During digestion, the muscles of the intestine contract simultaneously to move the food bolus through the intestine and excrete the undigested material in the stool.
Parkinson’s disease causes degeneration of the neurons that control the intestine muscles, which slows down the digestive process and stool elimination. As a result, stool stays longer in the last part of the intestine, the colon, where liquid is reabsorbed by the intestinal wall, making it dry and harder to evacuate.
Symptoms of the disease can increase constipation problems. For example, less physical activity leads to less bowel stimulation. Difficulty chewing and swallowing may reduce the tendency to adopt a fibre-based diet and to drink enough water and beverages (1.5 litres per day).
Some medications can also cause a decrease in the frequency of bowel movements. Talk about this with your doctor.