I think the biggest problem caused by the change in the cost and the availability of food was actually the cost itself. People can afford to consume more calories without going broke, and so they do - too many calories. I regularly eat fast food (probably 2-3x a week), but I get something that has a reasonable number of calories and skip the soda (which is just raw sugar) ... I have not become obese, and it has not had detrimental effects on my health. For example, a reasonable lunch at McDonad's might be a McDouble, and perhaps a small fry now and then. The McDouble is 390 Calories, and the small fry is 230. If you figure on fries every other time you get McDonalds, that averages out to a 505 Calorie lunch, which definitely will not add anything to your waistline.

The famous "documentary" by Michael Moore (who, personally, I can't stand) was based on the premise that he'd ALWAYS add any extras suggested by the restaurant (fries, coke, super-size, etc.) Eating like that, it's no surprise that he's morbidly obese.