Eat Your Fiber — Soluble and Insoluble

soluble vs insoluble fiber

Story at-a-glance -

  • Dietary fiber is made up of two types: soluble, which easily dissolves in water and becomes gel-like, and insoluble, which doesn’t dissolve but stays basically intact as it moves through your colon; both types are important for digestion
  • Fiber, especially that found in vegetables, is important for your diet on a daily basis because it promotes bowel movements and keeps your colon free of obstruction while benefiting your heart health and blood sugar control
  • Fiber helps optimize your gut microbiome and your immune system, while resistant starches act as prebiotics, helping to feed healthy bacteria in your gut

By Dr. Mercola

What constitutes soluble as opposed to insoluble fiber is something not everyone is clear on, especially in regard to their importance. In discussing fiber, there are two types: soluble and insoluble. Most consider fiber a tool to aid in healthy digestion, but it does so much more. Dietary fiber, the indigestible part of plant material, is considered an essential nutrient because only through eating it is fiber made available to your body, and you need both kinds. Some foods are made up of both soluble and insoluble fiber.

Any portion of a plant-based food that doesn't break down when you eat it or isn't readily absorbed into your body is referred to as indigestible. Fiber is also sometimes called roughage, a term that describes, to a degree, the function of fiber as it moves through your colon and, in the process, helps move along food particles that may tend to adhere to the sides. Food that remains stuck to your colon may cause bloating, pain and constipation, as well as other problems.

Vegetables, certain fruits, seeds and nuts are all important to include in your diet on a regular basis because, besides providing nutrients, they contain the fiber necessary to promote regular bowel movements and keep your colon free of obstruction, while providing countless other health benefits.

Aspects of Soluble Fiber

Soluble fiber is easily dissolved in water and becomes gel-like when it reaches your large intestine so it's more easily broken down by liquids and gastrointestinal fluids and releases certain gases. Some of the benefits of this type of fiber are that:1

  • The thick gel moves into every crevice in your colon and helps to slow down your digestion. This helps you to feel full longer and is one reason why fiber may help with weight control.
  • It prevents some dietary cholesterol from being broken down and digested, which serves to help optimize cholesterol levels.
  • It slows down the rate at which other nutrients are digested, including carbs, so they're not as likely to raise your blood glucose; it also helps stabilize your glucose levels and even prevent blood sugar spikes.
  • It helps lower your risks of developing heart disease and hypertension.
  • Some foods rich in soluble fiber help feed good bacteria in your gut.

Attributes of Insoluble Fiber and Overall Benefits of Both

Insoluble fiber does not dissolve and stays basically intact as it moves through your colon. Because insoluble fiber isn't digested, it's not a source of calories. This type of fiber:

  • Helps prevent constipation, as it absorbs fluid when it reaches your intestinal tract, which helps other byproducts stick to it, forming the waste you want to get rid of. In the process, it lessens the amount of time food spends in your colon and helps it to exit at the same time. Blockage and constipation are much less of a problem, and bowel movements become much more regular.
  • Decreases your risk of developing diverticulitis, which occurs when your colon forms pouches or folds and greatly exacerbates intestinal blockages that further lead to constipation.

What fiber does for you overall is twofold: Soluble fiber helps you feel full for longer after you eat by slowing digestion, while insoluble fiber fills up the space in your stomach and intestines, both of which help you manage portion size. Both types of fiber also serve to decrease your risk of several health-compromising conditions, including heart disease, diabetes and obesity. In fact, one study showed a 10 percent decreased risk for all-cause mortality for every 10 grams of fiber you increase in your overall fiber intake.2

Sources of Soluble and Insoluble Fiber

Many foods are good sources of fiber. Generally speaking, foods considered "good sources" contain at least 20 percent of the recommended daily value of dietary fiber per serving, but this doesn't tell the whole story of whether a food is healthy or not. Grains, for instance, are not recommended, even though they're high in fiber.3

Healthy foods with high amounts of fiber include green peas, artichoke, baked sweet potato with the peel intact, spinach, broccoli, Brussels sprouts, cauliflower and many other vegetables.

Additional foods that are also excellent in providing fiber include pears, raspberries, stewed prunes, dried figs or dates (eaten in moderation due to high sugar content), pumpkin, almonds, apples with the skin intact, bananas (also eaten in moderation) and oranges. Organic psyllium seed husk (nonorganic psyllium is typically loaded with pesticides) is another excellent way to optimize your fiber intake.

The Academy of Nutrition and Dietetics says 25 grams of fiber per day is enough for women, with a 38-gram target for men. However, my recommendation for daily fiber intake is 25 to 50 grams per 1,000 calories consumed. The Academy was right about one thing, though, for anyone wanting to increase their fiber intake:

"When increasing fiber, be sure to do it gradually and with plenty of fluids. As dietary fiber travels through the digestive tract, [it] is similar to a new sponge; it needs water to plump up [and] pass smoothly. If you consume more than your usual intake of fiber but not enough fluid, you may experience nausea or constipation."4

The importance of and the differences between soluble and insoluble fibers began emerging when prebiotic soluble fibers were discovered about 15 years ago.5 Nutritionists and medical researchers found that certain soluble fibers such as inulin, FOS (fructooligosaccharide) and oligofructose, which turned out to be prebiotics, brought about remarkable changes in the bacterial makeup of the colon.

Fiber Can Prevent Leaky Gut and Lower Blood Pressure Levels

The benefits continue through the effect fiber has on preventing leaky gut, which may be reaching epidemic proportions. In fact, 80 percent of the American population is said to be wrestling with this often-misdiagnosed problem, known to cause anxiety, joint pain, chronic fatigue, brain fog, hives and depression, not to mention irritable bowel syndrome (IBS), food allergies and bloating. According to Food Integrity Now:

"The wall of the intestine is considered semi-permeable. This means it only allows certain things to enter the bloodstream and blocks other things from entering the bloodstream. For instance, specific molecules and nutrients are allowed to pass through but toxins and large undigested food particles are blocked.

When you have leaky gut, the pores in your small intestine widen and this allows undigested food particles and toxins, that would normally be blocked, to enter your bloodstream. These particles and toxins aren't recognized and the immune system goes into attack mode because they are not supposed to be in the blood. In essence, the immune system literally recognizes these undigested particles as dangerous."6

In addition, gut health and a clean colon are aspects of well-being that many don't connect with cardiovascular health, but fiber is one aspect of nutrition that fills this role in several ways. For instance, an inverse association has been found between fiber intake and heart attack, and research shows that those eating a high-fiber diet have a 40 percent lower risk of heart disease.7

Fiber and Digestive-Resistant Starch for Gut Health

Besides providing fiber, some foods go even further in terms of improving your gut health, one case in point being not-quite-ripe tropical fruits such as banana, papaya and mango. That's because they also contain digestive-resistant starch. While fiber is crucial for your diet, what makes it even healthier is its potential for fermentation. According to Today's Dietician:

"Other properties, such as viscosity and fermentability, may be more important. Naturally occurring resistant starches are a group of low-viscous fibers that are slowly fermented in the large intestine."8

In your large intestine, resistant starches feed healthy bacteria, essentially acting as prebiotics. They also bulk up your bowel movements for easier, timelier disposal without making you feel bloated or gassy. Best of all, they don't spike your blood sugar the way the completely ripened fruit would do, so they're also much more likely to improve insulin regulation.

Unripe papaya, banana and mango aren't the only foods with this ability; seeds and tapioca starch also have the same capability, but the tropical fruits also contain higher percentages in certain vitamins and minerals. In many ways, resistant starch could be considered a third type of fiber.9

How to Improve Your Microbiome and Immune System

Keeping your immune system healthy is crucial for preventing illness and disease. That said, the environment we live in makes optimal health a challenge at times, especially in choosing foods that will help, not harm, your gut health, which plays an integral role in your immune system health. In the same vein, while grains such as wheat and corn are typically placed on the "do" list as good for increasing your fiber, there are multiple reasons why they shouldn't be.

In addition to their tendency to promote insulin and leptin resistance, and the fact that they contain lectins, one of the most important is the way these and other grains are grown, both in regard to the soil and how they're chemically treated as they grow to discourage weeds and maximize yield. Processing makes such grains even worse for your health, and they're implicated in worsening disease rates in numerous ways.

How you prepare foods to maintain their fiber content includes eating the peelings of appropriate fruits and vegetables, but make sure they're organic to avoid potential pesticide overload. As for improving your microbiome, including fermented vegetables and other foods in your diet, such as cabbage, carrots, turnips, parsnips and beets, is important. Nearly any vegetable can be used, but root vegetables seem to be the hardiest, with added herbs for flavor.

Take a look at these healthy, delicious fiber-rich recipes to obtain gut health, strengthen your immune system and maintain a clean colon. To work toward getting plenty of both soluble and insoluble fiber so you can stay regular and healthy, be sure to eat a wide variety of vegetables, seeds and nuts daily.

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