Americans' widening waistlines are the main force behind rising U.S. health care costs, a new study shows.

Between 1987 and 2002, the proportion of private health spending attributable to obesity increased more than tenfold, researchers report, from $3.6 billion to $36.5 billion.

In the year 2002, obesity-related medical care spending accounted for 11.6 percent of all private health care spending compared to just 2 percent in 1987, concludes an article published today in Health Affairs.

"We can focus on obesity and we should be," said study lead author, Kenneth Thorpe, the Robert W. Woodruff professor and chair of the department of health policy and management at Emory University in Atlanta. "We need to have the same type of societal attention on this issue that we gave to smoking 20 years ago," Thorpe added.

There's been no question that health care costs in this country are soaring. The question has been, "Why?"

"You can break health care spending into two things," Thorpe explained. "One is that we're treating more people who are sick, and second is that it costs more to treat those patients."

To find out which factor was predominant, Thorpe and his colleagues looked at data from the 1987 National Medical Expenditure Survey and the 2002 Medical Expenditure Panel Survey, both nationally representative samples. Specifically, they were concerned with spending among privately insured adults aged 18 to 64 for the top 20 medical conditions.

As it turned out, more than half of the growth in health care spending was attributable to increased prevalence of disease rather than an increase in how much it cost to treat each person.

"We found overwhelmingly that the rise in private insurance spending was traced to the fact that we were treating more and more people with a variety of chronic health conditions," Thorpe said.

In other words, more people are sick now than before; predominantly with conditions linked to obesity such as diabetes and high blood pressure. This means that in 2001, obese adults with private health insurance spent $1,244 more per person per year on health care than normal-weight adults. Back in 1987, that number was just $272.

Obese individuals are being treated for more conditions, too, the study found. In 2001, for example, 15.5 percent of obese adults were treated for six or more medical conditions, nearly double the 1987 percentage.

There have also been changes in the way clinicians treat patients. "They're more likely to give you a blood pressure pill today at a lower threshold than 15 years ago," Thorpe said.

Finally, new, expensive technology is also driving costs upwards, the study found.

"Obesity is a very expensive health problem and, unlike some other health problems, doesn't appear to be topping out. It appears to be accelerating," said Dr. Tom Farley, co-author of Prescription for a Healthy Nation and professor of community health sciences at Tulane University School of Public Health and Tropical Medicine in New Orleans.

"We all pay for these costs," Farley pointed out. "It's not just people who are obese. All of us have higher premiums, higher taxes. This is a society-wide problem and we need society-wide solutions."

Unfortunately, the debate on how to curb this growth in obesity and its related expense is focusing on the wrong issues, Thorpe said.

"The private insurers are doing nothing to deal with this issue," he said. "They're looking at consumer-driven plans, increasing co-pays. If you think about it, those are going to do nothing to affect the 60 percent increase in diabetes."

Rather than trying to decrease the cost per case, we need to look at the bigger picture, he said.

"We're looking under the wrong light here," Thorpe said. "We need to fundamentally focus our attention on how to control costs."