What is in beans that causes gas?

Answer 1

As simple as possible.
Beans, as well as such gassy goods as cabbage, soybeans, peas and onions, are naturally sweetened with a family of sugars called oligosaccharides. These sugars are big, clumsy molecules -- too big to slip into your body through the lining of the small intestine. Normally enzymes in the small intestine would rush in and snap these molecules apart like Legos. But due to a gross oversight, an anti-oligosaccharide enzyme is not standard equipment in a human being.
So these complex sugars pass unmolested through the small intestine and enter the large intestine still bearing valuable nutrients. Unabashed at digging into leftovers, the less reputable bacteria among the 200 strains in your large-intestine start to chow down. Their population grows as they divide into new generations to take advantage of the bounty. And as they gulp in the big sugars, they let out gas, and so will you.

Answer 2

Beans (legumes) cause gas because they contain a sugar, oligosaccharide, that the human body can not break down. Oligosaccharides are large molecules and are not broken down and absorbed by the lining of the small intestine as other sugars are.
This is because the human body does not produce the enzyme that breaks down oligosaccharides.
Oligosaccharides make it all the way through the GI tract to the large intestine still intact. The bacteria that live in the small intestine break down the oligosaccharides. This produces the gas that must eventually come out of the rectum.

By the same principal, other foods that come into the large intestine without being properly absorbed in the small intestine will cause gas. Stress, for example, can cause food to move through the GI tract too quickly to be properly digested, with the end result being gas in the large intestine.

Answer 3

Here's Steve's reference: http://ibdcrohns.about.com/od/otherdigestivediseases/f/beans.htm

If it's a problem for you, unless you're diabetic or have galactosemia, you can use Beano safely to aid in the digestion of these yummy legumes :)