Why do you feel two pounds lighter after a good bowel movement?

Probably because U R: Normal bowel frequency in america fits a bell-curve distribution--once daily stooling is the middle of the curve, but the normal range extends from 3 bowel movements daily to 1 bowel movement every 3-4 days. Stool volume, weight, consistency, color vary (see other healthtap answers on the subject), but you very well may be passing 1-2 pounds of stool a day appropriate to diet, transit time, etc.