In China, a largely rural lifestyle which makes heavy physical demands combined with a low-fat, rice-based diet keeps the population trim. In the US, where people commute to offices in air-conditioned cars and only break sweat when the pizza delivery is late, average seat sizes have increased.
Cultural factors play their part. In Samoa and neighbouring islands in the Pacific, obesity has long been regarded as a symbol of high status and prosperity and is seen as attractive as a result. In recent years, there have been signs that these traditional notions are changing as more Westernised ideas of an attractive body size take hold.
There are also sex differences. Women to be fatter than men, a legacy of child-bearing. But here, too, culture plays a part. In the United Arab Emirates, male obesity rates are similar to those in Britain but among women they are more than twice as high
Despite the enormous international range in rates of obesity, only about 20 per cent of differences in body shape can be attributed to genes, according to Professor Garrow.
"If it were the case that the Chinese and Japanese had something in their genes that kept them thin they wouldn't become taller and fatter when they migrated to the US. But migration studies show that they do.
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