Once you've finished your workout your body goes into what is known as a catabolic state. This means that it is breaking down your energy stores in order to make enough energy available, in the absence of food. The problem with this is that, when your body is in this catabolic state your body will begin to break down protein (muscle) as well as fat in order to make enough energy available. Therefore if you workout really hard and then don't give your body the nutrients it needs, you will start to break down the muscle that you just worked so hard to build, instead of your unwanted fat.
So you should have a decent, low fat, high protein meal within 40 minutes of working out, however the sooner the better. This makes your body realise that it needs to keep breaking down those fat stores, but it doesn't need to get desperate and start breaking down protein. A lot of people often take a protein shake to the gym with them, this is a great way of quickly replenishing your body with the supplies it needs.
Another problem with not eating after a workout, is that your body doesn't have enough nutrients to repair itself. This means that your recovery time after workouts will be slower, plus you will more sore the next day because your muscles are still tired from the workout. Most importantly this means that your body doesn't have enough nutrients to grow. This means you won't be able to gain muscle, and your progress will be much slower.
Many people think it is a good idea to workout hard in the morning, before breakfast. This is a VERY BAD IDEA. As I mentioned earlier your body needs nutrients to repair and grow, in the morning you probably haven't eaten for a long time, so your stomach is completely empty. This means that just being awake before breakfast is burning fat. When you add a long hard run or an intense workout, your body will resort to burning muscle much quicker than normal. This doesn't mean that you can't do anything before breakfast, as it would be a waste to ignore this prime fat burning period. What I recommend is a short, nicely paced walk. If you break a sweat, or can't talk normally then you're probably going too fast. You should do this as soon as you can after you wake up. If you have a dog, try taking it for a walk before breakfast and see how you feel after. You shouldn't be awake for more than half an hour before eating, as again your body will start to breakdown that muscle.
Breakfast is the most important meal of the day. You should try to get in some high fibre cereal, aswell as a good, low fat source of protein, such as eggs or a protein shake. If you don't eat breakfast, then your body will not function at it's best for the whole day. You will probably end up eating more throughout the day in snacks and other meals that you do have, which would exceed the amount of calories you would take in at breakfast. Ideally, you should eat 6 small meals a day, but we'll get to that in a later post.
You should eat within 40 minutes of finishing your workout (the sooner the better), alternatively bring a protein shake and drink that immediately after your workout and then have a large snack when you get home. You can exercise before breakfast but only at really low intensity for no more than 30 mins, and have breakfast IMMEDIATELY.
As Always, Good Luck