The culprit?

Doughnuts. That's right. Doughnuts are the worst food you can eat. The average glazed doughnut generally has 200+ calories (and some doughnuts have far more than that), but half of those calories are from fat. That's nearing 1/3 of your daily-recommended limit of saturated fat, and how often do you eat just one doughnut? Lets just hope the doughnut doesn't have trans fat, or it will be even more difficult to burn off.
In addition to being high in fat, most doughnuts are made with bleached white flour. This is a simple carbohydrate, and is digested more quickly than whole grains. Whatever your body doesn't use eventually gets stored as fat. For this reason, avoid anything with bleached white flour. This is another reason to avoid doughnuts.
Another simple carbohydrate in doughnuts? Sugar. And there is quite a bit of it. This is metabolized even faster, and therefore converted to fat even more quickly. Foods high in sugar also will give you quick energy, but you will crash later on. The last thing you need is to feel tired and sluggish later in the morning.
So think about that the next time you reach for that box of doughnuts at your office, or walk past tempting display at the bakery.