[www.ncbi.nlm.nih.gov]

Altered gastric microbiota, reduced mucosal protective mechanisms, decreased gastric blood flow, and consequently compromised repair mechanisms are the hallmarks of age-related gastric changes [40, 41].

Loss of cholinergic enteral neurons seems the most plausible explanation for decreased motility in elderly. While there is so much more to discover about enteric neurodegeneration that occurs with aging, it seems reactive oxygen species (ROS) may play a central role

Decrease in acid secretion as a consequence of chronic atrophic gastritis leads to two problems that are particularly prominent in the elderly population: small intestinal bacterial overgrowth (SIBO) and malabsorption [50, 51].

Emerging knowledge about the changes in human gastric microbiota associated with aging has shed new light on its association with gastric cancer. Pearson and co-authors [41] showed that gastric microbiota varies for patients with autoimmune and H. Pylori associated atrophic gastritis, with former expressing higher bacterial diversity and abundance. Both conditions are associated with reduction in gastric acid secretion and development of gastric cancer

The majority of ulcers in elderly is caused by H. pylori infection or is associated with the use of NSAIDs/aspirin [69, 70].

Angiodysplasias are acquired lesions associated with aging. They are characterized by the presence of a cluster of dilated, torturous, thin-walled vessels involving small capillaries, veins, and arteries [101]. The increase in incidence of angiodysplasia with age is thought to be due to changes in the composition and structure of extracellular matrix in the wall of the small intestine.

Small intestinal bacterial overgrowth (SIBO) implies excessive presence of bacteria, above 105-106 organism/mL in small bowel aspirate [108]. Common in elderly; it is associated with chronic diarrhea, malabsorption, weight loss, and secondary nutritional deficiencies.

Motility of the small intestine is not affected by age itself. Rather than age itself, slower motility in elderly is associated with medications, polypharmacy, and presence of concomitant diseases

Decreasing synthesis of neurotransmitters is another theory that attempted to explain potential decrease in colon transit time associated with aging and Takahashi et al. documented a significant decrease in nitric oxide synthase (NOS)-immunoreactive cells as well as NOS synthesis in colonic neurons [118].

There is a high degree of variability between infant microbiota (dominated by Bifidobacterium) and microbiota of an adult person (Bacteroidetes and Firmicutes dominate). Age-related changes in human microbiota have been associated with inflammatory bowel diseases (Crohn's disease and ulcerative colitis), irritable bowel syndrome, and metabolic disorders (diabetes mellitus types 1 and 2 and obesity).

Increase in prevalence of constipation in elderly is not related to decrease in colon transition time as much as it is to decreased mobility, cognitive impairment, comorbid medical problems, polypharmacy (especially opioid and anticholinergic medication use), and dietary changes.

Diverticulosis is an acquired condition referring to presence of diverticula—sac like outpouchings of mucosa and submucosa of colonic wall. They are believed to develop due to increased intraluminal colonic pressure at the points of least resistance in the muscular wall

Rectal bleeding is considered an alarm feature but is frequently found in patients with IBS of any age [155], whereas weight loss and poor appetite are “red flag symptoms” that are commonly found in elderly irrespective of the presence of IBS.

One recent theory argues that gut motility affected by 5-HT (serotonin) concentration might be responsible for IBS [157, 158]