There shouldn't be an extra tax on fatty foods or obese people because the government is not our mom. It's not its responsibility to prevent people from making bad choices.
Everyone agrees that obesity is a problem in the U.S., but I do not agree with this solution because it restricts the free market. It is our own responsibility to make healthy choices, and to promote healthy choices in our culture.
I do not believe there should be a fat tax because whether one chooses to consume unhealthy foods, it is that's person decision (except for young children, more or that later). So it is fine as long as they are held responsible for their choices. So how do we do this?
First, let's ensure we protect the children. If a children is obese (high body fat), then the parents need to be charged and monitored.
Second, all healthcare should be the responsibility of the individual (in the case of medicare, people must not obese, if they are then they need to pay a penalty). That means individual/family either buys their own insurance themselves or through employer when applicable. Hospitals will have the right to turn away uninsured patients.
Copied from my own opinions question on the fat tax: Too many reasons to write in the topic sentence. Firstly, fat tax is relevant mostly to people who won't be affected by fat tax. Not to say their demand for fatty foods is perfectly inelastic, but they tend to have higher income and an increase in price won't decrease their quantity demanded that much. Secondly, fat tax is regressive since the tax rate in terms of income decreases as taxable income increases. Worse still, the poor usually spend more on food (esp. McDonald's food), so they are hit the hardest. Income inequality will no doubt be widened, and the burden on low-income consumers will be unbearably heavy. Thirdly, if the fat tax is the same for all food items above a certain number of calories, by Alchian's generalisation, people will eat more of the more expensive food, increasing their spending on fatty foods. Fourthly, it will drive small-scale competitors out of the market since the marginal cost of each unit of food increases. Fifthly, it will create a black market or, in the case of Denmark (the first country to impose and subsequently abolish the tax, force citizens to cross the border to Belgium to buy junk food.
Now, if it's imposed on waistlines, that's a different matter; however, I think there will be the same problem with regressiveness (don't forget, many poor, busy parents are giving their kids McDonald's these days) and there are other factors causing obesity that are not always the person's fault.