Robbo87, all good advice given to you, except I disagree with the last sentence, the "light weights", as you won't activate as many muscle fibers. The primary focus is to make sure you don't aggravate the IBD though, so if light weights was recommended due to your disease, you best take it slow. But in the end, ultimately, you will see the best results by activating and calling into play the largest portion of fibers you can, and going to or close to failure (meaning you can't lift another rep, but be careful, only do this when form is understood very well...).... You call into play and activate the most fibers by lifting heavy. For example, a 100% maximum effort will in theory, call into play every possible muscle fiber, whereas a 30% effort (of your potential maximum) will call in a much smaller number of fibers. What happens is that your fibers in the 30% or so lifts will "tag team" so to say, and after a few reps, they will tire out and slowly the other fibers will be called into play for a while, when they get tired, it continues on....thus, your fibers are always being "benched" to some extent, and your movement in question never fully utilizes all the fibers at once, so the exercise in the end used much less of the muscle than it could have. On top of that, your CNS (Central Nervous System), which is responsible for the above mentioned "calling into play" or activating of the muscles, is important as well. You must remember to "train your CNS" as well as your muscles, because it's an integral part of the process and your progress as you gain muscle. You basically accustom your CNS to activate your fibers more efficiently over time, and slowly you can condition it to help your muscles in the lifting more and more.
Not to get to much more technical, but the last thing to note is that their are several types of muscle fibers, type 1 and type 2a and type 2b, and they all have different roles and growth potential....You lift a pop can to drink it, you are activating your type 1 fibers in your bicep (arm bicep, as to mention a random factoid, you have a "bicep" in your leg, because all it translates to is "two-headed" muscle, and you have one at your hamstring behind your leg, anyways...)
You lift a heavy weight, one you can only lift 8 times until you can't lift it a 9th time, you're using your type 2 (a and b are still divided even more as far as 1RM, or your 1 rep max)...it all comes down to how much resistance is being demanded of your skeletal muscles. Your type 2 have the most growth potential, so lifting heavy has the biggest impact on size gains, though type 1 can grow too. For proof, just look to runners: You have your marathoners, who are very lean and "scrawny" if you will, when compared to your sprinters, if you watched the Olympics, you saw the sprinters with massive tree trunks for legs, because sprinting asks your type 2 fibers for maximal loads to exert bursts of stress to propel you at great speeds. Your type 1, like the marathon or long distance runners use more, are asked to repeat low resistance reps many, many times over in endurance-based loads....
I will try to grab some links, but if you want try searching "weight gain" in the search for the forum and you might find them before me.
Remember, just out-eat your TCB so your body has no choice but to store the extra calories. **The most important thing is how your body 'stores' these excess calories, because if you are lifting weights and exercising hard, your body will see great benefit and almost a necessary adaptation, in making more muscle and "healthy weight". If you are very sedentary, your body sees no reason to store these calories as anything other than fat, and especially visceral fat (another word for abdominal fat)...
Eat tons of protein if you can tolerate it....About 0.8 grams or more per pound.