Since taking oatbran in a smoothie my bowel movements are getting harder, how much bran should you have daily?

Depends, more water!: How much fiber it takes to change your bowel habits varies from person to person. More fiber is great for you. Oat bran is fairly dense with it and it may need an increase in the amount of non-caloric, caffeine free liquids you take in. Chug-a-lug! And back off a little on the bran and increase more slowly if you're having problems with it. If you notice bloating, gas, simethicone helps.