What Is Fiber? A Complete Scientific Guide

"Eat more fiber." We’ve all heard this advice, so we assume fiber must be good for us. The problem is that fiber and flavor may 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 that 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.

Fiber is a macronutrient and a type of carbohydrate found naturally in plant-based foods that is not digestible in humans.

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

Although fiber cannot be digested, it moves down the digestive tract with nutrients as they're digested, and does some great things to positively impact our health along the way.

But fewer than 1 in 10 American adults meet their daily fiber recommendation, according to the American Society for Nutrition.

The appropriate level for most adults is between 22 and 34 grams (g) of fiber per day, depending on age and sex, as the 2020–2025 Dietary Guidelines for Americans state.

Fiber is a key nutrient that plays various roles in our health. Here are some examples.

May Improve Digestion

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

One benefit of sufficient fiber in your diet is a reduced risk of diverticulitis, a condition in which pouches in the colon become infected, per research published in the European Journal of Nutrition in 2020. Fiber keeps food clear of the pouches and moving through the digestive tract. If you’re at risk for diverticulitis, this study suggests aiming for 30 g of fiber per day to prevent the condition.

May Help With Healthy Weight Maintenance

Research suggests that fiber can play a major role in body weight. One study published in the October 2019 Journal of Nutrition found that, among overweight or obese adults, those who ate more fiber while restricting calories had greater weight loss success, and fiber helped them stick to their diet. Other research, in the March 2020 Journal of Chiropractic Medicine, found that fiber supplementation may also support efforts to lose weight. Fiber expands and bulks food in your GI tract, slowing digestion. This can increase your level of satisfaction and stabilize your blood sugar. Foods high in fiber also tend to be lower in energy density, meaning they’ll help you feel fuller without excessive calories, according to the U.S. Food and Drug Administration (FDA). This concept is at the core of why a higher-fiber diet is associated with a lower rate of obesity, as the Academy of Nutrition and Dietetics says in one paper.

May Lower High Cholesterol and Blood Pressure

Fiber’s wondrous effect on the body is a great example of medical nutrition therapy (MNT), a technique registered dietitian-nutritionists (RDNs) use to improve health outcomes when they counsel patients about healthy food choices.

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, per research.

When you eat food with fiber, the fiber binds to the bile salts, preventing them from being recirculated 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. A review in a 2019 issue of Nutrients noted that increased fiber intake complements a statin regimen (common cholesterol-lowering drugs), helping to lower total and “bad” LDL cholesterol levels. Potentially, this interaction may decrease the dose of statin needed.

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

May Help Prevent or Manage Certain Conditions

As the Mayo Clinic points out, you may improve or prevent health conditions such as prediabetes, diabetes, high cholesterol, obesity, and various digestion-related issues, like constipation and colon cancer, by simply increasing fiber in your diet, research in the October 2020 Nutrients suggests. More specifically, fiber may play a role in warding off colorectal cancer, according to one study.

May Increase Your Life Span

Fiber may actually help you live longer, per a study in the May 2020 American Journal of Clinical Nutrition. Studies note that people who eat a higher intake of fiber tend to have lower rates of heart disease, metabolic disease, mood disorders, and risk of colorectal cancer.

May Help With Other Bodily Functions

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

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, according to the FDA.

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, per the USDA.

According to the Dietary Guidelines for Americans 2020–2025, people should generally consume 14 g of fiber per 1,000 calories eaten. Here’s how the DGA breaks that down per group.

• Infants, ages 6 to 11 months: n/a
• Toddlers, ages 12 to 23 months: 19 g
• Children, ages 2–3 years: 14 g
• Children, ages 4–8: 17 g (female), 20 g (male)
• Children, ages 9–13: 22 g (female), 25 g (male)
• Adolescents, ages 14–18: 25 g (female), 31 g (male)
• Adults, ages 19–30: 28 g (female), 34 g (male)
• Adults, ages 31–50: 25 g (female), 31 g (male)
• Adults, ages 51+: 22 g (female), 28 g (male)
• Pregnant or nursing people: 25–34 g, depending on age and trimester

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 5 to 11 points, aim to take in at least 5 to 10 g of soluble fiber per day, per the National Lipid Association.

Here are some food sources of fiber, along with how much of the DV of fiber they offer, according to the 2020–2025 Dietary Guidelines for Americans.

Fruit

• Passion fruit (? cup): 3 g, 12 percent
• Raspberries (½ cup): 4 g, 16 percent
• Blackberries (½ cup): 4 g, 16 percent
• Kiwifruit (½ cup): 3 g, 12 percent
• Boysenberries and gooseberries (½ cup): 3.5 g, 14 percent
• Pear (1 medium): 6 g, 24 percent
• Prunes (? cup): 2 g, 8 percent

Vegetables

• Artichoke (½ cup): 5 g, 20 percent
• Lima beans (½ cup): 5 g, 20 percent
• Green peas (½ cup): 4 g, 16 percent
• Lentils (¼ cup): 4 g, 16 percent
• Kidney beans (¼ cup): 3 g, 12 percent
• Sweet potato (½ cup): 3 g, 12 percent

Nuts and Seeds

• Chia seeds (1 teaspoon [tsp]): 1 g, 4 percent
• Flaxseeds (1 tsp): 1 g, 4 percent
• Pumpkin seeds (½ ounce [oz]): 3 g, 12 percent
• Almonds (½ oz): 2 g, 8 percent

Grains

• Bran flakes cereal (¼ cup): 2g, 8 percent
• Shredded wheat (½ cup): 3g, 12 percent
• Popcorn (1 cup): 2g, 8 percent
• Whole-wheat crackers (½ oz): 2g, 8 percent

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

• Metamucil (psyllium) 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.
• Citracel (methylcellulose) and Fibercon (polycarbophil) are other fiber supplement options to keep you regular.

You can hit your fiber goal by incorporating fiber-rich foods into your meals and snacks. Here’s how.

Make Your Grains Whole

When you’re eating a carb, think, How can I make this higher in fiber? One sure bet: Swap out refined white versions for whole grains, suggests the Centers for Disease Control and Prevention (CDC). Choose whole-grain bread over white, brown rice instead of white rice, and whole-wheat pasta over traditional white pasta.

Make-Over Your Snacks

Midday snacks are a great opportunity to sneak in more fiber, such as with sliced, raw veggies, recommends the CDC. Buy them precut (or slice some over the weekend) to have them ready to grab when you need to munch; bring them out while you make dinner to get a few slices in then, too. While you’re at it, fold these sliced veggies into wraps and sandwiches, too, for extra crunch.

Rely on Beans

Beans are an excellent source of fiber, and they can be added to a variety of dishes, such as atop a salad and in soups and stews, suggests the Academy of Nutrition and Dietetics. Grab a few of those veggie slices and dip them into bean or hummus dip for a double-dose of fiber.

Sprinkle It On

Flaxseeds and chia seeds are foods that double as fiber supplements — and they’re easy to add to oatmeal, in smoothies, and on salads, toast, yogurt, or whatever else you can dream up. One tablespoon of ground flax offers 2 g of fiber, per the USDA. An ounce of chia seeds has a whopping 10 g of fiber.

Opt for Fruit as Dessert

Fruit is nature’s candy — and it happens to be packed with fiber. As the CDC recommends, you can fit in extra fiber (as well as vitamins) when you pair a whole fruit with your meal or eat it as dessert. Try a bowl of berries, a ripe, juicy pear, or grab a spoon and dig into a kiwi.

One of the main consequences of skimping on fiber is constipation, suggests research. In fact, it’s such a big public health problem, scientists say, that one report found if American adults upped their intake to 25 g of fiber per day, it could save $12.7 billion dollars in healthcare costs related to constipation.

As with everything else in life, too much fiber can be harmful to your health. The U.S. Dietary Guidelines don't specify an upper limit on fiber intake, but it’s well known that too much fiber causes gas, bloating, and diarrhea, according to the American Academy of Family Physicians.

A sudden increase in fiber, inadequate fluid intake, and inactivity may also increase the likelihood of these symptoms.

When you consume more than 50 g of fiber per day, you may face a risk of mineral binding, which essentially means your body excretes minerals instead of absorbing them. Some of the minerals at risk of binding with excess fiber intake include calcium and phosphorus, per research in Diabetes Care. This doesn’t mean you should limit fiber; rather, it brings home the importance of ensuring you’re consuming enough of the aforementioned minerals.

Bottom line: You’re likely not getting enough fiber, so consider eating more. The best way to get it 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.