The before and after meals are there to aid you do all of the above. They’re not something you need if all you are trying to do is lose weight. Your body has two fuel tanks: primary and secondary. Your primary fuel tank is the energy stored from your previous meal (glycogen stores) and the secondary fuel tank are your fat reserves (fat stored in fat cells). Unless the primary fuel tank is empty, your body will not access the secondary energy supply = your fat cells. So if your goal is to lose weight and/or streamline, you really want is to make sure you empty your glycogen stores first so you can access the fat stores.
To lose weight you can either train on an empty stomach or train with a light snack that you will burn shortly after you start your workout. You need a pre workout snack if you are so hungry you are feeling weak otherwise you won’t be able to push yourself hard enough - not enough to force your body to change and/or burn more reserves. Your goal is to train to raise your heart rate and body temperature sufficiently to speed up your metabolism. So the harder you work during your training session, the more you burn throughout the day after your workout. If you were too weak to work out at 100% it doesn't mean your workout was wasted - it’s just wasn’t as effective for the long-term burn.
A pre-workout snack is simply an aid to help you move faster and perform better. You can easily schedule your workouts around breakfast, lunch or dinner so you don’t require any pre-workout aid and hence consume no extra calories you then need to also burn off.
Some people train on empty early in the morning before breakfast. At that time, your body hasn't got any quick energy in the stores and will use your reserves to function. Your body will access fat stores straight away. Another plus of this type of training is since your body isn't busy digesting anything all of the energy will be directed into working out. You will burn more, you will burn fat but it is a brutal and a taxing way to exercise because of that so not many people can do that long-term.
Think of it this way: how long will you exercise and will you need extra energy for that or not? If your session is going to only last half an hour then you are unlikely to need any special pre-workout boost but if it’s going to be over an hour and it’s going to require everything you've got, then a pre-workout snack might be a good idea.
Running for under an hour for example, as demanding as it is, does not require any pre-workout snack.
A post-workout snack consumed roughly 30 minutes after your workout can help you repair the damage done to your cells and help you recover faster. It is usually a small protein snack or shake.
Weight loss is a deficit of available resources so if you give your body more food that it needs you’ll just end up gaining weight. On the other hand, if your goal is to gain weight than that’s exactly what you need to do - make sure your eat more than you burn.
