If you haven’t heard that fiber is beneficial to health, you evidently lack ears. The purported benefits of fiber range from bowel movement regularity to weight management, reduced risk of diabetes and cancer and lower cholesterol levels. There is some debate on whether all of these outcomes can be achieved with dietary roughage, but there certainly is a good body of evidence that suggests much of it is possible.

Still, there are some health advocates who question the value of fiber altogether.
Here we examine whether fiber can deliver on the claims. It also deals with the dread “gas factor.” But before you read any further, it’s important to realize one thing: Fiber does not exist on its own, except in commercial products. Naturally occurring fiber is in the complex carbohydrates we eat, which are fruits, vegetables and whole grains. Any discussion of fiber must also take into account the constellation of nutrients present in the whole-food form of plant-based foods.

The purported benefits of fiber and the evidence supporting or challenging them

Fiber prevents cancer: There was a long-held belief that fiber intake was related to a reduced risk of colon cancer; however, a paper published in the Journal of the American Medical Association (“Dietary Fiber Intake and Risk of Colorectal Cancer; A Pooled Analysis of Prospective Cohort Studies,” JAMA, [December 14, 2005]: 2849-2857) found no relationship between high dietary fiber intake and a reduced risk of colorectal cancer. Note that some studies in this report were conducted over 20 years’ time, so it is a pretty comprehensive look at the question. But this review can be misleading. People in the study who did not get cancer in fact also ate more fiber, but the study factored for other dietary risk factors — such as higher animal protein intake (generally associated with higher cancer rates) –that might be to blame for increased risk among those who did get cancer. So the most at-risk, for other reasons, were factored out of comparisons.

For my money, this report says that a higher-plant, lower-animal diet indeed helps prevent cancer. But the direct link cannot yet be made between fiber per se and cancer prevention.
Fiber lowers cholesterol: The Mayo Clinic Newsletter Web site says that “diet can play an important role in lowering your cholesterol.” Among the recommended foods, it lists oatmeal, oat bran and high-fiber foods (kidney beans, apples, pears, barley and prunes) first, followed by fish and Omega-3 fatty acids, walnuts, almonds, other nuts, olive oil, foods with plant sterols or stanols and a reduced intake of foods heavy in cholesterol, saturated and trans fat.

Fiber promotes regularity: Your grandmother ate prunes, but for marketing purposes they are now called dried plums (even though the sticky, sweet fruit still seems pretty wet to me). The prune’s insoluble fiber feeds friendly bacteria into the large intestine, which promotes a faster trip on the way out. It also produces something called butyric acid, which feeds the cells of the large intestine walls, supporting the physical mechanism of regularity. The prunes might have the added benefit (beyond its fiber) of propionic acid, which has been shown in animal studies to inhibit production of cholesterol by the liver.

In general, insoluble fiber in particular from bran, nuts and vegetables increases stool bulk, which helps things move along faster, so to speak.

Fiber prevents diabetes: Because fiber basically doesn’t digest, it slows the absorption of other nutrients in a meal. Importantly, this includes simple sugars that otherwise can quickly elevate blood sugar levels, which over time can lead to type II diabetes. It is the difference between eating an apple, with skin and pulp intact, and drinking apple juice, where there is nothing to slow the absorption of the sugary liquid.

Fiber prevents excess weight gain: Fibrous celery will take a lot of time and attention in the mouth to chew and swallow. A full cup of diced celery carries 19 calories and requires effort on the part of your stomach to digest (effort = calories used). In contrast, a 450-calorie gourmet ice coffee might be consumed in about 60 seconds. So if forced to choose between these two, which do you think would most likely boost weight gain?

Assuming some of these benefits are enough to convince you, how much fiber should a person get in a day? According to U.S. Dietary Guidelines for Americans (from the departments of Agriculture and Health and Human Services), the recommended daily intake of fiber for men under age 50 is 38 grams and over 50 is 30 grams. For women, it’s 25 grams up to age 50 and 21 grams after.

But on average, Americans’ consumption is woefully inadequate: We consume between 12 and 15 grams per day.

What about the gas caused by fiber foods?

A frequent complaint about doctors’ orders to increase fiber intake is the bloating and gas associated with high-fiber foods, including and especially that from beans and cabbage. It should be noted that according to the National Digestive Diseases Information Clearinghouse, a division of the National Institutes of Health, a healthy individual will pass one to four pints of gas per day, typically over 14 to 23 “incidents.”

This is of course problematic for people who work or otherwise spend time in closed spaces with others. The obvious solution is to take several private walks through the course of a day.
Understanding how to control it begins with knowing how gas forms in the digestive tract. Some comes from swallowing air, which happens to a greater degree from fast eating. It can also come from carbonated soft drinks. But a key culprit is oligosaccharides, found in beans and many vegetables. Oligosaccharides are sugars that are indigestible by humans, as is the fiber. But it makes perfect food for bacteria living in the large intestine, and the bacteria are what produce the gas.

So in fact, it’s not really you. It’s them. If only you could convincingly place blame where blame is due.

Products such as Beano and Gas-X help reduce this. So too does drinking water and other noncarbonated fluids, which dilutes gas and odors. And the body’s ability to process fiber increases with a graduated build of fiber-rich food (i.e., don’t do it all at once).

A contrarian’s viewpoint on Fiber

Mark Sisson, author of The Primal Blueprint (Primal Nutrition, Inc., 2009), begs to differ with conventional wisdom regarding fiber. He offers several perspectives on his Web site ( that, at the very least, lead the reader to consider how even the great, vaunted fiber has its limits:

  • “Soluble fiber (vegetables, fruit, oatmeal, and legumes that partially dissolve in water) enhances the thickness of the stomach’s contents. This slows stomach emptying. While this can give the body more time to absorb nutrients, it can also ‘trap’ minerals like calcium or zinc, binding them up in such a way that they don’t have the opportunity to be absorbed.”
  • “Some research shows that the very fiber we turn to with perfectly innocent intentions can become a serious monkey on our backs. It turns out, we may have to continually up the ante over time until we’re in over our heads — or behinds, so to speak.”
  • “The key to a healthy gastronomical tract is not roughage but bacteria. The large intestine’s natural bacteria, which help comprise stool bulk, maintain water content and soften the stool. Fiber, particularly excessive insoluble fiber, offering a quick jumpstart to things is not the natural catalyst for a healthy excretory system.”
  • “Researchers found that vegetable based fiber (as opposed to that from cereal and fruit) was the most cancer protective. [A study he cites] focused on those with a risk of prostate cancer, but other researchers and physicians extend this claim to suggest a vegetable based high fiber diet in lieu of carbohydrate fiber sources.”

We cannot entirely disagree with him. It seems as if he looks at the other components of high-fiber foods to see the benefits from the important nutrients they carry. And his point about calcium and zinc lost to soluble fiber suggests that the three old watchwords of nutrition — balance, variety and moderation — apply to fiber as much as anything else.

All that said, if you eat like a typical American, you need to seek out foods that will increase your fiber intake. Just like you’ve always heard.