If you are at all interested in optimal fitness, you need to be aware of what nutrients your body needs in order to build muscle and metabolize fat as well as optimize performance. This is better known as nutrient timing. Proper timing of the foods you eat will support hormone function and promote a lean and muscular physique, while eating the wrong foods at the wrong time will trigger fat deposition, loss of muscle, and longer post-workout recovery times.
When nutrient timing comes into play, the goal is to shift body composition so that fat loss and muscle gain are optimized and glycogen stores are maximized. Such shifts have long-standing health benefits which can effectively prevent development of major diseases such as diabetes and high blood pressure. One caveat is that consuming excessive amounts of food will result in fat gain regardless of nutrient timing.
During training days, you should consume higher amounts of protein and carbohydrate and decrease your fat intake. The general rule is to consume about 1 to 2 grams of carbohydrates per pound of body weight, with most of your carbs consumed post-workout. On your rest days, you should consume higher fat and protein while dropping your carb intake to about 0.5 grams per pound body weight. The rationale behind this is that you want to encourage higher insulin levels post-workout, but lower insulin levels at all other times.
You may want to bump up your carbohydrate intake a bit throughout the day if you are looking to gain muscle, since insulin stimulates growth or anabolism. You can also ingest carbs throughout the day if you are already lean and just want to maintain. However, if you are looking to lose body fat, you need to consume most of your carbs during and post-exercise, and consume mainly protein and fat during all other times.