[sirdarksol]

Actually, either diet or activity will do it (but having control over both is best)
The current American diet is one that we've had for over a century (okay, it was never quite this extreme, but "meat and potatoes" is an old concept here). However, when farmers ate meat and potatoes for every meal, they worked all day. We still eat tons of fatty meat and potatoes, but most of us now sit behind a desk or the like, and it shows.

Crazyhorse was on the right track with what to cut out, but the list was haphazard. In general, you want to cut out the densest fats, excessive amounts of proteins, and refined carbs (in other words, the most calorie-dense foods). Sugar, most white rice (basmati is relatively good, but brown rice is still better), anything with enriched flour, large cuts of meat, heavily marbled meat, most pork, all of these should be enjoyed in moderation. Aim for high-fiber, high nutrient veggies, a small amount of meat, and a moderate amount of grain, the closer to natural, the better.