By now you've probably been told that it's important you increase your fiber intake for a variety of reasons. Generally speaking fiber is not something that you can digest like normal food, so it helps to slow down the early parts of the digestion process and speed up the process in the later stages.
All of this sounds good, but what about gas (flatulence)? You may observe that if you increase your fiber intake, you tend to get extremely bad gas a few hours after you eat the product containing fiber. One great example is Fiber One bars. They are good source of fiber and made with natural ingredients, but many people end up with extreme cases of gas when they eat one, much less the poor souls who attempt to eat two bars at once.
So why is this and does it always occur with increased fiber intake? First of all, this is a symptom that varies for each person, but there are some commonalities we can explore. The most important is that gas is a byproduct of the digestion of soluble fiber in the colon. So that means increasing insoluble fiber should not cause an increase in gas.
But wait, I just said fiber isn't digestible and that gas is a byproduct of its digestion. You are very observant young grasshopper. It turns out that the stomach and intestines cannot digest either type of fiber (soluble or insoluble) but bacteria living in the colon can digest some of the soluble fiber you eat. This varies based on the person and the type of soluble fiber. When bacteria in the colon digests the fiber, gas is produced. Sometimes… extremely bad gas. (Please don't ask me to pull your finger…)
So should you attempt to avoid soluble fiber? No. Soluble fiber has many benefits, including removal of significant quantities of cholesterol from the body. Oatmeal and oat bran are prime sources of gas-producing soluble fiber, as is psyllium (the fiber in Metamucil and some supplements). Whole fruits (particularly prunes, apricots, raisins, and to a lesser extent bananas and apples) also contain relatively large quantities of soluble fiber and could produce the same effect. Beans and several vegetables (brussel sprouts, carrots, and onions), and the fruits mentioned above also contain sugars and starches that are hard to digest and may have a similar effect when they reach the colon without being fully digested.
It is also worthy of note that there are other foods that cause intestinal gas, including dairy products, fatty foods, and wheat products. Still, the gas is produced by bacterial digestion (except for a very tiny amount that comes from swallowed air). Much of the gas produced is actually reabsorbed by certain bacteria or through the intestine into the blood. There are also numerous medical conditions that can cause excessive gas, including lactose intolerance, celiac disease, and pancreatic insufficiency.
Unfortunately there is no simple solution to this problem, because it varies widely from individual to individual. The only thing you can really do is test each type of soluble fiber to determine which ones give you gas and limit your intake of those products to relieve the problem. Often when the fiber is coupled with sugars and starches that are difficult to digest, you really have a problem.
You'll want to test products with artificially added fiber as well as whole fruits and vegetables, legumes, and whole grain (especially wheat and oat) products. You may be able to tolerate some products and not others.
If you choose to try a very high-fiber product like a Fiber One bar, eat half a bar and see how you tolerate it. If that works out, go ahead and eat a whole one the next time. Be sure to wait until the next day just to be certain. Most people will not have any problems with half a bar, but some will experience gas when they eat one or more bars.
Some suggest that prolonged ingestion of these fibers (gradually adding more to your diet) will help the body tolerate each one more without the gassy aftermath, but that's not proven and in my experience is not necessarily true for everyone.
It's true that gas can be inconvenient, embarrassing, and sometimes even painful. But don't let a few episodes deter you from eating a healthy diet of whole, natural foods. Just test various foods and limit your intake of foods that cause excessive gas or seek medical attention if you think you have a disorder that is causing your gas. There are often simple cures for conditions such as lactose intolerance that can solve the problem.