The goal of many people we see in the gym is weight loss or, more importantly, fat loss. You see people aren't trying to lose body mass in general, just the unwanted fat. An unfortunate side effect of weight loss can be the depletion of muscle mass to the amount of 40-60% of overall weight lost.
Due to this loss of muscle mass the body is losing its fat burning powerhouses – mitochondria in the process. You see, mitochondria are the major source of energy production within our cells (the Krebs cycle and oxidative phosphorylation). In order to get these little guys producing energy we need to consume a balanced diet so we have the necessary molecules for energy production (Adenosine-triphosphate or ATP) – mostly fat in this case. Therefore the best way of depleting fat stores is by training aerobically. There are other systems for the production of ATP but we won’t go into them as they have no noticeable effect on fat metabolism.
This isn't to say that resistance training doesn't have its place in the fight against fat. It can play a key role in the amount of mitochondria in the body. The more you train, the more energy you need, which places demand on the mitochondria to self-replicate. The more mitochondria the more fat burning potential! Quick grab the weights!
So to prevent muscle breakdown (catabolism) we must ensure adequate protein intake to aid muscle repair. The amount needed for the majority of the population is 0.25g protein per Kg of bodyweight. Studies have shown that this amount needs to be doubled after the age of 55 in order to maintain muscle mass.
This is not to say go out and grab the nearest protein shake and smash it straight after a workout. There are plenty of food based options to achieve the needed amount, e.g. a 95g tin of tuna in olive oil has 16g of protein per serve. The other point to make is that protein intake doesn’t have a small window of opportunity post training. As long as you space out the servings – breakfast, snack, lunch, snack and a double dose at dinner (in order to maintain protein stores for your overnight fast) – you have covered all bases. The thing is even the act of protein intake is enough to get mTOR (mammalian target of rapamycin – the signal to begin production of protein) active, so the old theory of having it within one hour post training is a myth.
The protein source you choose can have an effect because you need to have enough Leucine (one of the amino acids) in order to get mTOR firing. For vegans and vegetarians you need to find a source fortified with leucine if you are going down the protein powder path (or if your protein source doesn’t contain this all important amino acid). The most effective protein is diary based products which are rich in leucine (whey protein isolate is perfect for the non-vegetarians/vegans out there).
Therefore to ensure muscle mass and fat burning ability isn't hindered, consume enough protein in your diet and space each serving out throughout the day.