IN MANY countries one of the side effects of the second world war was to breed a generation that could not abide waste. Newspapers, jars and string were diligently saved and reused. Glass bottles were returned to their makers. Most importantly, though, food was never, ever thrown away. Leftovers were recycled into new meals, day after day. Fast forward to today and things have changed. There are reports of rich countries throwing out 25-30% of what is bought. Add in what never even makes it to the cupboard or the refrigerator, and the scale of the problem is considerably larger.

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Reliable data, though, are scarce. Existing reports usually collate small-scale studies of households' leftovers and rubbish bins and then extrapolate the results across a country. So Kevin Hall and his colleagues at the National Institute of Diabetes and Digestive and Kidney Diseases, in Bethesda, Maryland, decided to look at the problem in a new way.

As they report in the Public Library of Science, they calculated the calorific consumption of America's population based on data in the National Health and Nutrition Examination Survey carried out by the country's Centres for Disease Control and Prevention. They then compared that figure with recorded levels of food production, modified for imports and exports.

They found that the average American wastes 1,400 kilocalories a day. That amounts to 150 trillion kilocalories a year for the country as a whole—about 40% of its food supply, up from 28% in 1974. Producing these wasted calories accounts for more than one-quarter of America's consumption of freshwater, and also uses about 300m barrels of oil a year. On top of that, a lot of methane (a far more potent greenhouse gas than carbon dioxide) emerges when all this food rots.

Food that is not eaten cannot, of course, make someone fat. Nevertheless, Dr Hall and his colleagues suspect the wastage they have discovered and America's rising levels of obesity are connected. They suggest what they call the “push effect” of increased food availability and marketing is responsible. The upshot is more food in the waste-bin, as well as more in the stomach.

That is probably not the whole story, however. The cheaper food is, the more likely it is to be thrown away even before it is sold to someone who might actually eat it. Such supply-chain waste can be built into the price, and usually makes economic sense. Throwing away leftovers is often better business than risking running out of stock. Yet any waste of a valuable resource is offensive at a visceral level. Just ask those who lived through the war.