Here's a pretty good link re: Stevia....
http://www.bentham.org/open/toobesj/articles/V002/101TOOBESJ.pdf
Stevia as Treatment for Inflammatory Bowel Disease
Stevioside and steviol have been shown to exert antiinflammatory
effects on colonic epithelial cells [42]. Animal
studies exhibit stevioside’s inhibitory effect on intestinal
smooth muscle contraction, stimulation of which is linked to
hypermotility-associated diarrhea [132]. The effects of stevioside
and its analogs, steviol, isosteviol, dihydroisosteviol,
and isosteviol 16-oxime, on cAMP-regulated Cl secretion
were studied in human T84 colonic epithelial cells and
in
vivo
for their antidiarrheal efficacy [133]. Results indicated
the major aglycone derivative, steviol, and its analogs,
inhibited cAMP activated Cl secretion in intact T84 cells in a
dose-dependent manner. The ineffectiveness of stevioside
could be due to its molecular bulkiness, rendering it
relatively impermeable to cell membranes, and thereby
exhibiting a promising agent in antidiarrheal treatment