added. People who have to avoid the sun should make sure that they get their vitamin D in their diet.