If there are still any doubts that obesity is a major public health problem in the U.S., a report from the CDC last week should put them to rest. According to the study, published in the Journal of the American Medical Association, rich diets and sedentary lifestyles led to 400,000 deaths in 2000, just behind the nation's No. 1 cause of preventable deaths: tobacco (435,000). By next year obesity will probably kill more Americans than smoking. Health and Human Services Secretary Tommy Thompson used the occasion to launch a series of clever public-service ads, but antiobesity activists calling for tough government action were not amused. Nor were they pleased by Republicans in the House who chose this week to push through the so-called cheeseburger bill, which protects fast-food restaurants from getting sued by obese Americans who blame the outlets' high-caloric fare for making them fat.