I'm pretty sure that Michael Phelps can out-exercise a bad diet, but not sure about my humble 7 hrs per week. So to be safe, I avoid packaged foods, processed meats, some preservatives, and most alcohol since:

1) Apparently, "0.5 grams of trans fat" food is still labelled 0 grams, so I avoid soups that say "cream of", things with crusts (meat pies, pizzas etc), and fried things like chips, fries, chicken, donuts, bisquits (darn it).

2) I used to get the "nitrate-free" processed meats (which didn't keep well), but then I simply quit processed meats altogether.

3) People are now saying that "Proprionate and Proprionic Acid" (preservatives used in some breads and cheeses) are the things that make Americans gain more weight than Europeans (whose bread and cheese have less shelf-life, but are apparently healthier). I guess the lesson learned might be "if it doesn't get moldy after 3-4 days, don't eat it"

4) This leaves fruits, veggies, unprocessed meats, whole milk, virgin olive oil, some soups, breads and cheeses (with short shelf life). No soft drinks, distilled spirits, a glass of wine on Fridays and one beer during Sunday football games.

At least ice cream is said to be relatively not bad for us.