from williams m. nutrition in health and fitness sport:
'"Muscle tissue consists of about 70% water, 22% protein and the remainder is fat, carbohydrate and minerals. Because the vast majority of muscle tissue is water, which has no caloric value, the total caloric value is only about 400-800 calories per pound of muscle. However, extra energy is needed to synthesize the muscle tissue.
It is not known exactly how many calories are necessary to form 1 pound of muscle tissue in human beings, nor it is known in what form these calories have to be consumed. The National research council notes that 5 calories are needed to support the addition of 1 gram of tissue during growth, while Forbes cites a value of 8 cal per gram in adults. Because 1 pound equals 454 grams, a range of 2300-3500 extra calories appears to be a reasonable amount....A study by Robert Bartels and his associates at Ohio State U revealed that an additional 500 calories per day resulted in nearly a 1 pound increase in lean body mass per week during a resistance-training program."