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BMJ 2011; 342 doi: (Published 01 March 2011) Cite this as: BMJ 2011;342:d1328

Exposure to a wide variety of microbes prevents asthma

Children who live on farms have less asthma and atopy than children who are growing up in other settings. Even indoors, they are exposed to a greater variety of bacteria and fungi, and this seems to explain a large part of the association with asthma, but not with atopy. These findings come from two large observational studies—PARSIFAL, with 6843 children aged 6–13 years in Bavaria, south Germany, and GABRIELA, with 9668 children aged 6–12 years in rural areas of Austria, south Germany, and Switzerland. Microbes were determined from house dust, in one study with a DNA technique that didn’t require culture growth, and in the other study by means of five different growth mediums. In both studies, the risk of asthma lowered with additional microbe bands detected in house dust. Atopy, which showed the strongest link with living on a farm, showed no association with microbe diversity in these studies.

It may be that exposure to a wide range of microbes prevents colonisation of lower airways with harmful bacteria, stopping them from contributing to development of asthma, but the mechanisms still elude us. There is some way to go before the health benefits of growing up on a farm are brought to those who don’t, concludes the accompanying editorial (p 769).

Obesity doesn’t pose excess mortality risk in Indians and Bangladeshis

U-shaped and J-shaped relations between body mass index (BMI) and risk of death have been reported, but largely in European and North American populations. A consortium compiled 19 cohort studies, for a total of more than 1.14 million Asian participants. About 120 700 deaths were recorded during the mean follow-up of 9.2 years.

As in European populations, the risk of death was lowest for people with a BMI between 22.6 and 27.5. This, according to the authors, argues against …

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