There are plenty of athletes who eat at McDonalds, quite regularly.
I remember this one linebacker from North Carolina, I think he's Bruce Carter, just got drafted by the Cowboys; but he was an athletic freak coming into his senior year in college. During one interview, he was asked what his daily meal was, and he basically said at least 5 Big Macs and 3 trays of McNuggets (college players aren't allowed to sign sponsorship agreements, before anyone thinks about money). Fact is, he works it out daily. Most athletes have regimens where they burn calories like crazy, and eating 100% healthy is a surefire way to lose your potential (unless, of course, you're in a sport that requires strict body management, such as gymnastics).
The irony is that McDonalds will fatten up anyone that doesn't work out, but the Olympics are all about the athletes.