Constipation and the High-Fiber Myth

For decades, fiber has been extolled as a vital component of a healthy diet. Doctors, dietitians, the food industry, and government have prominently touted a high-fiber diet as essential for preventing or relieving digestive disorders, including IBS-C (constipation-predominant IBS) and idiopathic constipation (constipation with no apparent cause). ​However, most of these health claims aren't supported by scientific research.

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In fact, a number of studies have demonstrated that an excess of dietary fiber may actually be harmful for gut health and may exacerbate the symptoms of IBS, specifically bloating, gas, pain, straining, and constipation. One invaluable study looked at whether idiopathic constipation and its associated symptoms can be effectively reduced by stopping or reducing the intake of dietary fiber. The researchers wanted to explore whether common beliefs about the use of fiber in reducing constipation were valid. 

What they found was that the subjects who followed a low-fiber diet for six months experienced more-frequent bowel movements! The results of decreasing fiber intake were statistically significant, not only in terms of increasing the frequency of bowel movements, but also in terms of decreasing other symptoms associated with constipation, such as bloating and straining. The patients who completely stopped consuming dietary fiber no longer suffered from bloating and abdominal pain. That's because these symptoms are caused by the fermentation of dietary fiber by bacteria in the colon, a process that produces hydrogen, carbon dioxide, and methane gases that get trapped in the colon and exert pressure on its walls. The researchers also found that decreasing the bulk and volume of feces immediately enables easier evacuation of smaller, thinner stools, eliminating straining, bleeding, and tearing of the anal sphincter caused by larger and more bulky fecal loads.

To better understand why this approach works so well, it's important to explore the myths about what poop is actually composed of. While most people think of feces as simply the used-up remains of the food we eat — the stuff that makes it through after digestion — the fact is that 50 to 80 percent of poop (excluding water) is bacteria that lives in the intestines and is ejected as food passed through. Poop also includes some indigestible plant matter — such as the cellulose in vegetables — but the exact amount depends on each individual's diet. Poop also contains small amounts of our own tissue: cells from the lining of the intestines that are sloughed off during digestion. And, of course, it contains some water.

Poop is brown because of a chemical called stercobilin, which is a by-product of the hemoglobin in broken-down red blood cells. It also comes from bile, the fluid secreted into the intestines to help digest fat. If poop is another color, it can be a sign of other conditions. For instance, yellow stool can be the result of a parasitic infection or pancreatic cancer. Black or dark-red stools can indicate bleeding in the upper GI tract (or that you recently ate beets). Green feces can be the sign of an infection. White or pale-gray poop can be an indication of liver disease or clogged bile ducts.

IBS-C and idiopathic constipation aren't caused by a lack of poop. They happen because the poop in the rectum isn't moving through the colon and on to the anal sphincter, the very narrow passageway that marks the end of the colon. If you eat a lot of fiber or take fiber supplements, you're bulking up the stool. While the pervading belief is that fiber, especially insoluble fiber, makes stool fluffy and easier to pass, what it actually does is make stool bigger. What do you think is easier to pass through a small opening: something small and thin or something big and bulky? If we try to pass a large stool through a small hole, it causes pain, straining, hemorrhoids, and bleeding. So the logical solution isn't bigger poops but smaller ones!

Now you may think that fiber makes stools more moist so they can more easily "slide" through the anal opening. That also isn't true. How much moisture stool absorbs is an involuntary physiological function, similar to how our body temperature is set. In general, stool is typically 75 percent water and 25 percent solid matter. Because fiber is the indigestible part of plants, it passes through us, creating more solid matter in the stool rather than water. This stool then gets stuck at the anal sphincter because it's too large to pass through it. As it sits there, it loses moisture and becomes hard and dry and extremely difficult to pass. If it sits too long and additional stool piles up behind it, it could eventually cause an impaction.

Although we often hear that we should poop one to three times a day, for many people, pooping three times a week is perfectly normal. As long as the stool passes smoothly and without straining, the quantity or size of the stool and the frequency you go (as long as it's no fewer than three times a week) isn't cause for alarm. With a low-fiber diet, the size of your stool might be smaller and thinner, but that makes it easier and more comfortable to pass. It should be soft as well. When you follow a low-fiber diet, you might find that you move your bowels more or less often than you do with a high-fiber diet, and this new bathroom schedule may take a little getting used to.

For many people with digestive disorders (including inflammatory bowel disease), a low-fiber diet is recommended during flare-ups. It gives the digestive tract a welcome break and time to heal. After a few weeks, you can try adding a little more fiber (preferably mostly soluble fiber) back into your diet until you determine how much you can tolerate. If you're concerned that your constipation will increase when you lower your fiber intake, remember that babies drink mother's milk and strained, pureed foods -- neither of which have fiber -- and they have no trouble pooping. Also bear in mind that people who go on juice fasts (juice contains no fiber) or water fasts don't get constipated.

Is it possible to follow a low-fiber vegan diet that's nutritionally sound? Yes, it is. I plan to post more about this in a future installment for those who would like to give a low-fiber diet a try.

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