Bit of history: I've recently come off a successful long-term (6 months) combination of low-calorie diet (~2000 cal/day) and exercise (4x/week) that has dropped 40 pounds. I'm now thinner and in better shape than I have been since college (I'm now 30, 6'4", 175lbs). Hurray! I'm very proud of this.
Trouble is, it's a bit TOO successful, in that now I'm just skinny. As a tall vegetarian, I don't expect to ever be huge, but I would like a bit more mass in the upper body so I'm not such a stick. So a couple weeks ago, I decided to go off the diet and start supplementing with protein - a 20g shake in the morning and a 20g bar in the afternoon, combined with simply eating more.
I shot up four pounds in 2 weeks! This has me worried, because I've been so focused on dropping weight for so long, fighting to get every pound off. I've been told that protein that isn't used to repair muscles gets stored as fat, and while I know muscle weighs more than fat, 2lbs/week of muscle gain seems unlikely. Of course, I have no idea what I'm talking about.
I DID see improvement at the gym during the last couple weeks, but not necessarily more than I've been seeing all during the diet. My workout is typically 2 spin classes a week with a long ride or hike (with weighted pack) on the weekend for cardio, and a weight circuit twice weekly and one or two session in the rockclimbing gym for strength.
OK, actual question time:
How do I gain weight while ensuring that the weight I'm gaining is the weight I want (ie, lean muscle mass)? I have no means of calculating bodyfat percentage. This feels like such a leap of faith to let my weight climb unchecked, it has me worried and thinking about dropping the protein supplements.