Nevertheless, until mid-Victorian times, the food even the poor ate was not actually dangerous. Fish, fruit and vegetables were cheap. Armies of hugely powerful navvies, paid only sixpence a day, built our canals and railways. They were not malnourished. If you survived the infections of childhood, your life expectancy was very similar to today's, despite all our medical advances. But over the last century, fish has become very expensive and even though fruit and vegetables remain relatively cheap, nobody wants to eat them. The reason? The industrialisation of food production has enabled tasty foods that were rare to become cheap and these have changed our eating habits drastically.

During our evolution, "thrifty" genes that gave their owners competitive advantage in times of scarcity, were selected. Animal fats, which are "saturated" rather than unsaturated fatty acids, provide the most energy calories of any foodstuffs. So genes tended to be selected that provided people with a taste for animal fats together with efficient fat stores to squirrel them away. The difficulty of hunting animals meant that meat was rarely on the menu. But those with thrifty genes would particularly benefit from those rare occasions. Hence they are prominent in our genetic heritage, particularly among races that evolved in harsh environments, such as native Americans or Australians. However, this works to their disadvantage nowadays because saturated fats derived from corn or soya are now so cheap and plentiful. So the genes that allowed a girl to accumulate fat in the summer to provide her and her baby with nourishment in the winter now cause her to put on more and more weight. That weight greatly increases her chances of diabetes, heart disease or a stroke, unfortunately not that far in her future.