Metabolism slows down with ageing, but there’s not much else that can slow it down acutely that I know of. A completely irrelevant one would be anaesthetics during surgery. Other than this, your metabolic rate is largely set by your activity and genetics. You can increase it acutely by doing exercise, and in the long term, your metabolic rate would also go up. Going down is another story. Of course, if you lose a good deal of muscle mass, then your metabolic rate will also slow down, but I would not suggest that you do this either. I’m actually not sure why you want your metabolic rate to be slower – most people want the opposite, so this is slightly confusing. However, the bottom line with metabolism is that you can increase it both acutely and chronically by regular exercise, and it only really decreases in extreme situations.

Regards