Nitrogen, oxygen, carbon dioxide, hydrogen, and methane make up more than 99% of the volume of intestinal gas. Their proportions vary widely in healthy people (Table 34-1). These five gases are odorless; the characteristic odor of flatus is conferred by a combination of trace gases that together constitute no more than 1% of the total volume of intestinal gas. The odoriferous trace gases include ammonia, hydrogen sulfide, volatile amino acids, and short-chain fatty acids.

All people swallow air in variable amounts. Swallowed air is the major source of nitrogen and oxygen (Fig. 34-1). Nitrogen and oxygen also diffuse from the blood into the intestinal lumen. Oxygen is largely consumed by intestinal aerobic bacteria.

Large amounts of carbon dioxide are generated by neutralization of gastric acid, fatty acids, and amino acids within the upper gastrointestinal tract. Carbonated drinks also provide an exogenous source of carbon dioxide. Most carbon dioxide within the upper gastrointestinal tract is absorbed and excreted by the lungs. Carbon dioxide in flatus is derived largely from the action of bacteria on intestinal substrates. Bacterial action on intestinal substrates also produces hydrogen and, in about one third of adults, methane.