Sure, but not all human tissue is either muscle fiber or fat. You gain other non-lipid mass in addition. Some of it is from increases in connective tissue. A tiny amount is from increased bone density. A large portion of it is from retaining more glycogen and water.

Did you see the articles about Zach, one of Mark Rippetoe's trainees? Based on skinfold caliper measurements, he gained 78 lbs in 6 months, 46 lbs of which was lean body mass, and 32 lbs of which was fat: http://startingstrength.com/resources/forum/showthread.php?t...