tl;DR : How farts are made Ever since my hearthmate banned me from eating black bean soup, I've been wanting to know the facts about flatulence. Why are some, uh, emissions appropriately labeled "silent but deadly"? Why do others cause great auditory but not olfactory embarrassment? To get answers I turned to Pitt gastroenterologist David H. Van Thiel, M.D., who promptly armed me with Chapter 19 of the textbook Gastrointestinal Disease, titled simply "Intestinal Gas." Between it and the good doctor, I am now the office expert on this awkward but interesting subject.
According to chapter authors Michael D. Levitt, M.D., and John H. Bond, M.D., "...Medical literature concerning bowel gas is relatively long on subjective impressions and short on scientific data." Some of the impressions, however (like the one about black beans), turn out to have good physiologic explanations. Here's what I learned.
Most of us pass somewhere between 200 and 2,000 ml of gas per day (average, about 600 ml) in roughly 13-14 passages. These emissions are composed of five gasses: nitrogen (N2), oxygen (O2), carbon dioxide (CO2), hydrogen (H2), and methane (CH4). Nitrogen usually predominates, followed by variable concentrations of carbon dioxide, hydrogen, and methane, and very low levels of oxygen. Gas gets into our guts by air swallowing (which accounts for very little), production within the bowel and colon (which accounts for a lot), and diffusion from the blood. As far as those who study these things can figure out, most air that is swallowed comes out the way it went in: it is belched or "eructed," as the textbook informed me. Most of the nitrogen is accounted for by diffusion from blood to bowel; it may give flatus some of its bang but little of its bite.
We are more interested in the gases produced in the bowel's interior space or lumen -- the carbon dioxide, hydrogen, and methane (all of which are negligible in the atmosphere). These gases accumulate when the digestive system's assortment of resident bacteria, acting like microscopic gas factories, begin the task of digesting our latest meal. It is unclear whether carbon dioxide is a direct or indirect product of bacterial metabolism, but CO2 can account for up to 50%-60% of the gas in flatus, usually in association with hydrogen, which is definitely a product of bacterial fermentation. Methane, too, is exclusively a bacterial leftover. However, to make the H2 and CH4, the bacteria need what Levitt and Bond call "exogenous fermentable substrates" -- in English, that's carbohydrate and protein. And here's where the science becomes useful in answering my questions.
"Gas is basically produced by foods that have undigestible or excess carbohydrates, which are not absorbed when they get to the colon, explains Van Thiel. "Beans, for example, have complex carbohydrates that are maldigested. And the same holds true for mushrooms. Many people don't realize that mushrooms contain a unique sugar called raffinose, which humans can't break down. Cabbages and onions do the same thing." As a group, vegetarians produce more gas than meat-eaters because the intestinal enzymes can't digest the cellulose in vegetables' cell walls. However, the colon's bacteria relish the stuff. The outcome: When a group of young men, in the service of science, feasted on a diet rich in pork and beans, their flatus elimination increased from 15 ml to 176 ml per hour!
Great, so this is where the gas comes from. But what about the odor, the noise, the frequency -- the embarrassing stuff? The amount and odor of flatus, says Van Thiel, are largely accounted for by the type of bacteria we have in our guts. Newborn infants and germ-free rats have no intestinal bacteria, although the infants acquire them almost immediately. "Unless you're hermetically sealed, which is essentially what those germ-free rats are, you take in bacteria within hours of being delivered; and they immediately begin to replicate in the advantageous environment of the intestinal system. And it appears as if the kind of bacteria you acquire in your neonatal life basically persist with you." While the bacteria that make up the intestinal flora are generally the same in everyone, "that's like saying people are generally the same," says Van Thiel. They do the same job but in their own way.
The reason this is important is that different bacteria digest different kinds of carbohydrates. Some make more methane; others help hydrogen to combine with sulfur (from other foods or water) to make hydrogen sulfide. According to Levitt and Bond, only about one-third of us excrete CH4, a capability that, again, seems to depend on the bacteria floating around us at birth. There's an easy way to determine whether you produce methane or not: if you do, your stools will float. This is because CH4 gets trapped in the stool, making it less dense than water. Better the relatively odorless methane, however, than hydrogen-sulfide, which has the unavoidable odor of rotten eggs. "The only notable fact about methane is that it burns with a blue flame," says Van Thiel, "and that's why crazy college kids who like to, uh, ignite their flatus have to be methane producers. And those who make more methane are more like flamethrowers than those who don't."
That answers the odor question, but what about the noise? "What makes the loud ones?" I asked the accommodating gastroenterologist. The loud ones are basically a function of three things, he told me: the volume of gas, the force with which it is expelled, and the presence of hemorrhoids or other anatomical features that could resonate. "People with large hemorrhoids, for example, will have louder flatus than people with lax sphincters. Vegetarians, who actually pass a lot of gas, frequently have quiet, frequent flatus because they have large, bulky stools and looser sphincters. They will be detected socially only if it's foul-smelling; whereas, carnivores may have less gas but, since they have tighter sphincters and may be more constipated, have high drama flatus."
So what's to be done if we wish to alter our flatulence profiles? Paradoxically, studies show that many people who complain of discomfort from "too much gas" have essentially the same gas volume and composition as normal controls. However, they seem to suffer from disordered intestinal motility, with more gas being refluxed back into the stomach, as well as an abnormal pain response to bowel distention. While anticholinergic agents have been prescribed for this problem, their efficacy has not been proven. Thus, the path of least resistance is to stop eating the offending foods.
The same advice is good for those who complain of frequent or excessive flatus. If much of the excreted gas is composed of hydrogen and carbon dioxide, which originate in the body, not the air, the problem may be a general malabsorption disorder or difficulty with particular foods. A low carbohydrate diet will often turn down the flatus volume.
In addition to the beans, mushrooms, onions, celery, and other foods containing complex carbohydrates and undigestible fiber, many patients with abdominal complaints also benefit from reducing their intake of lactose, the milk sugar in dairy products. Most adults lack, to some degree, the enzyme needed to digest lactose. "As long as you eat an amount below your tolerance threshold, you're asymptomatic," says Van Thiel. "If you go above your threshold, then you have gas, cramps, and diarrhea." Another culprit may be the sorbitol in diet foods like candy, chewing gum, and sugar-free lozenges. Sorbitol is another non-digestible carbohydrate; but, because of its form, people have a harder time making the association between consumption and later discomfort.
At this point I'm thinking, "Well, I don't want to stop being a vegetarian. I like cheese, and onions are a necessity. Maybe I can take some medicinal pesticide to kill my intestinal flora and recultivate with a less obnoxious set." Wrong, says Van Thiel. Even the strongest antibiotics won't seek and destroy all of the 107 to 1015 bacteria in my distal small intestine and colon; and I'm likely to make myself very sick in the process. Thus, after my journey into the dark recesses of my bowel, I can only come up with one solution: make sure that when I dine on black bean soup I dine alone.
Post edited at 5:56 pm on July 22, 2008 by Anonymous
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