A lot of people want to gain muscle and lose fat at the same time. They are often frustrated by the fact that when they start gaining muscle they start getting fat as well and when they start losing fat they losing muscle mass. The body is either in a catabolic or anabolic state and depending on the overall hormone and caloric levels you can either gain muscle or lose fat.
In order to achieve an efficient muscle-gaining mode you should have a caloric surplus (this means that you should be consuming more calories, than you need), your hormonal system should be functioning optimally (testosterone and GH levels on max) and you should constantly cause a progressive adaptation stimulus to the muscles by overloading them with weight training (trying to get stronger by lifting heavy).
In order for one to actively burn fat his adrenal and thyroid gland should be functioning to the max in order to release enough fat burning hormones (fat burning hormones speed up release of fatty acids from the your body). You should also sustain a negative caloric balance and will diet in order to stimulate the release of fat stores (eat less than needed).
Hormonal System: The human body under normal conditions has a limited capacity for hormone production. How the hormones will be used is up to the demands of the body. If you are actively training then a majority of the testosterone will be used for muscle protein synthesis and tissue regeneration. This holds true for all other hormones, thus you should choose a precise goal and follow it, in order to accumulate all the resource of your body towards achieving it.
Getting more calories than you need will make you gain mass and getting less calories than you need will make you lose mass. Simple as that. This means that there is no way to build new muscle tissue while losing fat. The body prefers to focus on one target and maintain it.
Training: When you actively build muscle you use very heavy weights and don’t do cardio. On the contrary when you burn fat you use little more repetitions and do cardio, which doesn’t not allow for muscle gain.
What’s the conclusion: When you burn fat – the whole body is focused and adjusted to fat burning mode, which does not allow for efficient muscle gain. On the opposite, when you are gaining muscle, the body is focused on creating new tissues and cannot actively burn fat.
You can do a bulking phase, followed by a cutting phase – this way it works best. However, remember that bulking for too long or cutting for too long will diminish your results.