I have a fast metabolism and am naturally on the slender side. I'm trying to gain weight/mass by eating twice as many calories as recommended for those trying to maintain weight-- eating 6 meals a day each containing dense calories. How long before I see a change physically and on the scale? I've also started working out again, doing some cardio and strength training.
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Surprisingly, I had once gained over 10 pounds without seeing any change in my appearance. Then again, I probably gained some lean muscle. I suggest that you stick to whole carbs, lean meats, fruits and vegetables etc. Yes, I know that you are trying to GAIN weight but you should still do it healthily. Kudos to you for doing cardio and strength training but don't forget to stretch! I find that my workouts are more effective if I stretch a little before the workout (dangerous to overextend muscles that haven't warmed up yet) and more at the end of one. Good Luck!