AFFLUENT, modern babies live in a sanitised world. This has already been blamed for a high incidence of asthma and allergies, but might also up the risk of developing a host of other conditions common in rich countries, such as stroke and heart disease.
According to the 鈥渉ygiene hypothesis鈥�, our immune system evolved to handle a germ-laden world. If we don鈥檛 encounter many pathogens during infancy, it doesn鈥檛 learn to keep itself in check, and turns on inflammation 鈥� normally a response to infection 鈥� in inappropriate situations. This reaction, the hypothesis goes, is responsible for the recent increase in asthma and allergies, both associated with inflammation.
Recently, it has emerged that chronic inflammation may also increase the risk of diabetes, stroke and heart diseases. So might the hygiene hypothesis be implicated here too?
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To find out, Tom McDade of Northwestern University in Evanston, Illinois, and colleagues turned to health surveys, which began at birth, of 1534 children in Cebu City in the Philippines, where western levels of sanitation are generally not found. When these people reached 20, McDade鈥檚 team were able to test their blood for C-reactive protein (CRP), a marker of chronic inflammation.
They found that the more pathogens the people had encountered before age 2, the less CRP they had at age 20. Every episode of diarrhoea back then cut the chance of higher CRP later by 11 per cent; every two months spent in a place with animal faeces cut it by 13 per cent. Being born in the dusty, dirty dry season cut the chance by a third (, DOI: 10.1098/rspb.2009.1795).
鈥淓very episode of diarrhoea as an infant cut the chance of high adult levels of an inflammatory protein鈥�
McDade suggests that early exposure to germs could reduce chronic inflammation later in life, and therefore the risk of developing a host of serious conditions. 鈥淭his takes the hygiene hypothesis well beyond allergy,鈥� he says. 鈥淭his is consistent with the effect of germs on immune development,鈥� says Richard Gallo of the University of California, San Diego.
McDade hopes that one day we may be able to safely expose babies to the protective elements of germs without incurring the risks that come with infections. In the meantime, he is taking a less high-tech approach: 鈥淚f my 2-year-old drops food on the floor, I just let him pick it up and eat it.鈥�
