I've occasionally been asked about gaining weight while skinny. This is because I used to be quite skinny, and while I'm not a big guy even now, I carry noticeable muscle.
In the past, I've lost weight and gained weight. I'm 6' 4". My heaviest, I was 217 and it wasn't comprised of a lot of muscle. My lightest, I was in the 160s and much weaker than at 217. My best size/fat level was around 198-200, which took constant maintenance. These days I float around 183-188 without too much work. This is because of years of playing with my diet, my exercise, and my habits. Making weight for MMA and grappling, where I did 83 kg and 170-179.9, respectively, trying to come up or down weight classes, and adjusting to totally different diets went into getting to where I am now.
I can say this with experience, and certainty: For generally lean, narrow framed guys, gaining weight can be hard.
First, you are skinny for a reason. Relatively low caloric intake, and/or more activity characterize the skinny guy. Doesn't eat too much, moves around a lot, and may tend towards higher-volume workouts (lots of running, lots of reps, lots of martial arts). Even skinny guys who eat "tons" of food tend to do it in shorts bursts - all you can eat sushi today, nothing for the rest of the day and less the day after than usual. If these things weren't true, you probably wouldn't be skinny.
Second, breaking a set point is tough - you have to lift enough, and eat enough, to convince your body it really needs to be 10 or 15 or 20 pounds heavier and stay there. You must do these consistently in order to succeed. The human body loves homeostasis - after all, right now, you're alive and things are generally working. It takes effort to convince your body to try something new. For weights, it's working hard in the gym. For eating, it's consistently eating appropriate - and large! - amounts of food.
Most people trying to lose weight will tell you'd they'd kill for your problem. But I've lost weight and I've gained weight. Dieting sucks, but eating 5000-6000 calories a day, day in, day out, isn't any more fun. Especially if you then get sick for a few days, can't keep food down, and your body drops right back to your starting weight. It's expensive, it's time consuming, and you eat like it is your job. You eat when you aren't hungry, and you eat even when you're sick of the foods on your eating plan. You weigh your foods and log them into a diet website to ensure you're eating enough. You have to know when to put down the weights (because you've done enough to gain, and not too much) and pick up the fork, and resist the temptation to get in some extra cardio to stay lean. You have to fight against what you've been doing in the past because it only got you to lean, not to muscular and lean.
Generally, this means eating like it's homework. You must do it, or you get an F in the gym. When it comes to weight loss, they say you can't out-train a bad diet. You also can't out-train insufficient nutrition when it comes to gaining weight, either.
Important Note: Forget About Your Abs
This needs to be said, because even skinny guys with no real abs are worried about losing them. Forget about them. Worrying about staying ultra-lean while getting more muscular is a trap. You won't succeed at the latter until you make it a priority over the former.
Trust me, if you start with abs you'll be back to them in no time (and you may not even lose them in the process.) Get stronger and bigger first, and then you can come back to leaning out. One thing at a time.
Tomorrow, I'll give some diet specifics that worked for me and clients in the past.
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