I have heard somewhere that if you drastically cut your calories that your metabolism will "slow down" and then you will gain weight more easily after you begin to eat again. Any basis in this. I like to see research to base things on.
That's generally true. In this case, you're not necessarily drastically cutting calories. It's a fast but a detox also. Some people want to gain weight during the Master-Cleanse and they can if they drink 7-12 glasses of lemonade gain daily or add additional syrup to their glasses.
I exercise by my usual walking each day during this cleanse and my metabolism remains high.
One very important fact to consider which should ease your mind is that the liver has a lot to do with your body's capacity to burn food efficiently, and just do it like it's supposed to.
Heck, there's even a book called: "The Liver Cleanse Diet" that involves drinking lemon juice in the morning, eating legumes, veggies, etc.. and taking milk thistle, and the whole point is to boost metabolism and lose weight.
I think you are doing your liver even better on MC.