Soya beans - Why they are not absorbed by the human body

The chief culprit, as with all beans, is the oligosaccharides in the carbohydrate portion. The word oligosaccharides =oligo (few) and saccharides (sugars). The best known oligosaccharides in beans are raffinose and stachyose. They require the enzyme alpha-galactosidase to be digested properly. Unfortunately, humans and other mammals do not come so equipped.

These two oligosaccharides make there way to the large intestine undigested by the small intestines, where hungry bacteria attack. The fermentation taking place always results in gas and odor. Increased availability of flatulent foods causes anaerobic bacteria to reproduce.

Malodorous methane (CH4) and hydrogen sulfide (H2S) gases are produced in greater amounts by infants fed soy formula. The toxic H2S has been linked to many intestinal disorders, particularly ulcerative colitis.

Carbohydrates in soy generally constitute 30 percent of the bean and break down into soluble sugars of sucrose (5 percent), stachyose (4 percent), raffinose (1 percent) and insoluble fiber (20 percent). The insoluble fiber consists of cellulose and pectins, which are not digested by the enzymes of the GI tract, and which absorb water and swell considerably. Unlike other beans, soybean carbohydrate contains very little starch (which humans can digest)—less than 1 percent.

Neither home cooking nor high-temperature industrial heating processes dispatch Raf and Stach. They are stubbornly heat stable. However, germination, which occurs during the fermentation process, will dramatically reduce the amount of these sugars, with a complete disappearance of the oligosaccharides on the third day. Incubation with microrganisms or enzymes derived from microorganisms also has this good effect.

Thus, old-fashioned soy products such as miso, tempeh and natto rarely cause gas but modern, heat-processed products that still contain the carbohydrate portion of the bean (soy flour, for example) create copious amounts.

Among the modern processed products, soy protein concentrate is said to produce the least gas because its carbohydrate portion has been extracted by alcohol. Soy protein isolate (SPI) is almost pure protein and thus considered practically free of "flatulence factors."

A study published in the American Journal of Clinical Nutrition showed no significant increase in flatus frequency after ingestion of 34 grams (about two tablespoons) of soymilk, but a major increase after 80 grams (about one-third cup). The researchers found that as the rate of gas production in the colon increased, smaller proportions were absorbed by the body and larger amounts expelled through the rectum. Thus, it is no wonder that soy consumption can so easily become a social problem. To make matters worse, soy inhibits a zinc-containing enzyme known as carbonic anhydrase, which helps transport gases across the intestinal wall. If carbonic anhydrase is neutralized, gas builds up in the colon. Hydrogen sulfide in the cecum has been reduced fivefold by supplementing with zinc, a mineral blocked by the phytates in soy and in short supply anyway in many soy-eaters' diets.

Soy-food eaters who suffer from truly excessive amounts of gas may be victims of undiagnosed soy allergies or sensitivities, and/or celiac disease. Obvious allergic symptoms to soy include sneezing, runny nose, hives, diarrhea, facial swelling, swollen tongue, shortness of breath and anaphylactic shock. Delayed allergic responses are less dramatic but even more common, and may manifest as gastrointestinal disturbances, including excess gas. Diarrhea, bloating and flatulence in celiac sufferers result not only from the consumption of wheat gluten and dairy products, but from even tiny amounts of soy. Soy saponins and lectins, which damage the mucosal lining of the intestine, may also be contributing to these gas and bloating problems.

The best solution for people who wish to eat soy is to choose old-fashioned fermented soy products like miso, tempeh and natto. With soaking and fermenting, the content of the oligosaccharides decreases while the levels of alpha-galactosidase increase. Proper preparation helps reduce trypsin inhibitors, saponins and other contributors to indigestion and to bowel disturbances, along with the gas-producing duo Raf and Stach.
For gas-afflicted folks who are addicted to the taste of tofu or to modern soy products, there is one other solution—a seat cushion packed with a charcoal filter. The medical journal Gut recently reviewed this product favorably, concluding that it "effectively limits the escape of these sulfur-containing gases into the environment."
As Mark Messina,.PhD, puts it, "there may be some beneficial effects associated with oligosaccharide consumption. Because of their growth-promoting effect on bifidobacteria, the oligosaccharides might promote the health of the colon, increase longevity and decrease colon cancer risk." This observation totally ignores research showing that the trypsin inhibitors present in soybeans adversely affect gut flora and allow more pathogenic strains to establish in the intestine and confuses the nasty oligosaccharides in soy with another type of oligosaccharides known as the fructooligo-saccharides consumers have used effectively to feed friendly bacteria and promote gastrointestinal health.
About the Author
"The Flatulence Factor" is excerpted from the forthcoming book, The Whole Soy Story (NewTrends, Spring 2004). In November, Kaayla Daniel, PhD, will start posting chapters on her new website.