in food from the external environment to the internal environment. During the course of the day, an adult usually consumes about 800 g of food and 1.5 liters of water and most of the nutrients and water are normally absorbed in the small intestine. Only approximately 150 g are eliminated from the body as feces. Secretion into the tract may amount to 7-8 liters of fluid, resulting in a total of 9-10 liters of fluid passing through the tract per day. Thus, a great amount of gases enter or leave the gastrointestinal tract with both solid and liquid substances.
On the other hand, although the intestines of normal subjects usually contain less than 200 ml of gas regardless of taking meals, the gastric bubble present in approximately 70% of normal chest and abdominal radiograph results from the 2-3 ml of air conveyed to the stomach with each swallow [1]. Gas within the stomach may exit via the esophagus by way of belching or via the duodenum, but most of the swallowed air should be regurgitated as a belch. The rate of excretion of gas per rectum is highly variable, ranging from 476 to 1,491 ml/day [2]. Gases are also produced while passing through the gastrointestinal tract, possibly resulting in abdominal symptoms although gas is continuously removed by eructation, anal evacuation, absorption through the intestinal mucosa, and bacterial consumption.
Esophagus
Gas passing through the esophagus may be derived from two sources: air swallowing and belching that is the passage of gaseous material from the stomach to the mouth. Nitrogen accounts for approximately 78% of swallowed air. Because little nitrogen is absorbed through the intestine and only about 400 ml of nitrogen is passed in flatus per day, most swallowed air should be eructated subconsciously or consciously [1]. It seems that 2-8 liters of air may be swallowed and eructated per day because of the 2-3 ml of air conveyed to the stomach with each swallow in an adult who usually consumes about 800 g of food and 1.5 liters of water.
Stomach and Duodenum
Since the esophagus enters the posterosuperior aspect of the stomach, gas is trapped above liquid overlying the gastroesophageal junction in the supine position. Therefore, most swallowed air is entrapped in the stomach and can travel along the small bowel.
In the upper gut, large quantities of gas are physiologically generated from chemical reactions of hydrogen ion and bicarbonate. For 1 mEq H+ neutralized by bicarbonate in pancreatic, biliary, or duodenal secretions, 22.4 ml CO2 is produced [3]. Fat digestion causes much gastric acid secretion and would result in an estimated CO2 production rate of 1,400 ml/h. Most CO2 produced during normal digestion is absorbed along the small bowel [4]. Thus, the luminal gas in the upper digestive tract is partly swallowed and in part originates from a series of chemical reactions of the foodstuffs within the gut. Theoretically, the calculated amount of CO2 produced in the upper gut is 33.6 liters per day (1,400 ml multiplied by 24 h). However, the actual volume of CO2 liberated as gas may be far below this value owing to the slow decomposition of H2CO3 to CO2 and H2O in the absence of carbonic anhydrase and the relatively high solubility of CO2 in water [5]. Presumably, approximately 5-10 liters of CO2 seems to be produced and released into the lumen by chemical reactions.
As mentioned above, the pCO2 rises dramatically in the duodenum, resulting in diffusion of CO2 from lumen to blood, whereas CO2 in swallowed air diffuses from the blood into the stomach [6]. Because the pO2 of swallowed air is greater than that of blood in the stomach, O2 is absorbed from the stomach. The pN2 of swallowed air is slightly higher than that of venous blood in the stomach, possibly resulting in very slow absorption. In the duodenum, N2 diffuses from blood into the lumen according to the partial pressure gradient because of the down gradient established by CO2 production. In contrast, since pH2 and pCH4 are always higher in the lumen than in the blood, gases are constantly diffusing from lumen to blood. In our previous study using endoscopy, H2 and CH4 gases are more frequently detected in the stomach than expected, regardless of the presence of abdominal symptoms [7] although it has been reported that the stomach and duodenum harbor very low numbers of microorganisms adhering to the mucosal surface, typically less than 103 bacteria cells per gram of counts [8]. Fried et al. [9] reported that most of the bacteria identified from the duodenal aspirates belonged to species colonizing the oral cavity and pharynx, suggesting a descending route of colonization. Also, Thompson et al. [10] indicated that fermentation of ingested carbohydrate by oropharyngeal bacteria could contribute to measured breath hydrogen values soon after meal ingestion. Moreover, intraduodenal H2 levels were higher in patients with severe atrophic gastritis than those without atrophic gastritis, and there was a progressive increase with the progression of atrophic gastritis [11]. In contrast, the intragastric H2 level was the highest in patients with gastric mucosa of the closed type and was significantly higher than in those with severe atrophic gastritis. These results suggest that extensive atrophic gastritis may be more closely related to bacterial overgrowth in the jejunum, compared to that in the stomach.
Small and Large Intestine
There is a progressive increase in numbers of bacteria along the jejunum and ileum, from approximately 104 in the jejunum to 107 colony-forming units per gram of contents at the ileal end, with a predominance of Gram-negative aerobes and some obligate anaerobes [8]. In contrast, the large intestine is heavily populated by anaerobes and bacteria counts reach densities around 1012 colony-forming units per gram of luminal contents because transit time is slow and microorganisms have the opportunity to proliferate by fermenting available substrates derived from either the diet or endogenous secretions. Because bacteria represent the sole source of gut H2 and CH4, fasting breath H2 and CH4 gases have been used as markers of colonic fermentation [12, 13]. As H2 production increases when a small amount of carbohydrate is supplied to colonic bacteria, the measurement of breath H2 concentration has been proposed as an indicator of carbohydrate malabsorption [14]. H2 gas is produced at a rate of 4 liters for every 12.5 g of undigested carbohydrate. Since it has been reported that 2-20% of carbohydrates escape small intestinal absorption [16] and men in their 40s consume 428 ± 72 g of carbohydrates [17], the calculated amount of H2 produced in the colon is 2.7-27 liters/day (428 g × 0.2 × 4 liters/12.5 g = 27.4). Approximately 20% of all H2 ingested or produced is eliminated via the lungs; the rest is either consumed or expelled via the rectum.
Several mechanisms of H2 utilization have been reported in the human large intestine including methanogenesis, dissimilating sulfate reduction, and acetogenesis. The latter process corresponds to the reduction of 2 mol of CO2 by 4
