Phenylketonuria (PKU) is a genetic disease which causes intellectual impairment in the developing brain. So we screen for it at birth. I say 'a genetic disease' but the impairment arises when someone with the PKU mutation eats a diet high in phenylalanine. Problem is, we live on a high phenylalanine planet: everyone with the PKU gene has a high phenylalanine diet and so gets the disease.
(Incidentally this also illustrates a limitation of case control studies. This methodology takes people with PKU (cases), compares them with people who do not have PKU (controls) and asks what is different. Answer: the cases have the gene mutation, the controls do not. Hence the gene 'causes' the disease. But what you miss is the fact that both the cases and the controls are drenched in phenylalanine. You can't spot that this too is a cause: without it the cases would not be ill.)
Now think about us as a low exercise society: what causes obesity? It might start to look like a genetic condition.
All this and more in an interesting paper by Neil Pearce.
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