Insoluble vs. Soluble FIber: What's the difference?
Understanding the difference between soluble vs. insoluble fiber is a key part of eating for optimal gut health. For starters, there are two types of fiber you should be eating regularly: soluble and insoluble. But what’s the difference, you may ask? Well, soluble fiber dissolves in water and forms a gel-like substance as it makes its way through the intestines.
- Marjorie Nolan Cohn, MS, RD, LDN, founder and owner of MNC Nutrition, LLC
What foods contain the highest amount of soluble fiber?
Think oats (oatmeal and oat bran), fruits, such as apples and pears (with the skin on) and berries, beans, and legumes (lentils, black beans, chickpeas), as well as most nuts and seeds.
What foods contain insoluble fiber?
Meanwhile, insoluble fiber does not dissolve in water and can actually speed up digestion. Think whole wheat and wheat bran, other whole grains like brown rice and barley, and many vegetables, including celery, carrots, and zucchini, plus leafy greens such as spinach or lettuce. Insoluble is pretty much the roughage from fruits and veggies that sweeps out your insides and bulks up your stools for regular BMs. On the other hand, “soluble fiber gets its name because it is soluble in water,” says Marjorie Nolan Cohn, RDN, founder and owner of MNC Nutrition, LLC in Philadelphia and spokesperson for the Academy of Nutrition and Dietetics.
What are the benefits of high soluble fiber foods?
Basically, soluble fiber absorbs water, swells, and creates a gel-like substance during digestion, Nolan Cohn says. That keeps BMs moving, but it also has another power: to keep your heart healthy. “Along with water, the gel-like structure can also absorb fatty acids, so it has an added benefit of reducing cholesterol,” she says.
Soluble fiber is also useful in slowing digestion, something that helps regulate blood sugar levels, says Nolan Cohn. A stable release of glucose into your blood helps prevent blood sugar dips and spikes that trigger hunger and mess with the hormones that play a role in appetite control. Finally, like all fiber, soluble feeds your GI bacteria, and we’re all about a healthy gut these days.
What are the best foods high in soluble fiber?
Here's the thing: Fiber-rich foods are going to include both insoluble and soluble fiber—some simply have more of one and less of the other. Don't stress too much about specific soluble fiber counts; it's difficult to tell how much soluble fiber specifically is in foods since nutrition labels usually just include total fiber. Aim for 25 grams of total fiber a day, says Nolan Cohn, and you'll be good.
However, if you want to hedge your bets and ensure you're getting lots of soluble fiber in the mix, here's a list of high-fiber foods that generally have a decent amount of soluble fiber, too:
What foods contain the highest amount of soluble fiber?
Fiber: 4 grams per cup (cooked)
There’s a reason why “reduces cholesterol” or “is good for heart health” is slapped on oatmeal labels: the cereal contains a type of soluble fiber called beta-glucan, which is what gives it its creamy consistency.
2. Black Beans
Fiber: 17 grams per cup
No matter what type of bean you love best, they’re all winners here. But black beans win out, says Nolan Cohn. “One cup of black beans has five grams of soluble fiber—that’s a lot,” she says. Others that get close are navy, red, and kidney beans.
Fiber: 16 grams per cup (cooked)
If you don’t routinely eat lentils, you’re missing out. Not only are they the perfect source of protein in those grain and veggie bowls you’ve been loving on lately (you know, the ones with the to-die tahini sauce), but they’re also packed with soluble fiber, says Nolan Cohn.
Fiber: 10 grams per 1-oz serving
Chia seeds are little fiber bombs. One tip-off is that it contains soluble fiber: when mixed with liquid, chia takes on the gel-like texture that makes it so excellent in chia puddings.
Fiber: 3 grams per tablespoon
Plant-based bakers know that by mixing water with ground flax, you can make a “flax egg.” That’s soluble fiber at work, folks. (Oh, BTW, if you’re on the keto diet, know that the tablespoon has 3 grams of carbs and 3 grams of fiber. That means 0 net carbs—making flaxseed a good way to get more fiber into a diet where fiber may be lacking.)
Fiber: 6 grams per cup (cooked)
7. Brussels sprouts
Fiber: 3 grams per cup (raw)
The veggie joins others like broccoli and cabbage as good sources of fiber. If you're not into the whole ordeal of chopping up your sprouts, buy pre-shredded bags of the veggie to sauté, throw on a pizza, or toss with olive oil and roast.
Fiber: 9.25 grams per fruit
Aside from being our favorite topping to spread on toast, avocados are loaded with health benefits like healthy monounsaturated fats, potassium, vitamin E, and dietary fiber. Plus, they’re filled with magnesium, which can help you get a better night’s sleep.
9. Sweet Potatoes
Fiber: 6.6 grams per cup
The humble sweet potato is one of a dietitian’s favorite foods for its boatload of health benefits, as they’re a great source of vitamin A, which helps keep skin glowing, and potassium, which helps regulate fluid balance, muscle contractions, and nerve signals. Not to mention, it’s one of the top 10 anti-inflammatory foods some of the longest-living people in the world eat daily in the Blue Zones, and it’s a great high-fiber food for a healthy heart.
If your parents always said that you should eat more fiber-rich vegetables like broccoli, they weren’t exactly wrong. After all, the veggie is loaded with essential nutrients like folate, vitamins A, C, B6, and K. Plus, the fiber helps support a healthy metabolism, too.
How can I add more soluble fiber (and fiber in general) to my diet?
Right. The goal may be 25 grams, but most people are getting only half of that amount. That's a big reason why nutrition experts tell you to fill half your plate with veggies (and fruits) and one-quarter with whole grains—these are all top-notch sources of fiber, and eating this way will help you reach that goal.
If you regularly say things like “I’m just not into vegetables” or “I’m trying to avoid grains,” then you may run low. If you’re really unsure, hook up with a registered dietitian to assess your needs. They may recommend a psyllium husk supplement (a supercharged source of fiber). It often comes in powder form, which you can then stir into yogurt or hot cereal or add to your smoothie to make it more palatable, says Nolan Cohn.
No matter what fiber source you're going for, the trick is to gradually—one more time for those in the back—gradually increase consumption. “If your body isn’t used to it, increasing fiber intake quickly can lead to GI distress,” Nolan Cohn says. Her recommendation: Don’t add more than three to five grams of fiber per meal to start; two to three grams per meal is on the safer side. Here’s to a happy heart and stomach.
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