General

My current plan is to drop thirty to forty pounds or so, and then I plan on giving gaining a shot. The thing is, I feel like my reason for gaining is just to lose it all again afterwards- I think it's really the whole "being extremely, unbelievably fat, and then losing it all over again" part of the equation that interests me the most. I'm extremely good at weight loss and my main method of doing it in between gaining phases might be fasting (48-72hr fasts). And if someone has enough bodyfat on them, it's possible to do those rapid weight loss protocols quickly while even lifting weights and building some muscle.

When I gain, I'm going to make sure that I do it as healthily as possible, but I also want to make it happen rather quickly so that I don't stay at a heavy weight for too long, since a lot of metabolic issues occur gradually overtime and not acutely. I hope this makes sense, and if I'm wrong, then someone can correct me on it.

The thing is, I still haven't quite decided on where I should draw the line. I don't know if it should be when I start experiencing mobility issues, or if I should set legitimate weight limits.

As I understand it, yo-yoing like this is actually not great for your health. I guess if it's just 30-40 pounds over and over, it's not super bad. Especially on a yearly cycle; I imagine we must be at least somewhat adapted to that as a species by now. I can't really avoid doing this myself; I have a labor-intensive job in the summers and usually lose about 30 pounds; it can't be helped. But I'm also the healthiest person in my family.

If you're worried about metabolic issues, just make sure that a decent part of your gains are muscle mass. That also helps with mobility issues. 40 pounds isn't going to make you immobile though. 400 pounds wouldn't even do that, assuming you're starting from under 200.

Thanks for your input.
As far as I know, the health effects of yo-yoing are somewhat a "myth." It's no worse for someone's health than it'd be to stay at a high BMI for extended periods. pmc.ncbi.nlm.nih.gov/articles/PMC4205264/
I'm still not sure about the kinds of weights I'd like to reach. Some of them are just a one-time thing that I'm thinking of hitting just as a once-in-a-lifetime personal milestone and then never reaching them again. But I'll have to spend some time thinking about if I really should reach them or if it should just stay as a fantasy, haha