Most people are well aware that fiber is an awesome nutrient we want to eat enough of. Fewer people know that there are two distinct types: soluble fiber and insoluble fiber. And both do a variety of different—but equally valuable—things for your body. Does fiber make you poop? You bet. But as glorious 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, 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 you can find them in, and the health benefits they can offer.
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 the way it does with other kinds of carbohydrates. So unlike sugar or starch, for instance, fiber is not actually 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. And almost 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 they’re listed separately in the nutrition facts, but often you’ll just see “fiber.” Take an apple, for instance. The flesh of the apple contains some soluble fiber, while the skin is full of insoluble fiber, Whitney Linsenmeyer, PhD, RD, instructor in the department of nutrition and dietetics at Saint Louis University and spokesperson for the Academy of Nutrition and Dietetics (AND), tells SELF.
Where you don’t always see both types of fiber is in fiber supplements (like psyllium husk products) and fiber-fortified foods (like high-fiber protein bars). Those often contain large amounts of added fiber, and often just one type or the other, Lisa Young, RDN, CDN, PhD, nutrition counselor and adjunct professor in the department of nutrition and food studies at New York University, tells SELF. And that can be less than ideal for your stomach, as we’ll get into, along with getting all the benefits of both types of fiber.
Soluble fiber is fiber that is able to dissolve in water. It is the main type of fiber found in grains (like barley and oats), legumes (like beans, lentils, and peas), seeds (like chia seeds), nuts, and some fruits and vegetables (like citrus fruits and carrots), according to the U.S. National Library of Medicine. It’s especially high in berries, artichokes, broccoli, and winter squash, board-certified health and wellness coach Kim Larson, RDN, tells SELF.
When you eat these foods, the soluble fiber pulls in and swells up with water in the stomach, partially dissolving within it to form a thick gel-like substance in the stomach 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, a process that ends up providing a small amount of calories, per the FDA.
So, what can this soluble stuff actually do for you? Quite a bit. Because of how it decelerates digestion, soluble fiber has a knack for slowing or lessening the absorption of several substances that can have negative effects on our health if their levels build up too high or too fast.
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 our blood glucose levels (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 more quickly. But if you eat a whole orange, which contains soluble fiber, the rate of 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 (low-density lipoprotein, the “bad” one) in the blood, according to the FDA—and, in turn, potentially lessen the risk of heart disease, according to the U.S. National Library of Medicine. That’s why Young recommends clients at elevated risk for heart disease incorporate plenty of soluble fiber in their diets.
If you’re guessing “insoluble” means this kind of fiber does not dissolve in water, bingo! Soluble fiber’s sister is found in the highest amounts in whole grains (like whole wheat flour and wheat bran), nuts, beans, and some vegetables (like cauliflower, potatoes, and green beans), according to the Mayo Clinic.
Insoluble fiber doesn’t pull in water to form a digestion-slowing gel like soluble fiber—its role is just the opposite, actually. This kind of fiber passes right through us looking pretty much the way it came in, hurrying along the movement of food through the digestive system and adding bulk to our stool, according to the FDA.
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) recommends people with diverticulosis—a disease in which little sacs bulge out of the weak areas of your colon wall—incorporate more fiber into their diet. 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. (For instance, certain fibrous foods like broccoli, and brussels sprouts may cause bloating and gas for people with IBS, according to the Cleveland Clinic.)
The added volume in your stomach provided by insoluble fiber can also help enhance the feeling of fullness you get after eating, Linsenmeyer says. Actually, according to the FDA, both soluble and insoluble fiber can help increase feelings of fullness for longer after a meal.
Insoluble fiber’s main claim to fame is its power to help push poops along. When it comes to soluble vs. insoluble fiber for constipation prevention and treatment, insoluble fiber takes the cake because of 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 getting things moving again). This type of fiber not only encourages consistent bowel movements to help you stay regular, but also helps create softer stools that are easier to pass, Christine Lee, MD, a gastroenterologist at Cleveland Clinic, previously told SELF. That’s why Young advises clients struggling with constipation—a common sign there’s not enough fiber in your diet—and complications like hemorrhoids to up their insoluble fiber intake in order to promote more regular digestion.
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.
Trick question! Fiber is generally awesome—and in most cases, it’s not really about soluble vs. 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.
So while all of this fiber goodness is fascinating and good to know, it’s not like you need to be tallying up how much insoluble versus soluble fiber you’re getting. (Besides, that’d be pretty difficult to do, given many foods don’t list them separately.) 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 (those without a digestive issue) is not the breakdown of grams of soluble vs. insoluble fiber but their overall fiber intake—as well as the nutrient balance in their diet in general.
Now that you know getting enough total fiber is generally more crucial than worrying about either type, you might be wondering just how much total fiber you should be getting. As with many nutrients, the optimal amount of fiber to eat in a day depends on your individual body—your dietary needs, any G.I. or other medical conditions you have, stress and activity levels, and what feels good for your body, as SELF has reported. But as a baseline, the USDA recommends about 14 grams of fiber per 1,000 calories in your diet. 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.
The best way to ensure you’re getting enough fiber, both soluble and insoluble? Simply aiming to consume a diverse array of plant foods every day, which naturally contain some of each kind. Think high-fiber vegetables, fruits, whole grains, legumes, nuts, and seeds. If you make it a habit to incorporate different types of plant foods that are good sources of fiber into your diet, you can ensure you’re getting a good amount of both types 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 vs. insoluble fiber foods for you as an individual.
One more thing to keep in mind here: While eating fiber is important, there’s no need to go HAM and overload your diet with high-fiber foods. In fact, going too hard on the fiber (especially by quickly increasing your intake or relying too heavily on fiber supplements or fiber-fortified foods) can end up causing bloating, cramps , and gas, according to the Mayo Clinic. Gradually increasing your intake of foods high in fiber, maintaining a balance of soluble and insoluble sources, and drinking lots of water can help.
