Everyones body is different. Even if there was hardcore research that suggests that Marijuana affects your metabolism one way or another, your bodys own physiology may react differently. I've gained weight while smoking and I've lost weight while smoking (and excersising). So IMO, there probably isn't much that's conclusive out there.

You sound like my brother...a metabolism that most of us would die for. If you want to gain weight you can try what he did...a glass of vodka and milk every night.