Bodybuilders, athletes, models, whoever needs to keep in shape, want a high metabolism. The faster the metabolism, the better we process calories for positive effects. Ganja DOES affect metabolism, fortunately or unfortunately. Smoking marijuana has been shown to lower thyroid hormones in the blood, although no study has ever shown it to have an impact on thyroid function. This potentially means that your metabolism may be lowered slightly in the short term when using marijuana, but there are unlikely to be any long lasting ill effects. Whew! So far, long lasting effects on metabolism have not been clearly noted, so take that how you want.