Trust me, the subject isn't taboo on here and various incarnations of bowel movement threads have taken place.
My own experience (and what seems to be many others) is that there is a definite decline in the frequency and volume, particularly if there is more protein consumption. Meat is processed very efficiently by our bodies and as such, leaves very little waste so there is less to eliminate. If a person eats a lot of vegetables it can increase the volume and frequency but it's still typically less than on a SAD diet.
Lots of grain products = lots of poop.