Um, there really isn't anything you can do to slow it down healthily and still build muscle.

Instead of trying to gain fat then to convert it (A process that will probably not work because of the extreme strain it puts on your body), eat more protein and eat about, 2x more then you usually do. Eat a lot of carbs and minimize the fat you eat, and make a work out plan that will push you everyday. Don't kill yourself, but if you aren't sore the next day you didn't work hard enough.

The converting fat to muscle is a very hard process and it's also extremely stressful, believe me, I know. Not to mention, that only works for people who already have fat, and you need a lot of fat, cuz fat is basically fluffy and takes up a lot of space, but it's not very dense where as muscle is really dense and that's why it weighs so much more.

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