Are vegan diets associated with more flatulence?

Question

Are diets that restrict or completely avoid animal products associated with a higher volume of flatulence? Here I am asking specifically about gas that is mostly produced in the colon and expelled in a healthy manner, which is different from bloating that arises when gas becomes trapped in the digestive system.

Anecdotally I have heard from people that they experience more flatulence when transitioning to a vegan diet, and for some people that change is persistent even months or years later.

Apparently it's common enough knowledge in the zeitgeist that the TV show Portlandia had an entire sketch based on flatulence in a raw vegan restaurant. And if you look through comment threads where vegans mention climate change, it won't take long to find a comment about the gassyness of vegans.

Are there any scientific studies showing a correlation (or causation) between vegan diets and flatulence?

Answer 1

Yes, a bean-laden vegan diet is likely to cause a larger volume of flatulence.

A 2011 study published in BMC Nutrition concluded that incorporating beans (an important staple food for vegans) has the potential to increase flatulence, but for many people this effect may only be temporary.

First, perception of flatulence increase is variable by bean type and across individuals. Second, after a few weeks of daily bean consumption, people perceive that flatulence occurrence returns to normal levels. Third, a small percentage of individuals may be bothered by increased flatulence regardless of the length of time they consume legumes.

The mechanism by which flatulence is affected is well understood.

Most legumes contain relatively high amounts of both dietary fiber and resistant starches. The soluble oligosaccharides found in legumes are not digestible by human intestinal enzymes alone. Instead, oligosaccharides such as raffinose and stachyose are broken down by bacterial fermentation in the intestines. Although some rectal gas is due to the ingestion of air, the majority of flatulence is produced from bacterial fermentation. The byproducts of this degradation are hydrogen, carbon dioxide, methane, and sometimes sulfur, depending upon the bacteria. Normal intestinal processes move these gases out of the body in the form of flatus. Removal or alteration of the oligosaccharide content of legumes will reduce the amount of gas produced.

A 1966 study called Effects of various soybean products on flatulence in the adult man showed that soy protein and soy fat are not significantly associated with flatulence production. Another 1966 study called Effects of Bean Diets on Concentration of Carbon Dioxide in Flatus concluded:

A direct relation exists between the volume of flatus produced and the volume of beans consumed in amounts of 27 per cent or more of the diet. The principal gas accountable for the increased flatus volume is carbon dioxide.

Beans can (and are) processed in various ways to reduce the amount of flatus produced.

• Fermenting soybeans to produce natto and tempeh greatly reduces oligosaccharides.
• Tofu has most (but not all) of the oligosaccharides removed, so that it causes less gas than eating an equivalent number of soybeans directly.

In conclusion, yes a vegan diet is very probably associated with a higher volume of flatulence due to the increased inclusion of beans, although results are likely to vary between individuals and even over time for a particular individual.

Answer 2

Beans are a common source of protein among people on a vegan diet. One aspect I can speak to is that when cooking dry beans, they're almost always soaked first. By discarding the soaking water prior to cooking, even though some nutrition (in the form of minerals) is lost, you get rid of up to 80% of the oligosaccharides that cause flatulence.

Answer 3

I grew up on a much more plant-oriented diet than most people in the US, and flatulence causing foods have been a big part of that since I was little. At one point in time, I frequently quipped that most of my favorite foods were listed on Beano bottles. But a few years after I became vegan, my flatulence level declined to the point that I've stopped taking Beano.

I would guess SquidInc.'s answer is part of this. I know that my favorite restaurant chain does that process (because they're post-modern, so their entire cooking process is on display. Part of that process only happens in their hub facility, but soaking their beans is one of the things they do in each store.)

However, I still consume significant quantities of most of the foods on the Beano website's list of foods to take Beano before eating. I don't have an explanation for this, but I think there's something more that goes on. I am personally less flatulent now than before I'd gone vegetarian, even comparing times when I was consuming 4-5 Beano tablets before meals versus today when I haven't taken Beano in years.

I realize that the OP's answer also indicated the temporary nature of the increase. But I'm not aware of anything that suggested there'd ever be a decrease.

Answer 4

Flatulence results from normal bacteria in the colon that produce gas during fermentation of soluble fiber or polyols from plant foods Natural animal foods do not contain fiber, so they should not cause flatulence.

Foods high in soluble fiber (NutrientsReview):

• Legumes: beans, peas, lentils
• Certain cereals: barley, oats, rye
• Other natural plants: figs, prunes, sweet potatoes, artichokes, eggplant
• Commercial foods with added oligosaccharide sweeteners (on the label check for inulin or oligofructose)

Natural or commercial foods high in polyols (check for maltitol, mannitol, sorbitol, xylitol...) (Monash University):

• Avocados, stone fruits (peaches, apricots, cherries...)
• "Sugar free" gum, yogurts, etc
• "Diet" beverages