There's much ado about fiber these days, and for good reason. But for all you hear about fiber's benefits, I've found that it remains one of the more misunderstood dietary components.
Most of the confusion about fiber rests in the fact that there are numerous different types of fiber, each with its own physical properties. These properties influence how fiber behaves in the body and account for its multiple – and often conflicting – functions. All fiber has one thing in common: By definition, it is indigestible by human enzymes. But beyond this, fiber can wear many different physiological hats.
For example, some types of fiber speed up digestive transit time – a polite way of saying that it helps move poop through your pipes faster. Other types of fiber can actually slow down digestive transit time, enabling the absorption of excess water in the colon, so that stool is less watery and more formed when it comes out. Some types of fiber are more likely to produce gas than others. And different types of fiber nourish different resident bacterial species in the gut, therefore, diets with a variety of different fibers from multiple food sources may promote a more diverse gut flora than diets with less fiber variety.
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In other words, not all fiber is created equal. In my practice, where I'm routinely called upon to help patients manage irregular bowel patterns like diarrhea or excessively urgent bowel movements, I've found that the most important distinction to make is that between soluble fiber and insoluble fiber.
Soluble fiber is a type of fiber that dissolves in water, and in so doing, transforms into a gooey, gummy, spongy mass that travels the length of the gut on its own timetable. But don't confuse this fiber's tendency to downshift digestive transit time with a constipating effect; rather, it is regulating.
Soluble fiber still contributes to fecal bulk and is able to hold onto water, therefore contributing to stools that are soft, well-formed and easy to pass – the Platonic ideal of a poop. Generally, soluble fiber is found in the flesh of fruits, root vegetables and in cooked grains. Examples include beta glucan in oats and barley and pectin in apples and citrus fruits.
Insoluble fiber is what you might call "roughage." When combined with water, its physical properties generally change very little; this is because it doesn't absorb water well. Therefore, insoluble fiber contributes bulk to the feces, helping to put pressure on the colon walls to stimulate a bowel movement. In this manner, insoluble fiber speeds up digestive transit time and is especially helpful when constipation is at issue. Insoluble fiber is found in the form of cellulose and hemicellulose from leafy veggies, fruit and vegetable skins or the bran layer of most whole grains; it's found in lignans from sesame or flax seeds and some mature vegetables. (A third type of fiber, called resistant starch, has some properties of both soluble and insoluble fiber. It's found in foods like beans, unripe bananas and potatoes that have been cooked and cooled.)
A variety of different fibers from different foods sources will deliver a variety of different and important health benefits. Therefore, in healthy people without bathroom woes, the ideal diet would contain a mix of both fiber types. Certain types of soluble fiber help lower cholesterol levels. Insoluble fiber has been credited with helping to prevent colon cancer, since it helps speed toxins and cancer-causing food compounds out of the body without haste. Some fermentable types of both soluble and insoluble fiber help increase calcium absorption from the diet and stimulate the growth and activity of the gut's most beneficial bacterial populations. Both types of fiber also help delay stomach emptying after a meal and therefore tend to help improve blood sugar control.
Generally, when constipation is a problem, a mix of both fiber types is helpful and well tolerated. But among people prone to diarrhea, urgent loose stools or "hyperdefecation" (increased frequency of bowel movements, even if they're not diarrhea), the mix of fiber in the diet may deserve more careful management. I often tell my patients with chronic diarrhea that soluble fiber in particular is their best friend: It helps slow down stool that's racing through the gut too quickly and can often consolidate trips to the bathroom for people who experience multiple, incomplete bowel movements in rapid succession.
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Lots of insoluble fiber, conversely, can make things a whole lot worse (particularly if it's not balanced out by some soluble fiber), exacerbating a pre-existing tendency toward urgent, loose bowel movements. For these patients, it can be helpful to think of soluble fiber as the "anchor" of each meal, ensuring that a fruit, veggie, grain and/or seed rich in soluble fiber plays a starring role in the mix.
At the risk of overstating the benefits of this approach to dietary fiber manipulation, I will share that my patients routinely describe the effect of so-called soluble fiber therapy as "life changing." Furthermore, the timing of fiber intake can play a role in managing symptoms. For people prone to urgent, loose bowels in the morning specifically, limiting insoluble-rich foods like salads at dinner and including a soluble-rich veggie (squash, skinless sweet potato), cooked grain (rice, pearled barley, quinoa) and/or soluble fiber supplement (Citrucel, Benefiber) can help smoothe out choppy morning bowel patterns. If symptoms tend to occur in the afternoon or evening, having a super-soluble breakfast (like oatmeal, chia seeds and banana) can help keep things flowing smoothly later on in the day.
As you can see, there's a lot more to fiber than meets the eye – and more to know than what a typical nutrition label reveals. If you need help figuring out how to tweak the fiber content of your diet to optimize your health and digestive tolerance, consult a registered dietitian to help you navigate the supermarket shelves to your advantage.
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Tamara Duker Freuman, MS, RD, CDN, is a NYC-based registered dietitian whose clinical practice specializes in digestive disorders, Celiac Disease, and food intolerances. Her personal blog, www.tamaraduker.com, focuses on healthy eating and gluten-free living.