Why would you want to gain fatty weight? Drink tons of soda, chips, lead a sedentary life style, cut down on your cigs.
If you want to gain muscle mass, there's nothing better than a full body workout at the gym and then eating protein sources until you're satisfied. Tuna is a great no-fat protein source.

I guess it also depends on your body type, some people have a hard time gaining weight and stay thin regardless of how much they eat. Personally, I find that I can build muscle pretty quickly, but it takes a lot of effort to burn fat.

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