17 - Intestinal Gas

Abstract

The digestive tract contains 100 to 200 mL of gas, which is under tight homeostatic control. Physiologic aerophagia and chemical reactions in the small bowel are sources of gas, but, in general, gas originates from fermentation of nonabsorbed meal residues by colonic microbiota. A substantial part of the gas produced is absorbed into the blood and eliminated by breath, and the rest is evacuated per anus. Patients with functional intestinal disorders frequently attribute their symptoms, particularly abdominal bloating, distension, and flatulence, to intestinal gas. Flatulogenic meals worsen these symptoms, but the source of the gas that produces symptoms is not clear. Intestinal gas production, intraluminal distribution, and evacuation in these patients is in the normal range, suggesting that symptoms are related to poor tolerance of intestinal gas. Diets low in fermentable residues decrease gas production and markedly improve symptoms, but in the long run such restrictive diets may have deleterious effects on intestinal microbiota. Other studies have shown beneficial effects of prebiotics, probably by a modification of intestinal microbiota, with proliferation of bacteria that metabolize residues without releasing gas. Excessive eructation has been related to compulsive aerophagia (air swallowing), impaired gas evacuation has been attributed to functional outlet obstruction, and visible abdominal distension has been explained by abdomino-phrenic dyssynergia (diaphragmatic contraction and descent), and these gas-related symptoms may benefit from behavioral therapies. Pneumatosis cystoides intestinalis is a specific entity characterized by gaseous cysts in the intestinal wall; its pathophysiology is not clear, but inhalation of high concentrations of O2 is an effective treatment.