Anyone talking about a diet first thinks of the word "Carbs." Often, carbohydrates are looked at from a negative point of view, with diets like Atkins and South Beach being the popular fads nowadays. HOWEVER, applied properly, carbs can be and are a great tool.

First, carbohydrates are necessary for energy. Your body takes in carbohydrates and converts them into glycogen, and stores them in the muscles for immediate use, and fats for long-term storage. Without carbohydrates, you wouldn't have the energy to work out at your fullest, and you would go through the process of converting protein into glycogen, which is less efficient, yada yada.

Anyway, the point is--if you use carbs right, you can increase energy, performance, and keep your bodyfat percentage low. The right carbohydrates are "complex carbohydrates", which basically means that it takes more for the body to process them, and is thus healthier and not converted into fat as easy. "Simple carbs" are processed very quickly and rush through the bloodstream, and take less work for the body to process, meaning there's more potential for it to be turned into fat.

Simple carbs often have a high sugar content. When sugar is taken up by the body, it spikes insulin levels, and when that happens, the brain tells the body to start storing more fat. Avoiding simple carbs and sugars will prevent the direct conversion of the carbs to fat, and instead store more in the muscles.

Here's a list of good carbs and bad carbs:

Good:
--Wheat bread/whole wheat products
--Fruits
--Vegetables
--Beans
--Nuts/seeds

Bad:
--White bread
--Carbs from sugar
--Fruit juice
--Soda
--processed grains (white rice, etc.)

Eating from the good list will ensure that while having high energy for your workouts, you'll keep bodyfat low.