Gaining Weight

Hello All,

For a while I have struggled with gaining weight. For most of my early highschool years, I did loads of cardio, and I think I was burning so much weight combined with the fact that I wasn't eating enough; I got a late start to the whole idea of really improving my physique. Therefore, I'm much smaller than I should be.

For some context Im 6'1 138, and I'm only going to get taller. Im 16 years old and I know I can improve my physique, but it's so difficult. Even with eating 3,500 calories a day, I feel like i'm only seeing marginal gains.

If anyone has been in my spot before, or knows anything about how to gain weight easy, and fast, please leave some tips below. (Ps. I don't go to the gym very often, and I'm really considering going regularly. There aren't any downsides, I'm just being lazy)

Thanks,

Sheriff