I read in the juiceman juicer book to never drink green juice without either some carrots, or apples, or you are apt to have gastric discomfort. He says to fill the glass with only 1/4 of green juice and then add the rest with either carrots, apples, or sometimes celery. He also says not to drink more than 4 oz. of beet juice or 2 oz. of wheatgrass juice at one time.
I always use some carrots and Celery in my green juice and I don't have a problem.