Gaining Muscle vs. Losing Fat
The Holy Grail of many a fitness fanatic is to gain muscle and lose fat simultaneously. While a select few can do both, at least to a small degree, the majority of people cannot. Here is why:
To gain muscle, a person needs to rigorously exercise their skeletal muscles on a regular basis while supplying those muscles with optimal amounts of nutrition to fuel muscle growth.
Generally, weight-training is the best option for stimulating muscle atrophy (growth), but various forms of calisthenics, plyometrics, and sport-specific activities can be used to good effect.
Once the muscles are shocked and stimulated by repeated forms of muscle-building exercise, appropriate nutrition, both in quality and quantity, is needed to supply the fuel needed to fuel muscle growth.
In terms of muscle-building, "appropriate nutrition" means eating a surplus of calories on a daily basis. To create new muscle mass, your body needs excess calories to fuel the growth.
Working out is only half the equation; optimum nutritional intake completes the other half. Exercise provides the stimulus; nutrition provides the fuel!
And in terms of trying to gain muscle mass and lose fat simultaneously, herein lies the problem: those two goals are antagonistic.
Just as adequate nutrition is necessary to fuel muscle growth, an optimal nutritional intake is required to shed body fat.
And for losing fat, "optimal nutrition" means less calories.
In order to lose fat, you must burn more calories through exercise and normal metabolic function than you ingest through diet.
This caloric deficit is what allows your body to pull fat from its fat stores to be burned as energy.
The larger the caloric deficit, the faster you will lose weight.
Although effective as a fat-loss tool, a caloric deficit prevents muscle building, since too few calories are available to fuel muscle growth.
All the weight-lifting in the world will not grow new muscle mass unless enough calories are available to fuel muscle repair and growth.
This is why many newcomers to the fitness game get discouraged when they don't achieve their goals of building muscle while losing fat.
The only people who sometimes can do both are people who have never done much resistance/weight-training before and whose muscles seem to grow simply by virtue of the "shock" or "newness" of any type of resistance training.
This effect, however, doesn't last long, and after experiencing some early muscle-building success, they hit a plateau.
The Best Strategy
For people seeking to both build muscle and lose fat, the best strategy for success would be to choose one goal at a time.
Either focus on gaining muscle mass, which means doing a lot of resistance training combined with a hyper-caloric intake, or focus on losing body fat, which means reducing caloric intake while increasing physical activity.
You can work to build as much muscle mass as possible, then you can change gears and shoot for fat loss. Or, you can try to get as lean as possible, then work towards building muscle mass.
Either way is great and will work better than trying to accomplish the nearly impossible task of trying to both at the same time!