The idea that “fiber is fiber” is a serious misconception. Dietary fiber comes in several forms that have different effects on your digestion and health.
What is Dietary Fiber?
Dietary fiber is the component of plant foods that humans cannot digest—in other words, our digestive enzymes cannot break it down. While fiber does not provide direct nutrition, it has other functions that are important for well-being and health. Fiber affects digestion, blood sugar, cholesterol, appetite, and your gut microbiome (beneficial microbes in the intestines). It can also influence inflammation and the immune system. Fiber is commonly described as either soluble or insoluble, but that’s an over-simplification. Fiber can also be understood by whether it is viscous, fermentable, or a resistant starch. These categories overlap, and most whole plant foods contain more than one kind.
Soluble and Insoluble Fiber
Soluble fiber dissolves or swells in water. Some soluble fibers, so-called viscous fiber, form a gel in the digestive tract, slowing the movement of food from the stomach and slowing the absorption of glucose into the bloodstream. Good sources of soluble fiber include oats, lentils, barley, beans, apples, citrus fruits, berries, chia seeds, flaxseed, and psyllium. Soluble fiber is associated with improved blood cholesterol and control of blood sugar. Oats and barley, for example, contain beta-glucan, a soluble fiber that helps lower LDL cholesterol when eaten regularly. Beans and lentils are particularly valuable because they combine soluble fiber with protein, minerals, and slowly digested carbohydrate.
Insoluble fiber does not dissolve in water. It adds bulk to stool and helps move waste through the intestine. It is useful for preventing or easing constipation and promoting regular bowel movements. Good sources include wheat bran, whole wheat, brown rice, many vegetables, nuts, seeds, and fruit/vegetable skins. By increasing stool volume and speeding transit, insoluble fiber helps maintain a healthy intestinal environment.
What is Fermentable Fiber?
Fermentable fiber can be used as food by bacteria in your intestines, mostly in the large intestine. These “good bacteria” produce a variety of substances that help maintain colon cells and play roles in other aspects of health such as inflammation, immunity, and metabolism. Fermentable fiber is often called prebiotic fiber when it encourages the growth of beneficial gut bacteria. Fermentable fiber is found in onions, garlic, leeks, asparagus, Jerusalem artichokes, bananas, apples, beans, lentils, oats, and some whole grains. By the way, this sort of fermentation has nothing to do with the fermentation involved in making beer and wine.
What About Resistant Starch?
Resistant starch is a form of starch that is not digested in the small intestine but instead moves to the color where it can be fermented by microbes. Thus, it behaves more like a fermentable fiber than a typical carbohydrate. The products of this fermentation have various beneficial health effects as described in the section above on fermentable fiber. In addition, because resistant starch is not digested (broken down into glucose) in the small intestine, it does not contribute to glucose load or insulin spikes. Some foods naturally rich in resistant starch are beans and lentils, peas, nuts, seeds, and whole grains.
Furthermore, some of the “regular” starch in foods like potatoes and pasta is converted to a resistant starch if the food is cooked and then cooled. This process is called retrogradation. The proportion of the starch that is retrograded by cooling after cooking varies widely from food to food. Reheating the foods after cooling only partially reverses this conversion to resistant starch.
These Other Health-Related Myths Might Interest You!
Think that GMO foods are bad for you? Think again!
Some veggies are actually more nutritious after cooking.
