[apmee]

It's much much harder to gain muscle and lose fat simultaneously, than it is to simply focus on one then the other. They are fundamentally at odds with each other: to lose fat, you need less calories; to gain muscle, you need more calories.

The advice I've heard is to build strength and size first, by eating lots and training hard until you have the muscles you're after. Then you can turn your attention to losing fat and maintaining muscle, by eating less (though you'll still need more calories than before, as larger muscles burn a lot more energy even at rest), lowering the intensity of your training, and focusing more on cardiovascular exercise like jogging and cycling.

[dkarl]

Up to a certain point, you can do both just by exercising more and harder. A complete beginner starting from zero volume will definitely lose fat and gain muscle if they start working out, as long as they eat no more than enough to maintain their weight. The only problem is that you can only do it if you're pretty poorly trained to start with. Most people who work out seriously are well past the point where they can do both at the same time.