What's the Difference Between Soluble and Insoluble Fiber?

An RD explains the benefits of each, plus how to get more of both in your diet.

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You've probably heard that eating more fiber is a good thing, but did you know that there's more than one type of dietary fiber? The two categories experts focus on most are soluble and insoluble. While they're sometimes found in the same foods, they play different roles in supporting good health. Here's a quick guide to what the two varieties do, plus the best ways to boost your intake.

Insoluble fiber

This is the type people think of as "roughage." It's the tough matter found in whole grains, nuts, and fruits and veggies (specifically in the stalks, skins, and seeds) that doesn't dissolve in water. Insoluble fiber isn't broken down by the gut and absorbed into the bloodstream. It adds bulk to waste in the digestive system, which helps keep you regular and prevent constipation (as well as any related problems, like hemorrhoids).

Soluble fiber

Soluble fiber is soft and sticky, and absorbs water to form a gel-like substance inside the digestive system. Top sources include beans, peas, oats, barley, fruits, and avocados. Soluble fiber helps soften stool so it can slide through the GI tract more easily. It also binds to substances like cholesterol and sugar, preventing or slowing their absorption into the blood. That's why it's known to help regulate blood sugar levels, and protect against heart disease by lowering blood cholesterol. What's more, soluble fiber boosts the population of good bacteria in the gut, which is linked to improved immunity, anti-inflammatory effects, and even enhanced mood. But that's not all: Soluble fiber also has middle-whittling benefits. For starters, it makes you feel full for longer, which helps with weight management. One study showed that for each additional 10 grams of daily soluble fiber eaten, participants had a 4% decrease in belly fat over a five-year period.

Why you need both

Both soluble and insoluble fiber are important for your health, which is why a lot of research has focused on total fiber intake. For example, a study published in the Archives of Internal Medicine found that over a nine-year period, consuming more dietary fiber lowered the risk of death from any cause. People who ate the most fiber (about 25 grams a day for women and 30 grams for men) were 22% less likely to die compared to those who consumed the least fiber (10 grams per day for women and 13 grams for men). The effect was even stronger when researchers looked at deaths from heart disease, infectious diseases, and respiratory diseases; people with high-fiber diets had as much as a 50% or greater reduction in risk.

How to get more

Don't worry about counting up total grams of insoluble and soluble fiber. Instead, use these tips to up your overall fiber intake, and reap the benefits of both varieties.

  1. Set a goal of eating a minimum of three servings of veggies and two servings of fruit every day. A good strategy is to have fruit at breakfast and as a snack; and veggies for lunch and dinner—and at breakfast when possible (think greens in an omelet or whipped into a smoothie).
  2. Choose fibrous veggies with tough stalks (like artichokes, Brussels sprouts, and broccoli) as well fruits with edible seeds, skins, and membranes (such as raspberries, apples, and citrus).
  3. Eat pulses (beans, lentils, and peas) at least three times a week, either as a plant-based protein in meatless dishes, or as the starch side in place of grains. For example, you could have fish on a bed of lentils rather than rice. Or check out my recipe for an omelet topped with chickpeas.
  4. Snack on nuts and seeds along with fruit. Or add them to Greek yogurt, oatmeal, salads, stir-fries, and cooked veggies. (One of my favorite side dishes is cooked spaghetti squash tossed with dairy-free pesto and topped with sliced almonds or roasted pumpkin seeds.)
  5. Put avocado in everything! Spread it on whole grain toast, whip it into smoothies, add it to eggs, salads, and bean dishes. You can also use avocado in place of mayo to coat chilled protein salads (like egg or tuna), and as a replacement for butter in baking.
  6. Swap out processed snacks with fiber-rich alternatives. For example, trade crackers or pretzels made with white flour for popcorn and roasted chickpeas.
  7. Opt for whole grains rather than refined grains. That means choosing brown or wild rice over white. And if you enjoy pasta, look for versions made from quinoa, or pulses like chickpeas and lentils.

Give your body time to adjust

While upping your fiber game is a very good thing, you may experience some gas and bloating at first. So be sure to balance that extra fiber with plenty of water–around 16 ounces, four times a day–to help move the fiber through your system. I explain to my clients that the discomfort is akin to the soreness you feel when you start working out: It's a response to a change, but a good change. Give it a little time, and those transitional side effects should subside.

Do you have a question about nutrition? Chat with us on Twitter by mentioning @CynthiaSass.

Cynthia Sass is a nutritionist and registered dietitian with master's degrees in both nutrition science and public health. Frequently seen on national TV, she's Health's contributing nutrition editor, and privately counsels clients in New York, Los Angeles, and long distance. Cynthia is currently the sports nutrition consultant to the New York Yankees, previously consulted for three other professional sports teams, and is board certified as a specialist in sports dietetics. Sass is a three-time New York Times best-selling author, and her newest book is Slim Down Now: Shed Pounds and Inches with Real Food, Real Fast. Connect with her on Facebook, Twitter and Pinterest.

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