The book is a warning against eating too much fast food. It shows the dangers on an individual whose only source of food is from McDonald's. All of the benchmarks of being a healthy person, from weight, to cholesterol, to blood pressure all rise dramatically as his intake from fast food increases.
I think the answer to this question lies within America's need for cheap, fast, and easy things. We are a country that is obssesed with material things. Our time has become so precious we refuse to waste a second or a dime on it. We have found ways to take short cuts in order to save money and time; despite the issues that may be caused. "Fast Food Nation" is a warning to America to "slow down and do things right". We are the most obese country in the world. We are also the most efficent.