It depends on your definition of those terms and your lifestyle.
You can be fairly heavy and active. Now extra weight does tax your joints and such but being stronger can help to combat that.
We have the habit as a society of demonising any ‘excess’ weight. So I suppose it is unlikely that being an Ssbbw is healthy, but I would say you can for sure be a bbw live a healthy lifestyle.

Of course the other issue is for a lot of People who get kicks from being a bbw part of that appeal of that is becoming less muscular and weaker. The lazy pig fetish if you will.
I would say that if living a healthy and long life is something you want to do, then that part of the kink should remain in the realm of role play. While you are still keeping yourself active and healthy you can allow the fantasy to remain.

Another great way to gain. Have a huge break fast. And while your still not hungry have an early lunch but nothing too big.

Delay dinner as long as you can so you are just starving which will make your body think it is starving and make it store more fat.
Then eat a hell of a lot like we talking twice ad much as usual for dinner, between being hungry and eating quickly that should be fairly easy,

If you don’t have the discipline to wait another great thing to do is have early dinner but then add a new meal right before bed. I say meal but it could just be sweets if you want just make sure it’s a lot and you go to bed stuffed because that will store more if it as fat.

Also cooking with lard instead of oil or butter can help.

You can mix in some corn syrup to milk for a nice lighter drink that really adds the calories too

cake, cream and ice cream.

A good shake can really do it.
The weight gain shake I used to make a feedee way back was.
1 full punnet double thickened cream.
4 tablespoons Nutella or peanut butter.
2-3 tablespoons of cookie dough
3 tablespoons Hershey choc fudge
3-4 scoops double choc ice cream
3 tablespoons Corn Syrup
Top blender with creamy milk

You gain add a weight gain mix if you want.
You can also add alcohol to which aids with the gaining a lot. I would often put in a shot of vodka and a shot of frangelico.

Have one of those a day and youlll gain soft fat in no time.