Soluble Fiber vs . Insoluble Fiber : What the Difference ?

self.com · Feb 24, 2026 · Collected from GDELT

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Most people are well aware that fiber is a nutrient we want to eat enough of. Fewer know that there are two distinct types: soluble fiber and insoluble fiber. Both play different—but equally valuable—roles in your body. Does fiber make you poop? You bet. But as hyped as fiber’s poop-promoting powers may be—and make no mistake, that function is indeed essential—there’s actually a lot more to appreciate about the stuff, both in its soluble and insoluble forms.In an effort to give fiber its full due, we broke it all down with the help of a few nutrition experts. Here’s everything you need to know about the two types of fiber, including what they do in your body, the foods where you can find them, and the health benefits they offer.First of all: What actually is fiber?Fiber, sometimes called dietary fiber, is a type of carbohydrate found in plant foods, according to the Food and Drug Administration (FDA). Its structure is formed by a bunch of sugar molecules, bound together in a way that makes it hard to readily break down and use as energy. The small intestine can’t digest fiber in the same way as other kinds of carbohydrates, so unlike sugar or starch, for instance, fiber is not a great source of fuel for the body. But it still plays a crucial role in a healthy diet.So what foods are high in fiber generally? Lots of kinds of plants. Nearly all plant foods (which include vegetables, fruits, whole grains, legumes, seeds, and nuts) contain a combination of both soluble and insoluble fiber, according to the FDA. Sometimes the two types are listed separately on nutrition labels, but often, they’re lumped together as “fiber.” Take an apple, for instance. Its flesh contains some soluble fiber, while the skin is full of insoluble fiber, Whitney Linsenmeyer, PhD, RD, an assistant professor in the department of nutrition and dietetics at Saint Louis University’s Doisy College of Health Sciences and a national spokesperson for the Academy of Nutrition and Dietetics (AND), tells SELF.Where both types of fiber don’t always show up is in fiber supplements (like psyllium husk products) and fiber-fortified foods (like high-fiber protein bars), which often contain large amounts of added fiber that consists of just one type or the other, Lisa Young, RDN, CDN, PhD, a nutrition counselor and adjunct professor in the department of nutrition and food studies at New York University, tells SELF. That not only denies you the benefits of both types, but can also be less than ideal for your stomach, as we’ll get into in a bit.What is soluble fiber, and what does it do?Soluble fiber is fiber that is able to dissolve in water. While there are “typically fewer rich sources of soluble fiber in our food supply than insoluble fiber,” Desiree Nielsen, RD, a recipe developer with a focus on plant-based nutrition, tells SELF, you probably still have plenty of decent options sitting in your pantry. Soluble fiber is found in grains (like barley and oats), legumes (like beans, lentils, and peas), seeds (like flaxseeds and chia seeds), nuts (like peanuts and macadamia nuts), and some fruits and vegetables (like bananas, avocados, citrus fruits, strawberries, and carrots), according to the US National Library of Medicine (plus okra and eggplant, Nielsen adds). It’s especially concentrated in berries, artichokes, broccoli, and winter squash, Kim Larson, RDN, a board-certified health and wellness coach, tells SELF.When you eat these foods, the soluble fiber pulls in and swells up with water in the stomach, partially dissolving to form a thick gel-like substance that slows down digestion, according to the U.S. National Library of Medicine. (You can even see gelling happening in real time when you cook oatmeal or hydrate chia seeds, Nielsen says.) This fibrous gel later gets broken down by bacteria in the large intestine, which ends up providing a small amount of calories, per the FDA.So what can it actually do for you? Quite a bit. Because of its ability to decelerate digestion, soluble fiber has a knack for slowing or lessening the absorption of several substances that can have negative effects on our health if they build up too fast or rise too high.For instance, soluble fiber puts the brakes on the rate at which carbohydrates enter into the bloodstream, according to the FDA, which helps prevent spikes in blood glucose (blood sugar) after eating. “It’s going to ‘trap’ sugar molecules so that they’re absorbed more slowly, which is helpful for keeping blood sugar levels more regular,” Linsenmeyer explains.If you drink a glass of pure orange juice, for instance, that sugar gets metabolized pretty much immediately, causing your blood sugar to climb rapidly. But if you eat a whole orange, which contains soluble fiber, the sugar uptake is more gradual, Linsenmeyer says. This is helpful for anyone trying to maintain steady blood sugar levels, such as those with prediabetes or type 2 diabetes, Young says.Soluble fiber also has a regulatory effect on the absorption of dietary fat and cholesterol. “It attaches to the cholesterol in food, so that it gets excreted from the body instead of absorbed by it,” Linsenmeyer says. (Remember, fiber doesn’t get digested the way other nutrients do.) This can help lower the level of LDL cholesterol (the “bad” kind) in your blood, according to the FDA—and, in turn, potentially lessen the risk of heart disease, according to the US National Library of Medicine. In fact, “consuming adequate soluble fiber is associated with healthy cholesterol levels,” Nielsen says. That’s why Young advises clients at high risk of heart disease to incorporate plenty of soluble fiber into their diets.Soluble fiber can also boost your gut microbiome—the vast population of bacteria and other microorganisms that live in your digestive tract and mediate multiple aspects of your health—in a big way. More specifically, many soluble fibers—like inulin and fructo- and galacto-oligosaccharides—can serve as prebiotics, or food sources for good bacteria (probiotics) that reside in the gut microbiome. Remember what we said about the fibrous gel being broken down by bacteria in the large intestine? Well, that involves fermentation, a process that provides fuel for beneficial bugs and supports the overall microbiome balance. Fermentation also yields beneficial compounds like short-chain fatty acids (SCFAs) as a byproduct. SCFAs (such as acetate, propionate, and butyrate) can have a lot of positive effects on your body, influencing “our immune function, our nervous system function, and the health of gut cells themselves,” Nielsen says, so that also represents a “really core benefit” of soluble fiber.Soluble fiber can also be helpful for slowing down digestion in some individuals with certain gastrointestinal issues. For example, people with irritable bowel syndrome (IBS) may find that fiber helps decrease symptoms like diarrhea, according to the Cleveland Clinic.What is insoluble fiber, and what does it do?If you’re guessing that “insoluble” means this kind of fiber does not dissolve in water, bingo! Insoluble fiber is found in the highest amounts in whole grains (like whole wheat flour, and wheat bran, quinoa, and brown rice), nuts (like almonds and walnuts), beans (like navy beans, pinto beans, and kidney beans), and some vegetables (like spinach, kale, cauliflower, potatoes, and green beans)—basically, the dense cell walls of plants, Nielsen says.Insoluble fiber doesn’t pull in water to form a digestion-slowing gel like soluble fiber. Rather, it has pretty much the opposite effect. Unlike its soluble sister, this kind of fiber passes right through us and exits virtually unchanged, speeding the movement of food through the digestive system and adding bulk to our stool, according to the FDA. Yet another difference: It’s also not fermentable. In contrast to soluble fiber, which is “inherently fermentable,” insoluble fiber typically isn’t, so it won’t do much to act as a prebiotic or promote a healthy gut microbiome (though there are exceptions to that rule), according to Nielsen.Instead, “you can think of insoluble fiber as the broom that sweeps the gut clear,” Nielsen says. “It helps to increase motility, and it helps to promote the turnover of gut cells.”Insoluble fiber can also be beneficial for various digestive conditions associated with sluggish or irregular bowel movements. For instance, the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) advises people with diverticulosis—a disease in which little sacs bulge out of the weak areas of your colon wall—to incorporate more fiber into their diets. Of course, if you have diverticulosis or any other digestive condition, always speak with your doctor to find out what the best diet is for you before making any radical changes. In some cases, doubling down on fiber could cause digestive discomfort: Certain fibrous foods like broccoli and brussels sprouts may cause bloating and gas for people with IBS, for example, according to the Cleveland Clinic.The added volume in your stomach provided by insoluble fiber can also help enhance the sensation of fullness you get after eating, Linsenmeyer says. In fact, according to the FDA, both soluble and insoluble fiber can help sustain post-meal fullness for longer.Which kind of fiber is best for constipation?Insoluble fiber’s main claim to fame is its power to push poop along. Because of this, insoluble fiber beats out soluble fiber for constipation prevention and treatment thanks to how it accelerates the passage of food and waste through the digestive system, per the FDA.While both kinds of fiber are needed for a balanced, well-functioning digestive system, insoluble fiber is especially critical for keeping you from getting backed up (or for getting things moving again). Not only does insoluble fiber encourage consistent bowel movements to help you stay regular, it also helps create softer stools that are easier to pass, Christine Lee, MD, a gastroent