After their biggest meal of the year, Americans might reflect on the fate of those moldering Thanksgiving leftovers. Nearly 40% of the food supply in the United States goes to waste, according to a new study, and the problem has been getting worse. "The numbers are pretty shocking," says Kevin Hall, a quantitative physiologist at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) in Bethesda, Maryland.
Food waste is usually estimated through consumer interviews or garbage inspections. The former method is inaccurate, and the latter isn't geographically comprehensive. Hall and his colleagues tried another approach: modeling human metabolism. They analyzed average body weight in the United States from 1974 to 2003 and figured out how much food people were eating during this period. Hall and Chow assumed that levels of physical activity haven't changed; some researchers think that activity has decreased, but Hall and Chow say their assumption is conservative. Then they compared that amount with estimates of the food available for U.S. consumers, as reported by the U.S. government to the Food and Agriculture Organization of the United Nations.
The difference between calories available and calories consumed, they say, is food wasted. "We called it the missing mass of American food," says co-author Carson Chow, a mathematician at NIDDK. In 2003, some 3750 calories were available daily per capita; 2300 were consumed, so 1450 were wasted, comprising 39% of the available food supply, the team reports  in the November issue of PLoS ONE. This figure exceeds the 27% estimated by the U.S. Department of Agriculture (USDA) from interviews with consumers and producers.
Much of the waste is probably happening at home, say experts. A study published earlier this year by Jeffery Sobal, a sociologist at Cornell University, and colleagues examined food waste in Tompkins County, New York, through interviews. They found that production accounted for 20% of waste, distribution for about another 20%, and consumers for the remaining 60%. "Food waste used to be a cultural sin," Sobal says.
There is apparently more sin now than ever. Chow and Hall's model suggests that the percentage of food wasted has risen sharply over the past 35 years, whereas the USDA figures that it has remained roughly constant. In 1974, Hall and Chow found, only 30% of calories in the U.S. food system went to waste. So exactly where are the extra losses occurring? "That's a very interesting question to which we don't have an answer," Hall admits.
Experts say they're not surprised that a greater percentage of food is being wasted, given that average food prices have declined in real terms. "If it was more expensive, waste would be reduced," says chemical engineer Greg Keoleian, who co-directs the Center for Sustainable Systems at the University of Michigan, Ann Arbor. He also notes that many Americans are also eating more than they need to. "Waste and overconsumption is the key issue affecting the sustainability of the U.S. food system," he says.
Hall and Chow hope to expand their analysis to other countries, including Japan, which has the reputation for being more frugal.