Okay, so my brother-in-law does fitness programs like CrossFit and is really into health foods so here is some advice I learned from him on the subject of tall guys and healthy weight:
Tall thin guys worry way too much about gaining weight. The best way to gain weight is to grab a bag of unlimited cookies, sodas, chocolate, and potato chips. Move little as possible and play video games all day long for months on end. If you do this until you're 40 you’ll weigh more......Ok that is a joke but don’t worry about the weight in lbs or Kgs. Measure yourself by your body fat percentages.
A healthy range for males is 6 to 20 percent of body fat. Most tall thin guys are around 6 to 12 % which is on the fit side of the spectrum. Yet they think they are way too skinny. Just forget about it.
If you're wanting to gain muscle mass look at the Zone Diet at
www.zonediet.com. This diet targets the growth of the body by balancing the macro nutrients—fats, carbs, portions. The downside to this diet is that it only works if you’re working out regularly and consistently, and you have a lot of measuring and pre-cooking to do to make it work.
Further, to build more muscle do few sets (2 or 3) and high reps (12 to 15) in weight training.
