After fast food, rising taxes on tobacco are the strongest driving force in America's obesity epidemic, according to new economic research.

The problem is that the taxes are doing precisely what they were intended to do. They discourage people from smoking, and so their appetite increases, their metabolism slows and they gain weight.

"Higher cigarette taxes and higher cigarette prices have caused more smokers to quit -- but these smokers seem to have begun eating more as a result," wrote Michael Grossman and Inas Rashad in the U.S. public policy journal The Public Interest.

In an interview, Dr. Grossman, a distinguished professor of economics at the City University of New York, said his work makes it doubly difficult for policy makers to decide which is the lesser of two evils: smoking or obesity.

"According to our research, each 10% increase in the real price of cigarettes produces a 2% increase in the number of obese people, other things being equal," the pair wrote.

Since U.S. obesity rates began to spike in 1980, the inflation-adjusted price of cigarettes has risen by approximately 164%. That rise accounts for almost 20% of the growth in obesity, the pair concluded, and is second only to the proliferation of cheap, unhealthy fast food. Their research is also detailed in the current issue of the Journal of Health Economics.

David Lau, a University of Calgary endocrinologist and president of Obesity Canada, said quitting smoking can cause sweeping metabolic changes that lead to weight gain.

"There is definitely a physiological explanation for why teenage girls resort to smoking as a means of weight control," he said.

Smoking inhibits a protein, lipoprotein lipase, that helps hoard fat for energy. When a smoker quits, the protein is back at full strength, helping to pack on the pounds.

"It's not just because they're substituting food for cigarettes," Dr. Lau said.

But Dr. Lau cast doubt on Dr. Grossman's theory, saying it would be "naive" to think taxes cause obesity. He called the relationship "distant and weak."

"I don't think this is causal," he said. "I think the two are not really related."

In Canada, 90% of people who quit smoking gain about five to seven pounds in weight, and 10% gain as much as 30 pounds, according to Health Canada data.

Canadian and American smoking rates are similar, and Canada's obesity epidemic is forecast to match that of the U.S. within a decade.

Dr. Grossman said his analysis aimed to factor out the effects of income, education, age, race, sex, family size, local cost of fast food and other socio-economic factors, in order to isolate the effect of tobacco taxes on obesity.

"But you can never say with 100% certainty that this is a causal relationship," he said.

He said his work illustrates the dangers that can be concealed in the best laid plans of social planners.

"Cigarette smoking is an important cause of a lot diseases, and we wouldn't suggest that taxes should be lowered in order to get people to smoke more so they won't gain weight. That's not our message. Our message is that there are unintended consequences of public policies," he said.

His findings recall the controversy over U.S. efforts to curb underage drinking by raising the legal drinking age.

In the main, those programs worked -- fewer young people purchased alcohol. But several studies also showed that, as the age of majority increased, so did teenage marijuana use.

Dr. Grossman said both cases represent failures of foresight.

"If you do want to discourage people from smoking, you might at the same time have to do other things in order to prevent these side effects" he said.

Health Canada considers tobacco taxing a "delicate balancing act" of discouraging smoking with high prices while simultaneously keeping it low enough to prevent contraband.

There is no mention of obesity, and for good reason, according to Carol Sutherland-Brown, manager of cessation and protection in the agency's Tobacco Control Program.

"It's a concern, but I think the benefits of quitting smoking far outweigh what is often a negligible weight gain," she said.

According to Health Canada data, smoking rates have been dropping slowly in Canada from about 33% in 1986 to just over 20% in 2002. However, this data is based on polls, and self-reported smoking figures are frequently as much as 30% lower than industry sales figures.

Tobacco taxes have been rising in this period, and now account for about 70% of the cost of cigarettes in Canada.

Ms. Sutherland-Brown said it is difficult to attribute lowered smoking rates to any one factor, but said taxes are an important part of the federal anti-smoking strategy.

Obesity levels, on the other hand, have risen in Canada since the 1980s so that today most men, one third of women and one in 10 children are clinically overweight.

They suffer increased levels of heart disease, type 2 diabetes and certain cancers.

National Post, Thursday, July 29, 2004. An accompanying photo, of a hand holding a lit cigarette with an out-of-focus face in the background, has not been reproduced here.