The Facts About Fiber

Fiber is the foundation of successful aging, yet a mere 5% of the U.S. population includes the suggested amount of fiber daily, and most adults eat just half of what they require. According to a 10-year study of 1,600 people ages 49 and older, those who ate the most fiber had the fewest chronic diseases, such as heart disease, type 2 diabetes and cancer, less disability and depression, and better brain function that those who consumed the least.

Fiber is a type of carbohydrate that's typically classified as soluble or insoluble depending on its ability to dissolve in water. Plant foods contain both kinds of fiber, although some supply more of one type. For example, legumes, lentils, berries and oats have higher levels of soluble fiber, while whole grains, including bread, cereal and pasta, tend to supply more of the insoluble kind. 

"No matter what the type, the benefits of fiber originate in the gut and affect your entire body," says Hillary Wright, RDN, author of "The Prediabetes Diet Plan." Insoluble fiber adds bulk to stools and helps reduce intestinal disorders that tend to increase with age, including constipation, hemorrhoids and diverticulosis.

"In stimulating the movement of food through the intestinal tract, insoluble fiber also limits your exposure to carcinogens and reduces the potential for colorectal cancer and cancers in other parts of the body, too," Wright says.

We don't digest either type of fiber, but our gut microbes can ferment the soluble kind. Microbes extract energy and nutrients from soluble fiber to produce vitamin K, several B vitamins and neurotransmitters.

Gut bacteria also produce short chain fatty acids, which help reduce the risk for infection, boost the absorption of calcium and magnesium, and regulate the immune system. Short chain fatty acids lower the likelihood of colorectal cancer by helping to maintain the integrity of the gut barrier which also hedges against inflammation and infection in the gut and rest of the body.  

Soluble fiber is key to eating satisfaction and better blood glucose control. Fiber is calorie-free, and if you have prediabetes or diabetes, it's important to note that adding fiber to your diet won't raise blood glucose levels and will likely lower them.

"Soluble fiber forms a gel with water that slows digestion to help you feel fuller longer and slow the rise of glucose in the blood after eating to help prevent and manage type 2 diabetes," Wright says.

Adding soluble fiber to your eating plan can result in weight loss independent of calorie restriction. In a 12-week study of adults with obesity, people were asked only to aim for 35 grams of fiber daily and consume adequate protein without counting calories. Researchers found that the participants consumed an average of 265 fewer calories daily. In another study, increased intake of soluble fiber slowed belly fat gain over a 5-year period.

A higher-fiber intake also benefits the brain, possibly by reducing the risk factors associated with stroke and dementia, including high blood pressure and elevated levels of glucose and cholesterol in the blood. 

In one study of more than 3,700 midlife men and women, researchers found that people consuming the most fiber, especially soluble fiber, had the lowest risk for dementia over a 20-year period. In addition to its other health benefits, soluble fiber binds with cholesterol in the small intestine and keeps it out of circulation.

Plant foods supply more than fiber for successful aging. "Food can provide benefits such as vitamins, minerals and phytochemicals called polyphenols that help foster the growth of health-promoting gut microbes," says Kate Scarlata, RDN, co-author of "Mind Your Gut: The Science-based, Whole-Body Guide to Living Well with IBS."

Adding more plants to your eating plan instantly improves the quality of your diet. Simply swapping just 1/4 cup of cooked pulses (lentils, chickpeas, beans) for one ounce of protein foods, such as meat, every day increases fiber and decreases dietary cholesterol, each by more than 10%.

Scarlata prefers plant foods for fiber, but if you consistently struggle to meet your fiber needs, a fiber supplement such as psyllium husk can help fill the gap. "Psyllium husk is a soluble fiber, but it has low fermentability," Scarlata says. "While it's not a key food for gut microbes, it can help soften stool for easier passage and also can solidify diarrheal stools with its gel-forming ability."

However, consider that fiber supplements don't contain all the beneficial compounds found in plants. In addition, high-fiber diets are more effective overall than fiber supplements when it comes to disease prevention.

Suggested daily fiber intakes are based on calories consumed – at least 14 grams of fiber for every 1,000 calories. That amounts to at least 25 grams of fiber on an 1,800-calorie eating plan and a minimum of 28 grams of fiber daily on a 2,000-calorie eating plan.

Scarlata suggests introducing fiber slowly to allow your gut to adjust. "I typically recommend increasing fiber by about five grams per week along with adequate water to help move it through your body," she says. More fiber without adequate fluid, including milk, coffee and tea, can be constipating.

• Add canned drained beans or cooked lentils to pasta dishes, soups, stews and salads.

• Top whole grain cereal with berries.

• Munch on roasted edamame or popcorn instead of pretzels or snack chips.

• Stir chia seeds into cooked oatmeal and sprinkle on salads.

• Substitute 3/4 cup 100% whole wheat flour for each cup of all-purpose flour in recipes for pancakes, muffins, and quick breads.

• Enjoy a variety of whole grains, such as quinoa, as side dishes.  

Elizabeth M. Ward is a registered dietitian nutritionist (RDN), writer, and award-winning nutrition communicator. Her work has appeared in WebMD, Men’s Health, and EatThis.com.

Ward is the author or co-author of eight nutrition, food, and health books. Her most recent book is The Menopause Diet Plan, A Natural Guide to Hormones, Health, and Happiness (co-author). Ward lives in the Boston, MA, area. Read More