This is a question that baffles many, but the truth is that the answer is not so straight forward. It’s certainly possible to do, however only in particular cases.
Traditionally, fat loss is attributed to a calorie deficit and muscle gain is achieved by eating in a calorie surplus. As both methods are conflicting many will claim that you can not do both concurrently, however, the human body works in weird and wonderful ways, often that we don’t always fully understand.
When is it possible?
If you’re a newbie to resistance training or even someone who has taken a significant amount of time off from training, you are the exception to the rule.
Studies have shown that even though the body is burning more energy than it is taking in, you can still synthesize new muscle proteins and achieve a significant increase in your lean muscle mass with regular resistance training.
Strength and muscle gains are very rapid for beginners or those returning to exercise due to the stimulation of the neuromuscular nervous system. Unfortunately though, if you are remaining in a calorie deficit for an extended period of time you will only be able to go so far before you reach a plateau.
If you really want to build a significant amount of muscle and at an optimal rate, a calorie surplus combined with regular resistance training is going to be the most effective mode of doing so.