There are two different types—and they serve completely separate purposes. Here’s everything you need to know about what soluble fiber and insoluble fiber do.
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 . 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 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 , tells SELF. Where both types of fiber don’t always show up is in fiber supplements and fiber-fortified foods , 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 , legumes , seeds , nuts , and some fruits and vegetables , according to the US National Library of Medicine . 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. 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 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. This can help lower the level of LDL cholesterol 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 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 as a byproduct. SCFAs 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 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 , nuts , beans , and some vegetables —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 , 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 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 . 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 gastroenterologist at the Cleveland Clinic, previously told SELF. In fact, constipation is a common sign there’s not enough fiber in your diet. That’s why Young advises clients struggling with constipation and complications like hemorrhoids to increase their insoluble fiber intake as a path to more regular digestion. Is psyllium soluble or insoluble fiber? Psyllium husk powder—the active ingredient in many fiber supplements, like Metamucil—is a highly concentrated source of soluble fiber. That said, psyllium does contain a little bit of insoluble fiber too. For instance, a serving of Metamucil contains six grams of soluble fiber and one gram of insoluble fiber. Psyllium—also called psyllium seed husk and ispaghula husk—is milled from the husks, or seed coats, of the seeds of the psyllium plant, the FDA explains. It’s used as a fiber supplement and added to some products to increase the fiber content. Psyllium is one of the types of added fiber ingredients that the FDA allows food companies to count toward a product’s total dietary fiber, as SELF has previously explained. What is better, soluble or insoluble fiber? Trick question! Fiber is generally awesome—and in most cases, it’s not really a contest between soluble and insoluble fiber. “Both types are very healthy,” Linsenmeyer says. “One isn’t better for you than the other, and we need both in our diets” for optimal digestive and overall health. And fundamentally, both “work together” to keep things moving smoothly—they’re teammates, not rivals, Nielsen says. So while all of this fiber info is fascinating and useful, it’s not like you need to be tallying up how much insoluble versus soluble fiber you’re getting. And again, while it’s common for foods to have more of one kind than the other, most plant foods have some of both. What really matters for most individuals is not the breakdown between soluble and insoluble fiber but overall fiber intake, not to mention dietary nutrient balance in general. Fiber type What it does Common food sources Why it matters Soluble fiber Forms a gel that slows down digestion and gets fermented by gut bacteria in the large intestine Barley, oats, beans, lentils, peas, flaxseeds, chia seeds, peanuts, macadamia nuts, bananas, avocados, citrus fruits, strawberries, carrots, okra, eggplant, berries, artichokes, broccoli, winter squash Prevents blood sugar spikes, modulates “bad” LDL cholesterol, supports gut microbiome health, reduces symptoms in people with certain digestive conditions, and increases fullness Insoluble fiber Speeds up digestion and softens and bulks up stool Whole wheat flour, wheat bran, quinoa, brown rice, almonds, navy beans, pinto beans, kidney beans, spinach, kale, cauliflower, potatoes, green beans Maintains bowel movement regularity, prevents and alleviates constipation, and increases fullness How much fiber should people eat in a day? Now that you know getting enough total fiber is generally more crucial than getting a certain amount of either type, you might be wondering what “enough” looks like. As with many nutrients, the optimal amount per day depends on your individual body—your dietary needs, your stress and activity levels, your GI or other medical conditions, and your own sense of well-being, as SELF has reported. But as a baseline, the USDA recommends about 14 grams of fiber per 1,000 calories in your diet. That said, many people aren’t in that ballpark. The average American gets just 16 grams a day when they should be getting more like 21 to 38 grams, per the U.S. National Library of Medicine. In fact, upward of 90% of Americans are considered fiber deficient. And in case you were wondering, there aren’t any “good clear-cut guidelines” for soluble versus insoluble fiber specifically, according to Nielsen, though Tufts University suggests a 2:1 ratio of insoluble to soluble. If you shoot for 30 grams of fiber per day, for example, that would come out to 20 grams of insoluble and 10 grams of soluble. How much fiber do women need daily? Delving more into the nitty-gritty, the USDA’s Dietary Guidelines for Americans 2020-2025 recommends 25 grams for women and girls ages 14 to 18, 28 grams for women ages 19 to 30, 25 grams again for women ages 31 to 50, and 22 grams for women ages 51 and over. To reiterate, however, every person is different. Invoking that 25-gram mark, “a smaller, less active woman might need less fiber than that, and a taller, more active woman might need more than that,” for example, Nielsen says. FWIW, she notes, “I probably consume 25 grams of fiber on a low-fiber day, and I can probably eat as much as 40 grams of fiber on a high-fiber day.” Much like the general population, though, most women don’t meet their goal. In a 2014 report by the USDA’s Food Surveys Research Group that examined data from the CDC’s National Health and Nutrition Examination Survey 2009-2010, researchers found that women consumed an average of 15 grams of fiber per day , though that amount was actually proportionally higher when adjusted for total energy intake. The researchers also noted that the diets of older women , as well as older men, were significantly more fiber dense than those of their younger counterparts. More recent NHANES data suggests that 12% of women in the US are consuming adequate fiber , according to a 2023 review published in the American Journal of Lifestyle Medicine. Regardless of your gender, the best way to ensure you’re getting enough fiber—both soluble and insoluble—is simply consuming a diverse array of plant foods every day, since they’ll naturally contain some of each kind. Think high-fiber vegetables, fruits, whole grains, legumes, nuts, and seeds. If you make this a habit, you’ll ensure you’re getting a good amount without overthinking it. You might find it helpful to work with a dietitian or health care provider if you’re having trouble incorporating fiber-rich foods into your diet and/or experiencing digestive issues in connection with your fiber intake. And if you are dealing with a digestive condition, absolutely speak with your doctor or a registered dietitian about the benefits and drawbacks of various soluble- and insoluble-fiber foods for you as an individual. How can you add more fiber safely? One final thing to keep in mind here: While eating fiber is important, there’s no need to dial it up to 11 and overload on the nutrient. In fact, going too hard on fiber can end up causing bloating, cramps, and gas, according to the Mayo Clinic. In that case, your gut “isn't habituated to a higher-fiber diet,” Nielsen explains. Just as you wouldn’t head straight for the 50-pound dumbbells your first time at the gym, “you also wouldn't go from consuming 10 grams of fiber a day to 40 grams of fiber a day overnight. Your gut is simply not equipped to handle it.” Here are three suggestions for leveling up on fiber that can help prevent or minimize digestive side effects: Start slow by adding one higher-fiber source to your diet per day. “In our practice, we call it ‘fiber training,’” Nielsen says. “We're talking only three to five grams of fiber”—say, a large apple or a tablespoon of chia seeds. Drink more water. “Fiber requires water to do its job,” especially when it comes to soluble fiber, which needs to take on H20 to work properly, Nielsen says. In its absence, you risk the risk of constipation, making hydration “super important” as you increase your fiber intake. Consider especially gentle fiber sources. Still dealing with indigestion even after trying the above steps? High-fiber foods that are low-FODMAP—low on fermentable oligosaccharides, disaccharides, monosaccharides and polyols, all of which are carbs that are absorbed poorly by the gut and can trigger distress as a result—could come to the rescue. Because these foods are less fermentable by gut bacteria, Nielsen says, “they're less likely to cause gas.” Examples of high-fiber, low-FODMAP foods include chia seeds and oats. Eating these allows you to “increase your fiber with a little less risk of getting extra gassy while you're doing it.” Still, you can never eliminate the possibility entirely, so be prepared to give your body some grace—and remember that a period of adjustment is totally normal, according to Nielsen. If you clock a bit more rumbliness than usual, she says, “just stay consistent and wait for that to pass.” Your gut—and your body as a whole—will thank you. Related: 12 High-Fiber Foods That Aren't Fruits and Veggies 5 High-Fiber Snacks That'll Keep You Satisfied Between Meals 5 High-Fiber Breakfast Recipes That Will Keep You Full Until Lunch Get more of SELF's great nutrition coverage delivered right to your inbox—for free.
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