Wednesday, April 13, 2005

A breathless inheritance

'A woman who smokes during pregnancy could increase her grandchild's risk of developing asthma', reports BBC News. Researchers in Southern California interviewed the parents or guardians of 908 children. At the age of five, 338 children had been diagnosed with asthma. Where both grandmother and mother had smoked during pregnancy, the children were two-and-a-half times more likely to have asthma than those where neither smoked. But even where only the grandmother had smoked during pregnancy, the risk of asthma was nearly doubled. The researchers suggest that this could be caused by damage to the eggs within the foetus, or perhaps due to damage to foetal DNA which only expresses itself in the next generation.

There is another explanation: that there is no such effect at all. Any epidemiological study will be subject to numerous confounding factors, including that the result is due to chance.

The odds of finding something that appears to be significant, but is actually pure coincidence, are increased by the way in which much of today's research is conducted. People are asked about many different factors - what they ate, smoked and drank, how much they exercised, what job they do, and so on - and these are then compared with the rates of a particular condition, in this case asthma.

If one factor seems to have a significant effect, then the researchers make educated guesses about why this might be, and declare their results to the world. Once in a while, something interesting is stumbled upon; but most of the time, the effect is more statistical than real. Before such speculations are promoted, the results should be replicated to confirm that they aren't just down to chance. As it is, this study offers no new evidence in relation to how grandmaternal smoking could cause asthma.

Moreover, the diagnosis of asthma in very young children is fraught with difficulty. Professor Martyn Partridge, chief medical advisor to Asthma UK, told the BBC: 'Several previous studies have shown an association between wheezing illnesses in early childhood and maternal, and to a lesser extent paternal, smoking. Some that have then followed such associations through to older childhood have shown less definite association suggesting that some of the early life wheezing was not necessarily due to asthma.'

And this study can't rule out a more obvious cause - that the kids are just breathing smoke from their parents or grandparents at home. Lead researcher Frank Gilliland admitted that 'second-hand smoke may also play a role that we could not separate in this study'.

It's not clear what practical application these results might have. Mothers-to-be are already warned for a variety of reasons that smoking during pregnancy is unwise, and most smoking women cut down or stop smoking for the duration of their pregnancy. Fretting about children to be born 20 or 30 years down the line seems excessive. But perhaps no guilt-trip is too excessive in an era where smokers have become public enemy number one.

Gran's smoking 'asthma risk', BBC News, 11 April 2005

Pregnant smokers increases grandkids' asthma risk, New Scientist, 11 April 2005

First published on Spiked's Don't panic page.

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