So the question of how many extra calories you should eat to gain muscle mass depends on how many calories you are burning each day.

CNN: How should I eat to build muscle mass?

When the bishop of Dijon was burning Santas, most Frenchmen still went regularly to mass.

ECONOMIST: Spoil the French and spare the Child

Before I answer your question, let me clarify that while metabolism does drop with age (about 3 percent per decade) due to muscle loss and hormonal changes, it does not drop with exercise, but rather increases due to direct calorie burning during your workouts and indirectly through an increase in muscle mass as a result of your workouts.

CNN: What's the best way to 'kick-start' my metabolism?