I read somewhere that since he's switched to a vegan diet his walking around weight pre-camp has dropped considerably. Fitch is one of the bigger welterweights, so I guess we'll see if he performs any differently in a few weeks against BJ. As far as I'm concerned, as a fighter, recovery should be his main concern. If he can recover properly and maintain strength and intensity in his training, then all the power to him. I know for a fact his endurance and stamina, something he's already known for, will only get better with his new diet. As a long term vegan myself, the overall feeling provided by that type of lifestyle is worth the limitations it imposes.
Also, Jake Shields is a lifetime vegetarian and he's no slouch.