It used to be when we thought about fiber, one word likely came to mind: “roughage.” These days, the benefits of what's essentially the nondigestible components of plant foods are seen as much bigger. Fiber's even been called “the new protein” for all the attention it's attracting.
Among other things, fiber-rich diets have been shown to reduce the risk for obesity, type 2 diabetes, cancer and, most significantly, cardiovascular disease, says Joanne Slavin, a registered dietitian and professor of food science and nutrition at the University of Minnesota.
The growing awareness of the critical role our gut microorganisms (microbiota) play in both physical and mental health has also brought heightened interest to the benefits of consuming fiber-rich foods.
Not all fiber is the same
We're also learning more about the different types and properties of dietary fiber, which has for some time been divided into soluble and insoluble types. Soluble fiber, which dissolves in water, can help lower glucose levels and blood cholesterol. Foods with soluble fiber include oatmeal, nuts, beans, lentils, apples and blueberries.
Insoluble fiber helps food move through your digestive system, preventing constipation. Foods with this type of fiber include wheat, whole wheat bread, whole grain couscous, brown rice, legumes, carrots, cucumbers and tomatoes.
More recently recognized, however, are factors such as viscosity and fermentability. For example, the viscous and soluble fibers found in oats, barley and psyllium form a gel in the intestinal tract, which slows digestion and binds with cholesterol and fats, thereby helping to control blood sugar and lower blood lipids.
"Fiber really is fuel for the gut,” says Slavin. Fermentable fibers, including fruit pectin, beta-glucans in barley and oats, and oligosaccharides in beans, are metabolized by gut bacteria to produce beneficial short chain fatty acids (SCFAs). SCFAs provide energy to cells in the colon, help improve mineral absorption, and lower the pH in the colon, which inhibits growth of disease-causing bacteria.
Many fermentable fibers are also considered “prebiotic” because they promote the growth and activity of beneficial gut bacteria.
Inulin, for example, is a prebiotic fiber found naturally in onions, garlic, leeks, wheat and oats. It's also extracted from chicory root, a plant that's part of the dandelion family. If there's a celebrity fiber type at the moment, it's this one, which has recently been added to things like beverages or power bars that wouldn't normally contain fiber.
While studies have linked inulin to weight loss and the lowering of blood sugar — among other things, it seems to slow appetite signals to the brain as well as slow stomach emptying, leaving you feeling fuller, longer — taking in too much inulin too fast, especially if you're not used consuming it, can lead to digestive upset.
Where to find it
- Eat whole fruits instead of drinking fruit juices.
- Replace white rice, bread and pasta with brown rice and whole grain products.
- For breakfast, choose cereals that have a whole grain as their first ingredient, or make your own bran muffins and add in more fiber (and taste) with chopped pecans, oat flakes, chopped prunes, apples and hemp seeds.
- Snack on raw vegetables instead of chips, crackers or chocolate bars. Or at least force yourself to start with the vegetables.
- Substitute beans or legumes for meat two to three times per week in things like burritos, chili and soups.
- Find a non-boring salad you enjoy eating and have it for lunch or dinner two or three times a week. And don't forget about quick and easy stir-fries — with options like brown rice, celery, cabbage, carrots, broccoli, cauliflower, snow peas, sweet red peppers, pineapple and sesame seeds — as dinner options that deliver lots of fiber.
How much do we need?
For folks over 50, the Institute of Medicine recommends 30 grams of dietary fiber per day for men, 21 grams for women. These numbers are based on the cardio-protective amount of 14 grams of fiber per 1,000 calories consumed. Unfortunately, most Americans get half this amount.
Teresa Martin, a registered dietitian based in Bend, Ore., who presented on fiber, gut microbes and disease prevention at the 2018 Academy of Nutrition and Dietetics annual meeting, recommends 35 to 50 grams of fiber per day to see therapeutic effects and to promote a healthy gut microbiota.
Even though fiber is naturally present in fruits, vegetables, beans, peas, whole grains, nuts and seeds, 95 percent of Americans come up short for a variety of reasons.
For one, highly processed low-fiber foods are ubiquitous in our environment. Two, the recent popularity of gluten-free, wheat-free, keto and paleo diets all limit grains; yet according to Slavin, whole grains are the No. 1 source of fiber in the American diet. Last, there's confusion about which foods are highest in fiber.
For example, just because a label says “whole grain” does not mean the food is high in fiber. Check the Nutrition Facts panel on packaged food labels to choose those providing at least 3 grams of fiber per serving.
And what about supplements? The Academy of Nutrition and Dietetics recommends getting your fiber foremost from foods such as fruits and veggies. This is because whole foods have benefits beyond fiber; they deliver vitamins, minerals, antioxidants and other beneficial plant compounds that often work synergistically. Fiber supplements can be helpful in bumping up fiber intake, but they deliver only one type of fiber.
Increasing your fiber intake
Martin takes a novel approach for getting more fiber. Specifically, she suggests trying to consume 30 different plant species each day. For example, instead of having a plain lettuce salad, add diversity to your bowl with any number of extra ingredients: radishes, spinach, onions, carrots, tomatoes, cabbage, cucumbers, celery, arugula, dandelion leaves, avocado, beans, sunflower seeds, nuts and fresh or dried fruit. Be adventurous!
For a list of fiber-rich food sources, check out the U.S. Dietary Guidelines, which list “high fiber bran ready-to-eat cereals” as the richest source of fiber, closely followed by cooked navy beans (though other options abound).
Melinda Hemmelgarn is a registered dietitian and host of Food Sleuth Radio.