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I mentioned earlier that I know almost nothing about how to gain weight intentionally. I've never had to struggle to put on weight in my life! But I know many people in the ex-gay community actually face a struggle putting weight on. This becomes difficult because gay men value extreme thinness while women generally do not.
I found a friend who is very well versed in how to gain weight, since he had to go from 140 to 215. He points out the following bullet points, which play out in the video below:
1. A healthy pace of weight gain is roughly 5 pounds of muscle every 2-3 months. Healthy weight gain is slow compared to healthy weight loss.
2. Exercise has to emphasize strength training. This means using your muscles against some resistance. The resistance has to increase so that your body has to put on more mass to increase strength.
3. It is important to work all muscle groups rather than only focus on the usual fixations, like chest and arms. Chest, arms, core (abdomen), back, shoulders, legs, and gluteals all matter. Legs, which are often neglected, actually account for a lot of muscle mass.
4. You have to increase how many calories you eat. You should ideally place most of the increased calorie intake on proteins and good fats, not on sugars.
5. My friend suggests that you find someone who can mentor you with strength training. I agree. Good luck!