Americans are eating themselves to death - soon poor diet and physical inactivity may overtake tobacco as the leading cause of death, according to a US government study.

About half of all deaths in the US can be attributed to preventable behaviours, shows the study by researchers at the Centers of Disease Control and Prevention in Atlanta, US, and led by Ali Mokdad.

Tobacco was the leading cause of death in 2000 - causing 435,000 or 18.1 per cent of total US deaths. Poor diet and physical activity followed closely behind, causing about 400,000 or 16.6 per cent of deaths.

However, compared with a 1990 study by Michael McGinnis and William Foege, the number of deaths due to obesity is rapidly catching up with those caused by tobacco. In 1990, 400,000 deaths could be attributed to tobacco, and 300,000 to poor diet and physical inactivity.

"These trends are going up due to changes in our environment. Simply we eat more and exercise less," Mokdad told New Scientist. "Tobacco is still going down in the US but at a slow rate, we hope it continues and perhaps declines faster."

If current trends continue, obesity will become the leading cause by 2005, with the toll surpassing 500,000 deaths annually. "This is a tragedy," says CDC director Julie Gerberding. "We are looking at this as a wake-up call."

Risk behaviours

To produce their report, Mokdad and colleagues first searched the medical literature to identify studies linking risk behaviours and mortality. They then combined their findings with 2000 mortality data reported to CDC in order to calculate the contribution of these behaviours to preventable deaths.

Smoking and poor diet and inactivity contribute to the three major killers - heart disease, cancer and stroke. The next biggest cause of death was alcohol - causing 85,000 or 3.5 per cent of the 2.4 million US deaths in 2000.

While the gap between deaths due to poor diet and inactivity and those due to smoking has narrowed substantially, this is not due to a decrease in smoking deaths. "The most disappointing finding may be the slow progress in reducing tobacco-related mortality," the team notes.

In response to the study's stark findings, the US Department of Health and Human Services launched a national education campaign and research strategy on Tuesday.

"Americans need to understand that overweight and obesity are literally killing us," said health secretary Tommy Thompson, adding that the study's findings "should motivate all Americans to take action to protect their health".

An estimated 130 million US citizens, or 64 per cent, are overweight or obese.

Journal reference: Journal of the American Medical Association (vol 291, p 1238)