Losing weight can be an expensive goal because replacing the empty calories of junk food with fruits and vegetables can 10 times the final cost to meet the nutrition requirements of 1,000 calories.
http://well.blogs.nytimes.com/2007/12/05/a-high-price-for-healthy-food/
If the governments wants its citizens to become healthier, and fight off the obesity epidemic then taxing junk food will lead nowhere while healthy food continues to be extremely expensive. The real solution is to subsidize fruits and vegetable farmers to the point of making them rich to produce excess fruits and vegetables for all Americans.
We would all be eating our apples and broccoli everyday if we can afford it.