Some iron is lost in sweat and. for unknown reasons. intense endurance exercise is sometimes associated with bleeding of the digestive system.(Para. 6) Some iron is lost when one sweats and for the reasons unknown to us. intense endurance exercise may sometimes result in bleeding of the digestive system the whole length of the pipe down which food travels through the body while it is being digested. See note 3 of the Background Information• Some iron is lost in sweat, and, for unknown reasons, intense endurance exercise is sometimes associated with bleeding of the digestive system. (Para. 6) Some iron is lost when one sweats, and for the reasons unknown to us, intense endurance exercise may sometimes result in bleeding of the digestive system. the whole length of the pipe down which food travels through the body while it is being digested. See Note 3 of the Background Information