As the upper body posture improves, the pressure on the stomach, small intestine and colon decreases and the stomach is no longer pressed between the pelvis and the chest. The increased muscle activity in the stomach area also increases intestine activity.
All this has a positive effect on the bowel movements (peristalsis and its efficiency) and intestine health. In our modern lifestyle the intestine health is weakened also by too little of muscle activity. One-part chairs (traditional or saddle) also press the anus and rectum, slowing down or postponing the evacuation reflex.
Slow intestine activity or long stay of food in the body is involved in common intestine illnesses from which a large part of the population suffers. Sitting with good posture would be a strong preventive measure.