All About Fiber: Nutrition, Food Sources, and More

Eat more fiber. We’ve all heard this advice, so we assume it must be good for us. The problem is that fiber and flavor might seem like opposites — and for many of us, flavor is the typical driver of food choice. But the reality is that fiber can have flavor, along with medicinal effects to potentially help reduce and prevent common diseases. In this article, let’s take a look at exactly what fiber is and how it functions in the body.

What Is Fiber Exactly, and Which Foods Contain This Carb?

Fiber is simply a type of carbohydrate found naturally in plant-based foods that is not completely digested by humans.

Plant-based foods that are rich in fiber — such as fruits, vegetables, whole grains, beans and legumes, and nuts and seeds, according to the U.S. Department of Agriculture’s MyPlate guidelines — also contain vitamins, minerals, and other powerful nutrients that the body can use for optimal health.

Although fiber cannot be completely digested, it is being moved down the digestive tract as nutrients are being digested, and can do some great things that positively impact our health.

The problem is that Americans are getting less than half of the daily intake recommendations of 14 grams (g) for every 1,000 calories of food. (1,2)

A simpler recommendation level for most adults is between 25 and 38 g per day (1). In fact, fiber is listed as a “nutrient of concern” due to the low overall intake and known health benefits. (3)

There are two types of fiber: soluble and insoluble. While both are important, the two function differently in the body. Here’s how:

Soluble Fiber

Soluble fiber is a type of fiber that attracts water and forms a gel. This gel causes a slowing of the digestion process, which can be beneficial for weight loss. Foods high in soluble fiber include oats, legumes, edible plant skins, and nuts. (4)

Insoluble Fiber 

Insoluble fiber is the type of fiber that, you guessed it, repels water. You can find insoluble fiber in foods such as veggies, fruits, nuts and seeds, wheat bran, and whole-grain foods like whole-wheat pasta and brown rice. Its primary benefit is to provide bulk to stool and aid in the movement through the digestive tract. (4)

A healthy, balanced diet contains a combination of soluble and insoluble fiber.

In short, fiber may actually help you live longer. (6) Studies suggest folks who eat a higher intake of fiber tend to have lower rates of heart disease, obesity, type 2 diabetes, stroke, high blood pressure (hypertension), and digestive diseases. (7)

You may help improve or prevent health conditions such as prediabetes, diabetes, high cholesterol, obesity, and various digestion problems, like constipation, colon cancer, and diverticulitis, by simply increasing fiber in your diet. (8)

Weight Loss

Research suggests nutrients like fiber can play a major role in body weight. (10) Dietary fiber intake, independently of macronutrient and calorie intake, promotes weight loss and dietary adherence in adults with overweight or obesity consuming a calorie-restricted diet, according to a study in The Journal of Nutrition.

Fiber expands and bulks food in your GI tract, slowing digestion. This can increase satisfaction of your food and helps stabilize blood sugar levels. (4) Foods high in fiber also tend to be lower in energy density, meaning they’ll help you feel fuller without consuming excessive calories. (4) This concept is at the core of why a higher fiber diet is associated with a lower rate of obesity. (11)

Digestive Disorders

Fiber is like nature’s scrub brush, keeping your body’s pipes clear and reducing carcinogenic activity.

One benefit of getting enough fiber in your diet is reducing the risk of diverticulitis, a condition in which pouches formed in the colon become infected. Fiber helps keep food clear from the pouches and moving through the digestive tract. Aim to take in 25 to 40 g of fiber per day to reduce your risk of diverticulitis. (12)

There’s an anti-cancer benefit to fiber, too: Both soluble and insoluble fiber can also play a role in warding off colon cancer. (13)

Cholesterol and Blood Pressure Reduction

Fiber’s wondrous effect on the body is a great example of medical nutrition therapy (MNT), a technique registered dietitian nutritionists (RDN) use on their patients to reduce the need for medication while improving health outcomes.

Here’s how it works: Your body uses bile salts, which are excreted by the gallbladder to break apart the fat content in food. Bile salts are made of cholesterol.

When you eat food with fiber, the fiber binds to the bile salts, preventing them from being recirculated for the next time you eat. As a result, your body must produce more bile salts by taking cholesterol from the liver. This is how soluble fiber reduces blood cholesterol. (14)

Fiber has a preventative role on blood pressure, too, but the reason is more associated with nutrients such as potassium, calcium, and magnesium contained in foods high in fiber. (11)

If you are not convinced to increase your fiber intake yet, know there’s also data emerging on fiber’s ability to impact the immune system, mood, and memory by the promotion of healthy gut bacteria. (15)

A recent study, for example, suggests that increased consumption of fiber can improve the overall health of the gut biome, which in turn boosts the availability of short-chain fatty acids — compounds that are believed to be instrumental in maintaining colon health, weight maintenance, and the prevention of metabolic diseases like diabetes.

The Food and Drug Administration (FDA) has established a daily value for foods and supplements with food labels. The daily value (DV) for fiber is 28 g. (4)

Note that natural foods containing fiber typically have a combination of soluble and insoluble fiber with the majority coming from soluble fiber. To lower your blood cholesterol by 3 to 5 percent, aim to take in at least 5 to 10 g of soluble fiber per day. (16)

Here are some food sources of fiber, along with how much of the DV of fiber they offer: (17)

Fruit

• Passion fruit (1 cup): 25g, 100 percent
• Breadfruit (1 cup): 11g, 44 percent
• Raspberries (1 cup): 8g, 32 percent
• Blackberries (1 cup): 8g, 32 percent
• Boysenberries and gooseberries (1 cup): 7g, 28 percent
• Pear (1 medium): 6g, 24 percent
• Prunes (5 pieces): 3g, 12 percent

Vegetables

• Artichoke (1 large): 9g, 36 percent
• Lima beans (1 cup): 9g, 36 percent
• Green peas (1 cup): 8g, 32 percent
• Lentils (½ cup): 8g, 32 percent
• Kidney beans (½ cup): 6g, 24 percent
• Sweet potato (½ cup, mashed): 4g, 16 percent

Nuts and Seeds

• Chia seeds (1 ounce): 10g, 40 percent
• Flaxseeds (1 oz): 6g, 40 percent
• Pumpkin seeds (1oz): 5g, 20 percent
• Almonds (1 oz): 4g, 16 percent

Grains

• Raisin Bran (1 cup): 7g, 28 percent
• Shredded wheat (2 biscuits): 6g, 24 percent
• Oat bran (1 cup): 6g, 24 percent
• Brown rice (1 cup): 4g, 16 percent

Just as with everything else in life, getting too much fiber can be harmful to your health. The U.S. Dietary Guidelines hasn’t established an upper limit on fiber intake, but it’s well known that eating too much fiber can cause gas, bloating, and diarrhea. (18) A sudden increase in fiber, inadequate fluid intake, and inactivity, along with a high-fiber diet, may increase the likeliness of these symptoms.

Doctors often prescribe fiber supplements for the treatment of irritable bowel syndrome or constipation. These supplements are considered functional fibers that are isolated from plant sources:

• Psyllium (Metamucil) is a type of soluble fiber supplement you can use to bulk stool and encourage regular bowel movements.
• Dextrin is a type of soluble, prebiotic fiber found in products such as Benefiber that promotes good bacteria for overall digestive health.
• Methylcellulose (Citracel) and polycarbophil (Fibercon) are other fiber supplement options to help keep you regular.

Check with your doctor before you add any supplement to your diet.

Whole-wheat food products, like brown rice, whole-grain bread, and whole-grain pasta, are a quick and easy source of fiber in many parts of the world. But these foods contain the binding protein gluten, and if you have the autoimmune condition celiac disease, or a wheat allergy, you need to avoid these types of foods to prevent serious health issues. (20,21)

Even if you don’t have one of these ailments, you may find that eating wheat causes gas, bloating, and abdominal pain. (22) If you and your doctor determine giving up whole wheat is best for you, you’ll need to add other whole grains to your diet in its place, as avoiding wheat can lead to a fiber deficiency.

If you have a wheat allergy, a gluten intolerance, or celiac disease, consider eating high-fiber gluten-free grains, like millet, amaranth, quinoa, and buckwheat. Fruits and vegetables that have skins are also good gluten-free sources of fiber, as are legumes and gluten-free oats. (21)

Although dextrins are derived from wheat, dextrin supplements meet the FDA’s guidelines for gluten-free of less than 20 parts per million. (23) People with celiac disease should always talk to their doctor before using any supplement.

Bottom line: You’re likely not getting enough fiber, so consider eating more. The best way to get fiber is through natural sources, such as fruits, vegetables, whole grains, nuts, seeds, and legumes, because these sources also include important vitamins, minerals, and phytonutrients your body needs for optimal health.

Ask your doctor or RDN if a fiber supplement would be right for you if you are trying to treat or prevent a health condition. Don’t have an RDN? You can find one at EatRight.org.