By Markham Heid
October 18, 2018

Your body doesn’t like things to be too easy. Challenging it from time to time—with exercise, with the elements, and even with short periods of going without food—is often associated with better health outcomes.

The same is true of your gut and the foods it digests. Foods that break down and slip through too quickly (namely, refined starches and sugars) tend to promote overeating, out-of-control blood sugar surges, and other disease-linked side effects. Meanwhile, foods that put up a bit of a fight against digestion are often the best ones for you. That’s certainly true in the case of fiber, which is the edible part of a plant that resists breakdown and absorption in your small intestine.

“The evidence from prospective studies is remarkably consistent that a higher intake of fiber is related to lower risk of type 2 diabetes, cardiovascular disease and weight gain,” says Dr. Walter Willett, a professor of nutrition and epidemiology at Harvard School of Public Health.

Almost every year, a new long-term review paper reaffirms the links between dietary fiber and lower rates of disease and death. Earlier this year, a review published in the American Journal of Clinical Nutrition found that the consumption of dietary fiber was “convincingly” associated with lower risks for pancreatic cancer, heart disease-related death, and death from any cause.

But not all fiber is equal.

“Our FDA now allows purified and synthetic fibers to be included on the fiber line on [a food label’s] Nutrition Facts,” Willett says. For example, polydextrose is a synthetic fiber added to many packaged foods in order to boost the food’s fiber content and cut down its levels of sugar, fat and calories. Synthetic fibers also tend to pop up in nutrition bars or drinks, some breakfast cereals, and other ready-to-eat products. While the FDA has collected some evidence that suggests replacing unhealthy sugars and refined starches with polydextrose may lead to lower blood-sugar spikes and reduced appetite, Willett says synthetic fibers do not contain the minerals, vitamins and phytochemicals found in natural sources of fiber—and so aren’t nearly as good for you.

Fiber can be broken down into two subtypes: soluble and insoluble. Soluble fiber dissolves in water, and the healthiest varieties of it tend to become viscous or “gel-like” during digestion, says Nicola McKeown, a fiber researcher and associate professor at Tufts University’s School of Nutrition Science and Policy. McKeown says soluble, viscous fiber is associated with lower blood cholesterol and better control of blood sugar levels.

Insoluble fibers, on the other hand, do not dissolve in water and so tend to pass through the digestive system largely intact. This is a good thing. “Insoluble fiber acts like little scrubbies on the inside of your colon to remove old and damaged cells, thus reducing risk for colon cancer,” says Dr. Robert Lustig, a metabolism researcher and professor emeritus of pediatrics at the University of California, San Francisco. Lustig says insoluble fiber also slows digestion and helps support the health of the microbiome.

These are just a handful of the many ways fiber is good for you. Unfortunately, most people aren’t getting nearly enough of the stuff. While the average American eats about 15 grams of fiber each day, the Institute of Medicine recommends that adult men eat 38 grams of fiber each day while women should aim for 25 grams. “I would say 25 is the bare minimum, actually,” says Wendy Dahl, an associate professor of food science and human nutrition at the University of Florida. As long as it comes from whole foods or whole grains, “there is no upper limit on fiber—you can’t get too much.”

The best foods to eat to up your fiber intake are those that naturally include both soluble and insoluble fiber. “That’s everything that comes out of the ground and is not processed,” Lustig says. Think whole fruits, vegetables, seeds and legumes (beans and peanuts). Beans in particular are a cheap, eco-friendly and plentiful source of dietary fiber, Dahl says: “We should all eat more beans.”

Whole grains, too, are a particularly good source of fiber. If the inclusion of whole grains surprises you, you’re not alone. Many popular low-carbohydrate diets call for the elimination of whole grains and other fiber-rich foods. Willett says this is a concern. “We have no long-term studies of these diets,” he says. Meanwhile, “the evidence of benefits for dietary fiber, especially from grains, is strong. If we really consume our grains as whole grains, we can have a relatively low carbohydrate intake and still get plenty of fiber.”

The healthiest whole-grain foods are the ones that can be eaten more or less intact, such as brown rice, wheat berries or steel-cut oats. Other experts add barley, rye and popcorn to his list.

But while whole grains are great, “fiber from a variety of sources is desirable to minimize the chance of missing something important,” Willett says. For example, a breakfast of unsweetened oatmeal and berries is one healthy, fiber-rich way to start your day. (A cup of oatmeal and half a cup of berries include roughly 15 grams of fiber.) But eating other fruits and whole grains—as well as legumes, seeds, nuts and other fiber-packed plant foods—is optimal.

“A variety of plant-based foods ensures the fiber you get in your diet is not exclusively soluble or insoluble, so you can reap the benefits of both,” McKeown adds.

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