Food & Nutrition

What’s the Difference Between Soluble and Insoluble Fiber?

If you know one thing about fiber, it’s probably the fact that it can really help get things moving down there. 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.

For starters, there are actually two different kinds of fiber: soluble and insoluble. And both do different—but equally valuable—things for your body.

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.

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 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.

Now, let’s talk about those two types: soluble and insoluble fiber. Almost all plant foods (which include vegetables, fruits, whole grains, legumes, seeds, and nuts) contain a combination of both, 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, Ph.D., R.D., 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 and fiber-fortified foods (like protein bars). Those often contain large amounts of added fiber, and often just one type or the other, Young notes.

Soluble 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, R.D.N., 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, Lisa Young, R.D.N., C.D.N., Ph.D., adjunct professor in the department of nutrition and food studies at New York University and author of Finally Full, Finally Slim, tells SELF.

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.

Insoluble Fiber

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—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.

Yes, this is the the poop-powering kind of fiber you’ve heard so much about. Because of how it moves digestion along, insoluble fiber can help prevent and treat constipation, per the FDA. Young advises clients struggling with constipation and complications like hemorrhoids to up the insoluble fiber in their diets.

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.

The added volume in your stomach provided by insoluble fiber can also help enhance the feeling of fullness you get after eating, so it may help people with weight management, Linsenmeyer says. According to the FDA, both soluble and insoluble fiber can help increase feelings of fullness for longer after a meal. Larson says people who have been advised to lose weight by their doctor may find that adding more of either kind of fiber to their diets helps.

One more thing to consider when it comes to soluble vs. insoluble: Scientists are still studying the links we’ve observed between overall fiber intake and reduced risk of a number of health issues. For instance, research suggests a negative correlation between fiber intake and how likely you are to get colorectal cancer, according to the AND, but isn’t conclusive about whether soluble or insoluble fiber is to thank. It may be both.

So, how can you make sure you’re getting enough of both?

The topline takeaway here is that fiber is generally great. “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.)

What really matters for most people without a digestive condition is overall fiber consumption. And the easiest way to ensure you’re getting enough of both types is by aiming to eat a wide variety of plant foods—like whole grains, legumes, nuts, fruits, and veggies—every day, as they naturally contain some of each, Young reminds us. If you make it a habit to incorporate different types of fiber-rich plant foods into your diet, you can ensure you’re getting a good amount of both types without overthinking it.

Just like many other nutrients, the best amount of fiber to eat daily depends on your body and your personal dietary needs (and let’s be real, what helps keep you regular). But as a baseline, the USDA recommends about 14 grams of fiber per 1,000 calories in your diet.

And while eating fiber is important, you don’t need to overload your diet with it. In fact, going too hard on the fiber (especially quickly increasing your intake) can end up causing stomach cramps, bloating, and gas, according to the Mayo Clinic. These side effects can also happen when people who eat a low-fiber diet try to boost their intake with fiber supplements and fiber-fortified foods, which often contain large amounts of only one type of fiber. Plus, if you’re relying on those options to meet the recommended fiber intake, that means you’re missing out on all the other nutritional benefits that fiber-rich foods have to offer.

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