what herbs can do this and only answer if you know about this

I am thin and as you age your metabolism can slow. I want to keep my metabolism the way it is now which is in working order so I need to know what herbs can raise metabolism. Can Kelp do this? And I dont believe in megadosing so if I take kelp (or another herb) should I take one or two tablets per day? Please dont give me names of herbs that are unattainable or uncommon.