As many of you know, there are really only two times of the day to take in simple carbohydrates: first thing in the morning and after your workout.
First thing in the morning because you are coming off a "fast" - which is how ever long you slept the night before. So, at that time you want a simple carbohydrate source and a quick digesting protein source. Now, if you plan on doing morning cardio, skip the carbs and have a small protein shake, then when you do your cardio you will be burning mostly fat for fuel. When you're done, have your carbs and protein.
After the workout is the other time to take in simple carbs: this is critical because it starts the whole recovery/muscle growth process. Following a hard workout, your body is severely depleted of glycogen and glucose.
During the workout hard working muscles use glucose (usable energy) and glycogen (stored energy) for energy. As such, there is a point at which blood glucose levels (available energy) and glycogen levels (stored energy) get so low that intense exercise can't continue. There just isn't enough available energy for your muscles to use.
So what happens is that the hormone cortisol is secreted, this is your body's "stress" hormone and it has very catabolic effects. What cortisol does is eat up muscle tissue for protein and convert it into glucose. A process called gluconeogenesis ensues, producing glucose from these amino acids in the liver. The net result is a loss of muscle tissue.
The post-workout shake prevents this. It also allows insulin to be released, this is, as most of you know, one of several anabolic hormones in the body (if you are a natural trainer especially, you want to maximize the release of all your body's anabolic hormones through all available methods).
So, whey protein is your best protein source at this time because it is absorbed quickly, what is the best carbohydrate source? Well, we want a high glycemic carbohydrate source. This term refers to carbs that are high on the glycemic index (70 and above rates as high).
The Glycemic Index is a measure of how quickly a food raises blood sugar and hence insulin levels. Normally, it is best to eat lower glycemic foods so as not to initiate an insulin spike (55 and under rates as low). But post-workout, the exact opposite is true.
It is critical to get the carbs (and protein) to the muscle cells as fast as possible. As well, the elevated insulin levels will help to drive nutrients into the muscle cells. And again, high-glycemic carbs are best for this purpose.