Why in the world would you want to do this? Take in more calories if you are having trouble gaining, as richardjst mentioned. This is the key to gaining. Your metabolism will slow down soon enough on its own unfortunately.
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Quoting from F.ZaneFan:
I want to slow my metabolism down and I thought about foods that would slow it down. What do you think?
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-------------- Great people never want it easier, they just want to be better!
Ron