As a dentist, I have treated many patients with retrusion of the lower jaw (commonly referred to as "TMJ or TMD" - poor term). This retrusion results in forward head posture affecting the neck and shoulders with a probable referral to the Vagus nerve in the spinal cord. An X-ray of the head and neck, in profile, will show the status of the cervical spine which normally has a curvature of about 17 degrees. In FHP, this curvature is lost and the cervical spine is straight, or even bent slightly forward. since the Vagal nerve has effect on the intestines, this is a probable cause although I am unaware of any definitive studies. There are also other conditions/symptoms related to this condition.