Usually, aside from water retention, it’s the appetite, I would assume. Lower metabolic rate by itself would lower the appetite because the person would feel less hungry.
Metabolic rate and appetite are loosely correlated at best. Most stimulants simultaneously reduce appetite, and increase metabolic rate. (in fact, that's where a significant portion of their negative side effects come from. Habitual meth users tend to become malnourished, mostly because of the appetite suppression, which combined with teeth grinding jitters, causes the iconic "meth mouth")