Borborygmi. One of my favourite words. Borborygmi are the gurgling sounds of gas travelling along your intestines. The rumble can be most pronounced during a conversation lull at the dinner table. Hark, I hear borborygmi.
Gas inside your digestive system is normal. It comes from two main sources: 1) the air you swallow during eating or drinking; and 2) gas produced by bacterial fermentation in the large bowel, the back end of your digestive system. Smaller amounts come from CO2 production when your stomach acid is neutralised in the small intestine.
One obvious way of swallowing air is through aerated drinks such as soft drink, champagne and beer. This usually comes back into the atmosphere via eructation (burping to you). Air can also be swallowed during eating, more commonly in those that eat quickly or talk a lot during the meal. If it isn’t burped out then it will pass from your stomach and into the small intestine.
Once the contents of your small intestine pass the appendix and into the large intestine, the resident bacteria feast on the fibre and small amounts of starch and protein that doesn’t get digested. When bacteria feed they produce gas. In fact, 75% of flatus is bacterial gas. As the contents move along the large intestine (aka colon) water is absorbed so the waste becomes more solid and the gas becomes entrapped in large bubbles.
Some bacterial gas is absorbed back into the blood and exhaled. This is the basis of a breath hydrogen test. If you have, say, suspected lactose intolerance your breath is tested for its hydrogen content because if lactose has reached the colon the bacteria will consume the lactose and generate far more hydrogen than normal.
Most of flatus is oxygen, nitrogen (the two main components of air), hydrogen, methane and carbon dioxide. Some sulphur containing gases provide the head-jolting aroma. You want to know: why baked beans? The legume family have plenty of fibre and two types of carbohydrate that don’t get digested well – stachyose and raffinose – which then become bacteria food with the end product being gas.
Here’s the physics of what happens to gas down the back end. The abdominal muscles and the anal sphincter contract at the same time, thereby raising the pressure of the gas against the anal sphincter and then, at the magic moment, the pressure gradient sends out the air at high speed through a very narrow slit too small for solid waste to escape. Hopefully. The high speed gas causes the edges of the anus to vibrate. Maybe fart should be pronounced with a distinctive rolled “r”.
What does it all mean?
Gas is normal. You can reduce the amount of swallowed air by avoiding fizzy drinks and eating slower. Colonic gas is from bacteria eating undigested food ie fibre, which is exactly what the health authorities want you to eat. As one author said: “Flatulence rich in bacterial gases might be the price for the large bowel water reabsorption. It seems that little can be done to reduce flatulence.”
So if you eat plenty of fibre-containing food, you will release more methane into the air, vis-à-vis, don’t think you can solely blame global warming on the cows.
Medical Hypotheses 2006; 67: 235-239
American Journal of Gastroenterology 2007; 102: 842-849