It must be understood that when fat cells are burned, the stored THC metabolites are released back into the bloodstream. The kidneys filter the THC metabolites from the bloodstream and deposits them into the bladder for the eventual exit. This is the main reason why THC carries a longer detection period than other types of drugs.

Any metabolic benefits resulting from niacin are sloooww in coming (we're talking months, not days or weeks). For your time frame, you'll accomplish more by just sticking to exercise. However, if your scheduled U/A is 3 days or less away, then fat-burning activities can work against you for the reason stated in the first paragraph. Therefore you want to curtail all exercise during this period and be as dormant as possible to avoid releasing a bunch of THCA into the bloodstream, which will end up in the urine to be detected.