i duno how body fat % plays a role but im assuming that it is directly linked with how fast ur matabolism is cuz im 5'10" and 155 lbs at 5% body fat aswell and my metabolism is extremely fast so for people like us it doesn't stay in our system nearly as long as a bigger person with slower metabolism especially if your drinking a lot of water, and as for niacin i've heard that its a complete myth, that it doesn't work at all, if your into dilution try a product called Certo, i put up a thread with peoples experiences with the product, u can find it under my profile. best of luck. stay high.