What does your body need after working out?

First of all, I don't recommend working out on an empty stomach. When you exercise, you have to breakdown glucose first in order to have pyruvate to be able to burn body fat. When you go overnight without food, your natural glycogen stores are used to fuel the brain because it only uses glycose/glycogen for fuel. After a while of fasting where you don't have intake of more carbs to replenish the glycogen stores, your natural body process is to produce cortisol to tell the body to breakdown protein to make glucose to fuel the brain. This is fine if you aren't trying to maintain muscle and burn body fat. For those of us trying to lose body fat, however, its not a good thing because it actually slows the metabolism from the muscle breakdown. If you exercise first thing in the morning, without replacing that glucose/glycogen, then the cortisol level stays elevated and you keep breaking down muscle to produce the glucose/glycogen needed to fuel the exercise. You will burn some fat, too, because the breakdown of the protein produced glucose provides the pyruvate just like breaking down carbs so you can go into fat burning. But the glycogen is provided by breaking down your hard earned muscle mass which slows the metabolism in the long run. Muscle burns calories just to maintain itself so you want to keep that muscle to be able to consume more food in the long run. Not to mention the benefits of having the added muscle in increasing bone density (decreasing osteoporosis risk), increasing strength, decreasing risk of injury, etc.

Now, for fueling exercise and recovery, you need a mix of carbs and protein. Before exercise you need mostly carbohydrates. Like I said earlier, you breakdown carbs for fuel to provide the pyruvate for fat burning. If you are doing a short, high intensity workout, simple carbs are sufficient because they are available almost immediately. If you are doing a longer, lower intensity activity, complex carbs with a little simple carbs are the way to go because the simple carbs are available for the beginning of the workout and the complex carbs are available for the end of the workout. The protein that you consume before the workout isn't actually available until after the workout, but that is good because that is when they are needed to repair and rebuild the muscle from the microscopic damage that exercise causes. After the activity, you need a mix of carbs and protein with more focus on protein. The carbs are to help replace any glycogen stores the body used (in addition to the carbs you consumed beforehand) to fuel the activity and the protein is to help continue the repair and rebuild process throughout the next 24-48 hours because you actually gain strength in those muscles during the repair and rebuild process.

Because you need carbs and protein, that is why chocolate milk is recommended because it has both. I prefer a sandwich, but that is just a personal preference. Anything that provides both protein and carbs is fine.