Soy: The King of Musical Fruits

beans carbohydrates flatulence gas national colon awareness month oligosaccharides soy soybeans Mar 05, 2013

In honor of National Colon Awareness Month, let’s talk about soy — soy as  an incomparable gas producer, as the very king of musical fruits.

Abdominal bloating, rumbling and flatus experienced by vegetarians and other heavy soyfood eaters make soy the butt of a great deal of bathroom humor. Unfortunately it is no laughing matter for the many people struggling with health problems who have been advised to eat more soy but cannot abide the consequences to their marriages, relationships, jobs and self image. Such people often ask Dr. Andrew Weil and other soy proponents to help them choose the types and brands of soy that will give them the supposed health benefits of soy minus the killer gas.1


In fact, neither Dr. Weil nor anyone else has completely solved this problem. The obvious solution is to steer clear of soy. Since the average American prefers to do just that, the soy industry has acknowledged that the “flatulencefactor” must be overcome if soyfoods are ever to become a major part of the American diet.2,3

Accordingly, research dollars have poured into studies with titles such as “Flavor and flatulence factors in soybean protein products,” “Effects of various soybean products on flatulence in the adult man,” “Development of a technique for the in vivo assessment of flatulence in dogs” and so forth. Studies comparing types of soyfoods (tempeh, tofu, soy protein isolate, etc.) and/or different strains of soybeans (hybrid or genetically engineered) in terms of their flatulence potential are commonplace.

Test subjects have included rats, college students and other animals. “Containment devices” have included gas-tight pantaloons sealed to the skin at the waist and thighs using duct tape and equipped with two ports. Qualified scientists have measured numbers of incidences per hour and day: the quantities of gas ejected per incident, the proportions of hydrogen, methane, carbon dioxide, hydrogen sulfide, methanethiol and other gases; and even propulsion force and noise levels. In addition, researchers have called on professional “odor judges” to make subjective measurements of bodily emissions.

Despite these fine efforts, scientists have not completely identified the “flatulence factor” in soybeans and can propose only partial solutions.


The chief culprit, as with all beans, is the oligosaccharides in the carbohydrate portion. The word oligosaccharides comes from 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.

The result is that the pair–whom we’ll call Raf and Stach–pass through the small intestine unscathed to arrive in the large intestine, where they are attacked by armies of hungry bacteria. The digestive fermentation that takes place always results in gas and sometimes in odor. The precise amount and specific smell varies widely from person to person and also depends upon gender, age and the demographics of each individual’s gut population.4 Several reports indicate that the increased availability of flatulent foods causes anaerobic bacteria to reproduce. Eating more such foods results in a “rapid rate of gas production,” with the possibility of faster, more explosive results every time additional foods of this ilk appear in the intestine.5-7

Although a few people seem able to eat soy without gassing up, studies on soybean digestion often refer to “excessive volume” and “noxious odor.” Malodorous methane (CH4) and hydrogen sulfide (H2S) gases are produced in greater amounts by infants fed soy formula.8 The highly volatile and toxic H2S has been linked to many intestinal disorders, particularly ulcerative colitis.9

Over the years scientists have done their darndest to find a way to either reduce the presence of Raf and Stach in soybean products or to cut out the entire carbohydrate load. 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.10,11

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.12 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 “flatulencefactors.” 13-17

In theory, tofu should be a low gas producer because oligosaccharides concentrate in the whey (the soaking liquid) and not the curds (the part sold as tofu).18 Some Raf and Stach remain, however, and tofu is a gas producer for many consumers. The probable reason is that the product is eaten in such large quantities that even the small proportion of Raf and Stach that remain in the curd are enough to set off a feeding frenzy among colon bacteria.

In fact, science confirms the anecdotes of many soy consumers–that eating a little soy produces minimal gas, but eating just a bit more can result in discomfort or embarrassment. 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.19 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.20

The question remains why certain individuals experience stupendous amounts of gas even when they consume soyfoods that are virtually devoid of Raf and Stach. Imbalances in gut flora caused by trypsin inhibitors (which inhibit protein digestion) may be part of the problem, though undigested protein itself is not. Circulating levels of insulin, gastrin, gastric inhibitory polypeptide, pancreatic polypeptide and neurotensin are affected by trypsin inhibitors, but do not seem involved in flatulent dyspepsia.21

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.22 Soy saponins and lectins, which damage the mucosal lining of the intestine, may also be contributing to these gas and bloating problems.


One solution proposed by the soy industry is genetically modified strains of soybeans that are low in the two stooges Raf and Stach. Plant scientists have already developed a strain known as “High Sucrose Soybeans” that contains more sucrose and less indigestible carbohydrates than ordinary beans. It also lacks the lipoxygenase-2 enyzme that gives soy its infamous “beany” taste. The industry hopes that the modified bean, with taste improved and flatulence eliminated, will be popular with makers of soy milk and tofu.23,24

Another possibility–not seriously proposed for humans–is antibiotics. Animal studies have shown that antibiotics destroy anaerobic bacteria in the intestinal tract that eat Raf and Stach and cause gas, thus improving the smell of chicken coops and barnyards.25


Until such “low gas” beans come on the market, soy proponents recommend that afflicted parties take Beano™ with their soy. This was the solution proposed by soy industry spokeswoman Clare Hasler, PhD, to a consumer who said he enjoyed eating tofu and drinking soymilk but wondered what to do about levels of gas that were “almost too embarrassing to discuss” and which made him unable to “stand the smell of myself.”26 Beano™ is an over-the-counter supplement containing alpha galactosidase, the enzyme required to break down the raffish oligosaccharides into simple digestible sugars. Sometimes this works, but many times it doesn’t. Beano™ will not reduce gas caused by soy allergies or intolerances, or by celiac disease.

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.27 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.”28 Current Treatment Options in Gastroenterology further recommended the cushion as a viable solution for “the noxious odor associated with flatus,” saying that “the charcoal cushion may improve patients’ symptoms.”29 Taking charcoal internally will not do the trick. 30


Meanwhile, the soy industry has begun singing its version of the popular childhood song “The more you toot, the better you feel. Let’s eat soy with every meal.” Gas–we are being told–could be a good thing, and consumers might wish to reconsider their long-standing request for a new and improved “low gas” soy.

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.”31 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 intestine32 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. Despite considerable evidence to the contrary, Dr. Messina would prefer to believe that since soy is a good thing, then the soy constituents Raf and Stach help feed good (never bad) bacteria and produce only the finest, healthiest gas.

Should consumers remain unconvinced, the industry still proposes to benefit. Japanese researchers have come up with a new miracle supplement– soybean oligosaccharides in powder form to be used as a substitute for table sugar and sprinkled directly on foods.33

Do hold your breath.

*  *  *  *  *

The image of the dog and cat comes from “Gassiest Dogs: German Shepherds Rank Number One,” which can be read here:

The human explosion photo has widely circulated on the internet.  I found it at: and

Copyright: This is chapter 15 m The Whole Soy Story: The Dark Side of America’s Favorite Health Food by Kaayla T. Daniel, PhD, CCN. NewTrends Publishing, 2005.


  1. Question sent on November 2, 1998 by Lynn Willeford, Associate Editor of Dr. Andrew Weil’s Self Healingnewsletter, to Clare Hasler at the “Ask an Expert” StratSoy website, which is sponsored by the United Soybean Board and developed at the University of Illinois at Champaign.
  2. Suarez FL, Springfield J, et al. Gas production in humans ingesting a soybean flour derived from beans naturally low in oligosaccharides, Am J Clin Nutr, 1999, 69,1, 135-139.
  3. Visser A, Thomas A. Review: soya protein products, their processing, functionality and application aspects. Food Rev Inter, 1987, 3 (1&2), 1-32.
  4. Liener IE. Implications of antinutritional components in soybean foods, Crit Rev Food Sci Nutr, 1994, 34, 1, 49.
  5. Suarez F et al. Insights into human colonic physiology obtained from the study of flatus composition. Am J Physiol, 1997, 272, 5, pt 1, G1028-1033.
  6. Smith Allan K and Circle, Sidney J. Soybeans; Chemistry and Technology, Volume 1 Proteins (Westport, CT, Avi Publishing, 1972), p. 181.
  7. Jiang T et al. Gas production by feces of infants, J Pediatric Gastroenterol Nutr, 2001, 32, 5, 534-541.
  8. Levine J et al. Fecal hydrogen sulfide production in ulcerative colitis, Am J Gastroenterol, 1998, 93, 1, 83-87.
  9. Suarez F et al. Production and elimination of sulfur-containing gases in the rat colon, Am J Physiol, 1998, 274, (4, pt1) G727-733.
  10. Liu, KeShun. Soybeans: Chemistry, Technology and Utilization (Aspen, 1999) 72,76
  11. Berk, Zeki. Technology of production of edible flours and protein products from soybeans, Food and Agric Organ of the United Nations, Rome, 1993 FAO Agricultural Services Bulletin, 97, 15.
  12. Jimenez MJ et al. Biochemical and nutritional studies of germinated soybean seeds (article in Spanish), Arch Lationoam Nutr, 1985, 35, 3, 480-490.
  13. Rackis JJ. Flatulence caused by soya and its control through processing, J Amer Oil Chem Soc, 1981, 58, 503.
  14. Rackis JJ. Flavor and flatulence factors in soybean protein products. J Agric Food Chem, 1970, 18, 977.
  15. Calloway DH, Hickey CA, Murphy EL. Reduction of intestinal gas-forming properties of legumes by traditional and experimental food processing methods, J Food Sci, 1971, 36, 251.
  16. Jood S et al. Effect of flatus producing factors in legumes, J Agri Food Chem, 1985, 33, 268.
  17. Liu, 74
  18. Olson AC et al. Flatus-causing factors in legumes in Ory RI, ed. Antinutrients and Natural Toxicants in Foods (Westport CT, Food and Nutrition Press, 1981, p. 275.
  19. Suarez FL et al. Gas production in humans ingesting a soybean flour derived from beans naturally low in oligosaccharides, Am J Clin Nutr, 1999, 69, 1, 135-139.
  20. Smith and Circle, p. 182.
  21. Watson RG et al. Circulating gastrointestinal hormones in patients with flatulent dyspepsis, with and without gallbladder disease, Digestion, 1986, 35,4, 211-216.
  22. Faulkner-Hogg KB, Selby WS, Loblay RH. Dietary analysis in symptomatic patients with coeliac disease on a gluten-free diet: the role of trace amounts of gluten and non-gluten food intolerances. Scand J Gastroentrol, 1999, 34, 8, 784-789.
  23. Parsons CM, Zhang Y, Araba M. Nutritional evaluation of soybean meals varying in oligosaccharide content. Poultry Sci, 2000, 79,8, 1127-1131.
  24. Kane, Janice Roma. Chemical companies fortify with soy: soy receives heavy investment in functional foods from DuPont, ADM and Henkel. Chemical Market Reporter, November 8, 1999, 256, 19, FR14.
  25. Smith and Circle, p. 181.
  26. Response by Clare Hasler on January 18, 1999 to a question sent to the “Ask and Expert” part of the StratSoy website funded by the United Soybean Board and developed by the University of Illinois.
  27. Guimaraes VM, de Rezende ST et al. Characterization of alpha-galactosidases from germinating soybean seed and their use for hydrolysis of oligosaccharides, Phytochem, 2001, 58, 1, 67-73.
  28. Suarez, FL, Springfield J, Levitt MD. Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odor. Gut, 1998, 43, 100-104.
  29. Fink RN, Lembo AJ. Intestinal gas. Curr Treat Options Gastroenterol, 2001, 4, 4, 333-337.
  30. Suarez et al. Failure of activated charcoal to reduce the release of gases produced by colonic flora. Am J Gastroenterol, 1999, 94, 1, 208-212.
  31. Messina, Mark. Legumes and soybeans: overview of their nutritional profiles and health effects. Amer J Clin Nutr, 1999, 70, 3, 439S-450S.
  32. Grant, 319.
  33. Hata Y, Yamamoto M, Nakajima K. Effects of soybean oligosaccharides on human digestive organs: estimate of fifty percent effective dose and maximum non-effective dose based on diarrhea. J Clin Biochem Nutr, 1991, 10, 135-144.


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