Lifestyle: When allergies go west : Nature : Nature Publishing Group
Modern living seems somehow to make our immune systems overly sensitive. Is cleanliness at fault — or something else?
Germs can be good for us. This ‘hygiene hypothesis’ claims the urban ‘Western’ lifestyle, with its relatively limited exposure to infectious agents during childhood, might be behind the post-war epi-demics of asthma, eczema and food allergies. But can the explosion in allergies in the developed world really be explained so simply? A more complicated theory is emerging.
For, while it is apparent that there is something about Western living that increases the risk of developing allergies, research has shown that it’s not just a culture of antibacterial soaps, antibiotics and excessive cleanliness that’s to blame. There is evidence to suggest there is more about the way children are raised — where they are raised, what they are fed and how many siblings they have — that influences their burgeoning immune systems. It’s not just that their exposure to microbes or potential allergens seems to be limited. Sanitary living conditions disrupt the delicate balance between our bodies and a complex ecology of microbes and parasites with which we co-evolved, and through which our immune systems are balanced and regulated (see ‘Gut reaction’, page S5).
In 1989, epidemiologist David Strachan proposed the theory that a reduced exposure to dirt could render a person prone to allergy. The idea originally had a far narrower scope and, as Strachan, then at the London School of Hygiene and Tropical Medicine, but now at St George’s University of London, is at pains to point out, he didn’t actually use the term ‘hygiene hypothesis’. Strachan, like others, was trying to understand a perplexing public health paradox. With clean drinking water, sanitation, vaccinations and advances in medicine, many diseases were on the wane. So why was it that allergic diseases were becoming more prevalent, particularly in industrialized and urban parts of the world?
The prevalence of allergies varies considerably, but many Western countries experienced a twentyfold increase in incidence. Asthma affects as much as 40% of the population in regions of New Zealand, Australia and the United States1, 2. Cases of eczema identified as atopic, meaning that they are associated with a propensity for allergies, doubled and even tripled in some industrialized countries. While these increases appeared to plateau, allergies rose rapidly in developing nations where living conditions and hygiene standards were becoming more like those in the West. It was starting to look as though the causes of allergies had something to do with the nature of Western lifestyles.
Strachan’s insight was to link increases in allergic disease to the declining size of families. In his landmark study, which involved following more than 17,000 British children born in 1958, he found that there was an inverse correlation between allergic rhinitis, or hay fever, and the number of older siblings. This, he suggested, “could be explained if allergic diseases were prevented by infection in early childhood, transmitted by unhygienic contact with older siblings or acquired prenatally”. Older siblings appeared to increase the range of bugs that either a pregnant mother or a younger sibling would be exposed to, boosting a younger sibling’s protection either indirectly or directly.