[yummyfajitas]

Food that is good for you is just more expensive than food that is bad for you.

Any person who is obese is eating more food than they need. With the same amount of money, they can eat fewer calories but get better nutrition.

Incidentally, poverty is not the issue. Most graduate students are poor (or very close to it), but obesity is not particularly prevalent in grad school. Based on my personal experience, I'd guess that more poor students eat overpriced organic granola than big macs.