From "Go Ask Alice!" at Columbia University:http://www.goaskalice.columbia.edu/2946.html
Scientific research has yet to document whether or not smoking marijuana (cannabis) has a direct effect on metabolism; however, it appears that the body processes food similarly when high on pot as when sober.
Studies have shown, though, that cannabis does trigger an increase in appetite, hence "having the munchies." "The munchies" is usually a desire for sweet and fatty treats, foods that, when not consumed in moderation within a balanced, varied eating and exercise plan, promote weight gain. Studies also show that individuals who regularly smoke pot gain weight over time. The increase in appetite leads to extra caloric intake, contributing to weight gain. In fact, even though the use of marijuana as medicine remains a controversial issue, the appetite-stimulating effects of marijuana have been documented. HIV/AIDS patients who need to gain weight, for example, eat more when they smoke cannabis.
Based on what is known, marijuana does not influence metabolism. In other words, what you eat matters just as it would if you were not high on pot.