Since the 1990s, medical researchers have discovered more and more benefits when dietary fiber is significantly increased in our diet. Fiber is a substance in plants. Dietary fiber, also called bulk or roughage, is the kind we eat. It is the edible portions of plant cell walls; hence, it is found only in plant foods like fruits and vegetables, whole grains, nuts and seeds, as well as beans and legumes.
Fiber is a carbohydrate and is usually listed under “Total Carbohydrates” on the “Nutrition Facts” label. Humans lack the digestive enzymes to breakdown fiber. Therefore, it is undigested and not absorbed into the bloodstream and it arrives at the colon pretty much intact. Fiber has zero calories. Instead of being used for energy, it is excreted from the body.
The current recommended daily intake for adults who are 50 years or younger is 25 grams/day for women and 38 grams/day for men. For adults over 50 years of age, the recommendation is 21 grams/day for women and 30 grams/day for men. Unfortunately, for many who eat a typical American diet, it can be a huge challenge to consume that much fiber everyday. Most people top out at an average of 15 grams/day, regardless of how many calories they eat.
Maybe if we understand more about the different types of fiber and how they can immensely contribute to better health and lower disease risks, there will be more incentives to increase the daily fiber intake. Fiber is an important part of a healthy, balanced diet. Apart from helping us stay regular, fiber has a long list of other health benefits. The following will distinguish the different types of fiber, their specific health advantages, and which foods contain these fiber.
Classifications Of Fiber
There are several ways to classify the different types of fiber. However, as their characteristics do overlap, experts have yet to agree on the best categorization. For decades, the most commonly used classification is soluble and insoluble fiber. These days, as researchers discover the benefits of fermented fiber, another classification – fermentable and non-fermentable fiber – is also used. However, do know that both soluble and insoluble fiber have some that are fermentable and some that are non-fermentable, though soluble fiber is more easily fermented.
Soluble and Insoluble Fiber
The major difference between soluble and insoluble fiber is that they have different properties when mixed with water, hence the designation between the two.
- Soluble fiber is soluble in water. When mixed with water, it forms a gel and swells.
- Insoluble fiber does not absorb or dissolve in water. It passes through the digestive system in close to its original form.
Both types of fiber serve their own purposes and have different health benefits. Most plant foods contain both soluble and insoluble fiber, just in different proportions. For instance, wheat is about 90% insoluble fiber. Oats are 50/50. Psyllium plant is mostly soluble fiber.
Fermentable and Non-Fermentable Fiber
Some fibers are readily fermented by bacteria that colonize the colon, others are not. Fermentable fiber is used by the colon’s friendly bacteria as a food source. Fermentation results in the formation of short-chain fatty acids (acetate, butyrate, and propionate) and gases. Epithelial cells that line the colon use butyrate as the main source of energy.
Researchers found that butyrate exerts a wide range of health benefits. It:
- Decreases inflammation and oxidative stress,
- Prevents colon cancer, irritable bowel syndrome (IBS), and Crohn’s disease,
- Strengthens the bowel wall,
- Improves the body’s ability to absorb essential nutrients such as calcium,
- Makes hormones that control appetite and anxiety.
Foods High in Soluble Fiber
Fruits: blueberries, apple, oranges
Grains: barley, oats
Legumes: beans, lentils, peas
Vegetables: Brussels sprouts, carrots
- Digestion and weight control. When soluble fiber dissolves in water and becomes gel-like, it helps prolong stomach emptying and slows down digestion, making you feel full longer and have less room for other not-so-healthy food cravings.
- Blood sugar regulation. Soluble fiber slows down the digestion rate of many nutrients, including carbohydrates, so it helps stabilize glucose levels and prevent after-meal blood sugar spikes.
- Cholesterol and heart health. Soluble fiber binds to cholesterol and bile acids (made by the liver and stored in the gall bladder for the digestion of fats) in the small intestine and promotes their excretion. Studies found that consuming more soluble fiber leads to a decrease in LDL (bad) cholesterol levels, hence, reducing the overall risk of cardiovascular disease.
- Healthy bowel movements. Soluble fiber soaks up water as it passes through your system, which helps bulk up the stool and guard against constipation.
- Colon health. Prebiotic fiber is a type of fermentable and soluble fiber that is used by the colon’s friendly bacteria (probiotics) as a food source. Prebiotics and probiotics work together to maintain the balance and diversity of intestinal bacteria, especially increasing the good bacteria like lactobacilli and bifidobacteria.
Foods High in Soluble Prebiotic Fiber
(In parenthesis is the name of the fiber)
Asparagus (inulin and oligofructose)
Banana (inulin and oligofructose)
Burdock root (inulin and oligofructose)
Chicory root (inulin and oligofructose)
Cocoa (flavanol compounds)
Dandelion greens (inulin and oligofructose)
Garlic (inulin and oligofructose)
Jerusalem artichoke or sunchoke (inulin and oligofructose)
Jicama root (inulin)
Konjac root or glucomannan fiber (see Note)
Leeks (inulin and oligofructose)
Onion (inulin and FOS)
Psyllium (mucilage) – used as a fiber supplement, always buy organic due to pesticides
Wheat bran (arabinoxylan oligosaccharides or AXOS)
Yacon root (FOS and inulin)
Konjac is native to Asia. Its fiber, known as glucomannan, has been used as both food and in traditional Chinese medicine for thousands of years. Today, glucomannan is used as a fiber supplement to promote colon health, lower cholesterol, and improve carbohydrate metabolism. It is also used in a food product called Shirataki noodles. These noodles are made of 97% water and 3% glucomannan, so the fiber content is very diluted. Though not a high source of prebiotic fiber, the noodles have zero carbohydrates, fats, protein, and calories. Thus, these noodles are suitable for diabetics and people who are looking to lose weight.
Fiber and Gas
Everyone has some intestinal gas and that is normal. The amount of flatus passed each day depends on your sex (men have more frequent flatus) and what is eaten. The normal number of flatus may range from 10-20 times a day.
If you are not used to eating a high amount of fermentable fiber, eating too much at a time can lead to excess intestinal gas, bloating, and mild cramping. So, increase gradually. When you consume vegetables with prebiotics or take a prebiotic fiber supplement, the flatus is often non-odoriferous. The foods that cause smelly flatus are usually the ones that contain high sulfur content, such as eggs and cruciferous vegetables.
If you experience severe gas and gut discomfort after gradually introducing more fermentable fiber, you may have small intestinal bacterial overgrowth (SIBO) or yeast overgrowth. In this case, you need to work with a healthcare professional to address your gut issues.
Foods High in Insoluble Fiber
Fruits: skins of fruit
Fruits (dried): dates, prunes
Grains: corn bran, oat bran, wheat bran, whole grains (e.g. whole wheat, brown rice)
Nuts and seeds
Vegetables: green beans, green leafy vegetables, root vegetable skins
- Weight management. Like soluble fiber, insoluble fiber can play a key role in controlling weight by starving off hunger pangs.
- Digestive health. Insoluble fiber helps to move bulk through the intestines. It lessens the amount of time food spends in the colon, hence, constipation and hemorrhoids are much less of a problem and bowel movements become more regular.
- Diverticulitis. This condition is characterized by inflammation and infection of pouches or folds that form in the colon walls. Development of diverticulitis is often associated with a low-fiber diet and becomes increasingly common after the age of 45. It exacerbates intestinal blockages and constipation. Eating more insoluble fiber can decrease the risk of having diverticulitis.
- Colon cancer. Insoluble fiber increases the rate at which waste is being removed from the body, therefore, it reduces the amount of time toxic substances stay inside the body. Insoluble fiber also helps to maintain an optimal pH (acid-alkaline) balance in the intestine, making it less likely for cancer cells to grow and prosper.
Resistant Starch is a type of fermentable insoluble fiber. It is a kind of starch that is not digested in the small intestine. Instead, it feeds the beneficial gut bacteria in the colon, just like the prebiotic soluble fiber with the same remarkable health benefits. When you eat resistant starch, it resists digestion and does not spike blood sugar or insulin.
Foods High in Resistant Starch
Cooked and cooled beans and legumes (properly soaked or sprouted), oatmeal, pasta, potato, rice, and yam (see Note 1)
Green (unripe) bananas, mangos, papayas, and plantains (see Note 2)
Hi-maize flour or Hi-maize resistant starch
Raw unmodified potato starch (not potato flour) – ideally organic or at least non-GMO
- By cooking and cooling these foods, the carb load is reduced by around 20-30% due to the formation of retrograde resistant starch. However, if you have trouble with blood sugar control or if you are looking to lose weight, you will still need to be very cautious with these foods as the majority of the absorbable carb is still present.
- The unripe version of these foods have very little digestible carbohydrates. You know you are eating the resistant starch when the fruit is crispy or crunchy, and sometimes a little chalky.
Dietary fiber is an essential component of a healthy diet. A high-fiber diet has many benefits: it normalizes bowel movements, maintains colon health, lowers cholesterol levels, regulates blood sugar balance, and helps in weight control.
Fiber is commonly classified as soluble, which dissolves in water, or insoluble, which does not dissolve in water. Most plant foods contain soluble as well as insoluble fiber, just in different proportions.
Both soluble and insoluble fiber can also be fermentable. Prebiotic fiber (from soluble fiber) and resistant starch (from insoluble fiber) ferment and feed the friendly bacteria in the colon. They have positive impacts on the diversity and number of beneficial intestinal bacteria in our gut. The more good bacteria we have, the harder it is for the bad bacteria to flourish. The by-products of fiber fermentation produces short-chain fatty acids like butyrate which has powerful anti-inflammatory effects that translate into protection against colon cancer, gastrointestinal disorders like constipation, diverticulitis, and IBS, and also obesity.
All dietary fiber is good for you. Ideally, you would want to get it from different sources of plant foods to reap their distinct benefits. Hopefully, this has given you more incentives to increase your daily intake of high fiber foods, like fruits and vegetables, whole grains, nuts and seeds, as well as beans and legumes. However, for individuals with blood sugar and weight issues, go easy on the carb-heavy fruits, whole grains, beans and legumes.
Carol Chuang is a Certified Nutrition Specialist and a Metabolic Typing Advisor. She has a Masters degree in Nutrition and is the founder of CC Health Counseling, LLC. Her passion in life is to stay healthy and to help others become healthy. She believes that a key ingredient to optimal health is to eat a diet that is right for one’s specific body type. Eating organic or eating healthy is not enough to guarantee good health. The truth is that there is no one diet that is right for everyone. Our metabolisms are different, so should our diets. Carol specializes in Metabolic Typing,