Fiber Fundamentals

Make no mistake: Dietary fiber is good for your health. Yet most Americans only get about half as much fiber as they should each day—and this nutritional deficit has been the case for decades. Even with all its overt health benefits, fiber is consistently overlooked. In fact, in our 2022 Food and Health Survey participants said that when grocery shopping, the only information on the Nutrition Facts label that they looked at less often than fiber was servings per container and sugar alcohols. If you need inspiration for increasing your fiber intake, read on for a refresher on the fundamentals of fiber and its unique role in human health.

Fiber Facts

Carbohydrate, fat, and protein—collectively called macronutrients—are the over-arching nutrient groups that provide calories in the foods and beverages we consume. Dietary fiber is a non-digestible form of carbohydrate—but that doesn’t mean we shouldn’t eat it! In actuality, our inability to completely digest fiber is part of what makes it so good for us.

Fiber has numerous health benefits confirmed through decades of scientific research. Fiber may be best known for its ability to relieve constipation, but it does more than keep you “regular.” Fiber also helps us feel full for longer periods of time; maintain a healthy gut microbiome; stabilize blood sugar; and lower LDL (“bad”) cholesterol, blood pressure, and inflammation, which collectively reduces our risk for developing cardiovascular disease and heart disease.

Fiber is naturally found in plants and plant-based foods, including fruits, legumes (like beans, peanuts, peas, pulses, and soybeans), nuts, seeds, vegetables, and whole grains. Most of what we eat and drink is thoroughly broken down during digestion. Fiber, however, avoids the full breakdown process. Because we lack the enzymes needed to completely digest fiber, it remains mostly intact throughout its journey through the stomach and small intestine, until it reaches the large intestine, where gut bacteria can ferment it. Fermentation, in this instance, is a method by which the bacteria living in our gut can partially digest fiber. Fermentability is the degree to which our gut bacteria can digest fiber.

You may have heard about benefits associated with some types of fiber such as soluble and insoluble fiber. However, the properties of fiber that provide health benefits go beyond solubility. In 2001 the National Academies of Sciences, Engineering and Medicine recommended advancing our understanding of fiber by emphasizing the health-promoting characteristics of fiber’s viscosity and fermentability rather than solubility.

Viscous fiber thickens in the stomach, becoming a gel-like substance which can later be fermented by bacteria in the colon. Viscous fiber—for example beta-glucan, pectin, and psyllium—helps to reduce digestion speed, stabilize blood glucose, and lower cholesterol. Consuming viscous fiber can also help delay the emptying of our stomach into the small intestine which extends the length of time that we feel full, potentially reducing total calorie intake.

Fiber that is partially fermented—like wheat bran—and fiber that is not fermented—like psyllium—in the large intestine improves digestive health by providing bulk to stools, which helps to keep things moving regularly through the gastrointestinal tract. Fermentable fibers—including oligosaccharides, beta-glucans, gums, some hemicelluloses and some resistant starches—are fermented by gut bacteria to make the short chain fatty acids acetate, propionate and butyrate which lowers the pH in the colon thereby increasing the bioavailability of certain minerals and inhibiting the growth of certain pathogenic bacteria.

Here is a detailed flowchart grouping types of fiber by health-promoting characteristics.

Fiber: Fitting It In

A healthy eating pattern includes adequate amounts of fiber from diverse sources. But despite the well-known health benefits of fiber, less than 10% of women and 3% of men in the U.S. get the recommended amount, mainly due to chronically low fruit, vegetable, and whole-grain consumption. Fortunately, most high-fiber foods contain a mix of fiber types, so it’s relatively easy to get diverse varieties in the same meal or snack once you start focusing on increasing your overall fiber intake. Foods that have especially high fiber-to-calorie ratios include artichokes, asparagus, beans, bell peppers, berries, bran cereals and oats, Brussel sprouts, bulgur, cabbage, carrots, chestnuts, guava, jicama, kale, lentils, passionfruit, pears, peas, seeds (chia, flax and pumpkin), spinach, starfruit, and turnips.

Regularly consuming foods with high fiber-to-calorie ratios can help to increase fiber intake without overdoing it on calories. Making more nutrient dense food choices aligns with advice from U.S. nutrition experts to get 14 grams of fiber for every 1,000 calories we consume. Because this guidance is calorie-dependent, the total daily fiber goal will not be the same for everyone—there will be differences between men and women as well as by age. For example, adult women are recommended to get at least 22 to 28 grams of fiber each day, while adult men should get at least 28 to 34 grams.

If you’re looking to add more fiber to your diet, consider this handful of helpful tips:

  • Eat more types and amounts of whole plant foods.
  • Add fruit to your cereal, oatmeal or yogurt; seeds to your salad; or beans to your evening routine.
  • Reach for whole fruits and vegetables before juice.
  • Drink plenty of water, especially while increasing your fiber intake.
  • Prioritize whole grains over refined grains by aiming for at least half of all the grains you eat to be whole grains.
  • Slowly incorporate diverse sources of fiber that you enjoy.
  • Check Nutrition Facts labels for fiber content when grocery shopping.
  • If you are having difficulty consuming adequate amounts of fiber from foods, consider a fiber supplement such as psyllium powder.

Synthetic and Isolated Fibers: The FDA’s Take

In addition to getting fiber from whole-plant foods, we also consume fiber that is added to packaged foods and beverages during manufacturing. Added fibers can be isolated or synthetic. Isolated fibers, such as beta-glucan found in barley and oats, are extracted directly from plant sources. Synthetic fibers, such as polydextrose made from glucose or sorbitol, are modified or manufactured from ingredients of plant origin.

According to the U.S. Food and Drug Administration (FDA), only “certain naturally occurring fibers that are ‘intrinsic and intact’ in plants, and added isolated or synthetic non-digestible soluble and insoluble carbohydrates that FDA has determined have beneficial physiological effects to human health” can be counted in the grams of fiber displayed on a Nutrition Facts label. The FDA cites numerous examples of fiber’s health benefits that guide the food labeling of isolated and synthetic fibers. The labeling criteria differ between naturally occurring fibers that are intrinsic and intact in plants and fibers that are added in isolated and/or synthetic form. These distinctions are intended to ensure that the anticipated health benefits of a fiber still exist and can be clearly demonstrated after its isolation or synthesis.

The FDA has approved seven isolated and synthetic fibers for inclusion in the current FDA definition of dietary fiber, which was established in 2016. These fibers include beta-glucan soluble fiber, cellulose, guar gum, hydroxypropylmethylcellulose, locust bean gum, pectin, and psyllium husk. Further, the FDA has scientifically reviewed 26 types of non-digestible carbohydrates and proposed that ten of them (plus the category of mixed plant cell wall fibers) be included in the pending update to their definition of dietary fiber.

The Final Word on Fiber

Fiber is one of the most important nutritional features that food has to offer. But most people don’t get enough fiber and would benefit from consuming more. The diverse health benefits of fiber are well-established but will vary depending on how much, how often, and what type you consume. If you are looking to increase your fiber intake, slowly add diverse sources of fiber and space them throughout the day, with the goal of making these foods a regular part of your diet.

This article includes contributions from Alyssa Pike, RD and Gaby McPherson MS, RDN, LDN from Nutrition on Demand.