F iber gets a lot of well-deserved credit for keeping the digestive system in good working order—but it does plenty more. In fact, it’s a major player in so many of your body’s systems that getting enough can actually help keep you youthful. For example, older people who ate fiber-rich diets were 80 percent more likely to live longer and stay healthier than those who didn’t, according to a recent study in the Journals of Gerontology.

The trouble is, few Americans consume the amount they should. For people age 51 and older, government guidelines recommend at least 28 grams per day for men and 22 grams for women. But the Department of Agriculture says adults in this age group average just about 16 grams per day.

What Is Fiber Anyway?

Fiber is a carbohydrate found in plant foods: beans, fruit, grains, nuts, and vegetables. Technically, it isn’t a nutrient because it isn’t broken down and absorbed. But that’s what makes it so beneficial.

There are several types of fiber, but they all fall into two broad categories: soluble and insoluble.

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Soluble fiber is soft and dissolves in water, forming a gel-like substance. It bulks up your stool, making it easier to pass. Sources include beans, oats, sweet potatoes, and the flesh of some fruit.

Insoluble fiber is found in whole grains, vegetables, and fruit skin. “This kind of fiber promotes contractions of the digestive tract that move food and waste through the body,” says Lindsay Malone, R.D., a dietitian at the Cleveland Clinic Center for Functional Medicine.

Many plant foods contain both, so by eating a variety, you’ll cover all your bases.

How It Keeps You Young

The study mentioned earlier followed over 1,600 healthy adults for 10 years. Those who had “aged successfully” after a decade (meaning they were free of cancer, heart disease, and diabetes, and had good overall cognitive, physical, and cardiovascular function) consumed an average of 29 grams of fiber per day. How is it that this simple substance can have such a powerful effect on health and longevity? It turns out there are many ways that fiber works its anti-aging magic.

Cutting cholesterol. Soluble fiber binds to bile acids, substances produced by the liver that aid in digestion and fat absorption, and it helps your body excrete them. “The body then needs to produce more bile acids, and it pulls cholesterol from the blood to do it,” says JoAnn E. Manson, M.D., chief of the division of preventive medicine at Brigham and Women’s Hospital. A 2016 Cochrane Review of 23 studies found that increasing fiber led to a 7.7 mg/dL reduction in total cholesterol and a 5.4 mg/dL drop in LDL (“bad”) cholesterol.

Protecting against diabetes. A study published in 2009 in Diabetes Care found that people who got less than 20 grams of fiber per day had about a 50 percent greater risk of developing type 2 diabetes than those who got 31 grams or more per day. “Eating a food that’s high in fiber slows the absorption of carbohydrates into your bloodstream,” Manson says, “so blood sugar levels rise more slowly and the pancreas has more time to react and produce insulin.”

Controlling weight. Fiber adds bulk, so you feel full faster and stay full longer. And many high-fiber foods are low in calories.

Lowering colorectal cancer risk. A recent report by the World Cancer Research Fund International/American Institute for Cancer Research found that eating 90 grams of fiber-rich whole grains daily could lower colorectal cancer risk by 17 percent.

Reducing inflammation. Chronic inflammation has been linked to many diseases, such as arthritis, certain cancers, and even Alzheimer’s. “Many studies have shown that increased insoluble fiber intake leads to reduced inflammation,” says Qi Sun, M.D., Sc.D., an assistant professor in the department of nutrition at the Harvard T. H. Chan School of Public Health. This may be because other beneficial components of whole grains, such as polyphenols and magnesium, also have anti-inflammatory properties.

Protecting joints. If fiber can reduce inflammation, it stands to reason that it may help reduce the risk of arthritis. And a recent study, published in the Annals of the Rheumatic Diseases, offers some proof. Researchers evaluated two groups of people and found that in one group, those whose daily fiber intake averaged 20 grams had a 30 percent lower risk of knee osteoarthritis than people who ate about 8 grams. In the other group, those who averaged about 25 grams of fiber per day had a 61 percent lower risk compared with those who consumed about 14 grams.

Boosting good bacteria in the gut. “Fiber doesn’t digest, it ferments,” Malone says. “By the time it reaches the colon, the fermented material supplies food to help those good bacteria multiply and thrive.” A healthy supply of good bacteria can have far-reaching health effects, such as strengthening the immune system and helping to control inflammation.

Natural or Not?

Beta glucan, cellulose, chicory root, inulin, pectin, psyllium, and xanthan gum are types of fiber that are added to some packaged foods. The Food and Drug Administration is reviewing of some of those ingredients to determine whether to allow manufacturers to continue to count them as part of a product’s total fiber content.

The question is whether there’s enough evidence to prove they have the same physiological benefits as natural fiber, says Joanne Slavin, Ph.D., a professor of food science at the University of Minnesota. “The advantage of adding fiber into foods and beverages is to increase fiber without increasing calories,” she says. But critics worry that this practice may make something that’s essentially junk food appear to be healthy because the label touts its fiber content. “Foods that are naturally high in fiber are some of the healthiest foods,” Manson says. “You’re not going to get the same health effects from eating highly processed foods with a sprinkling of added fiber.” 

Tips for Boosting Fiber Intake

Getting fiber from foods naturally rich in it is your best bet. “Using a supplement as a replacement means missing out on all the other benefits of fruits, vegetables, and whole grains,” Manson says.

And getting more fiber doesn't limit you to eating only prunes and wheat bran. Some top sources include avocados (9 grams in one medium fruit), green peas (9 grams per cup), raspberries (8 grams per cup), sweet potatoes (6 grams in one large potato), and pears (5.5 grams per medium fruit).

People often cite gas and bloating as reasons for not adding more fiber-rich foods to their diets. That concern is warranted. “You don’t want to go from 5 grams a day to 30 all at once,” Manson says. “There are enzymes that need to be cultivated so that the intestines are ready to handle the increased load.”

So up your fiber intake gradually, spread it across meals, and be sure to drink more water simultaneously. “Fiber in the presence of fluid adds bulk and softens stool,” Malone says. Without enough water, fiber can actually be constipating. It’s also worth experimenting with a variety of high-fiber foods to find which ones your digestive system tolerates best. 

Editor's Note: This article also appeared in the March 2018 issue of Consumer Reports On Health