If You Really Want to Optimize Your Diet, Focus on Fiber

Fiber isn’t sexy, but it’s the key to health

Photo: the_burtons/Getty Images

IIt’s a puzzle that doctors have long struggled to solve: Inflammatory bowel disease, a gut condition that was once vanishingly rare, now affects roughly three million Americans. Between 1999 and 2015 alone, rates of inflammatory bowel disease (IBD) surged by 44% in the U.S., according to the Centers for Disease Control and Prevention. No one is quite sure why.

The term “inflammatory bowel disease” encompasses two conditions — Crohn’s disease and ulcerative colitis. Both are associated with improper and damaging inflammation of the gastrointestinal tract. “IBD is a relatively new disease,” says Dr. Gilaad Kaplan, a gastroenterologist and professor of medicine and the University of Calgary’s Cumming School of Medicine. “We didn’t really begin to see it until the 1800s, and it’s almost exclusive to industrialized countries.”

Kaplan says the causes of America’s escalating IBD rates are almost certainly multiple and may include the injudicious use of bacteria-slaying antibiotics, which can disrupt the function of the human GI tract and microbiome. But while the harms of antibiotic overuse are on a lot of people’s radars, Kaplan says another, less talked about issue may be contributing to IBD’s rise: American’s paltry fiber intakes.

While national nutrition guidelines advise men and women to consume between 22 and 34 grams of fiber a day, the average American only swallows about 15 grams.

“There are many cohort studies that have shown people who eat less fiber are at high risk for developing Crohn’s disease,” Kaplan says. One of the most robust of these studies was a 2013 report from researchers at Harvard Medical School that looked at diet and health data collected from roughly 170,000 women over a period of more than 26 years. It found that those women who landed in the top fifth of the population in terms of fiber intakes were 40% less likely to have been diagnosed with Crohn’s disease than women who landed in the bottom fifth.

“The average person is not even remotely aware of the importance of fiber.”

“IBD is complicated — it’s not driven by any one factor,” Kaplan says. “But where all the research is taking us today is toward the microbiome.” And fiber is integral to a healthy and properly functioning microbiome, he says.

Fiber is not a vitamin or a mineral. It’s not an essential nutrient that the human body absorbs and utilizes to shore up the walls of cells or to solidify the strength of nerves. Fiber is a group of carbohydrates — found almost exclusively in plant foods — that resist breakdown in the small intestine. But it’s exactly that resistance to breakdown that makes fiber so indispensable to the gut and the trillions of microorganisms that it houses. While low fiber intakes are implicated in IBD, the research on dietary fiber stretches back decades and encompasses the health of the GI tract, the brain, the lungs, and the heart.

“The average person is not even remotely aware of the importance of fiber,” says Dr. Robert Lustig, an emeritus professor of pediatrics and endocrinology at the University of California, San Francisco, and author of Fat Chance: Beating the Odds Against Sugar, Processed Food, Obesity, and Disease. “They don’t have a clue.”

Fiber and health

When considered alongside discussions of biohacking, ketogenic diets, and some of today’s other complicated-sounding nutritional fads, talk about fiber can seem fusty and antiquated. Your grandmother drinks a fiber supplement to help her poop. Yawn.

But while fiber isn’t new or sexy, it may have more evidence backing its benefits than any other single component of the human diet. A 2018 review in the Journal of Nutrition compiled decades of evidence linking high-fiber diets — and, in particular, the consumption of insoluble fibers from whole grains — to a reduced risk of Type 2 diabetes. Other research efforts have found that eating fiber prevents weight gain and obesity in both kids and adults.

While fiber isn’t new or sexy, it may have more evidence backing its benefits than any other single component of the human diet.

Fiber’s benefits aren’t confined to the gut. A 2018 research review published in the American Journal of Clinical Nutrition found that adequate fiber intakes were “convincingly” associated with a reduced risk of heart disease, coronary artery disease, and all-cause mortality. That review also found evidence that fiber may fend off various types of cancer — including cancer of the pancreas and stomach. There’s even research that finds fiber supports deep, restorative sleep (though it’s unclear why).

To understand how fiber could have all these effects, it’s important to understand just what fiber is and what it does in the human gut. “There are two kinds of fiber, and you need both,” Lustig says. “One without the other is not nearly as effective.”

The two kinds of fiber he’s referring to are soluble and insoluble. Soluble fibers partially dissolve in water, and many form a gel-like substance during digestion in the small intestine. Insoluble fiber, on the other hand, maintains its fibrous, stringy structure as it passes through the GI tract. “When you have both soluble and insoluble fiber, the stringy stuff forms a latticework on the inside of the small intestine and duodenum, and the gel stuff plugs the holes of that latticework,” Lustig explains. The resulting barrier acts as a natural governor — limiting the speed with which the small intestine absorbs the sugar molecules it derives from food. This moderating effect prevents the big insulin spikes that occur when too much sugar floods the bloodstream and liver and that are associated with weight gain and metabolic diseases such as Type 2 diabetes.

But fiber’s ability to helpfully slow down sugar absorption is arguably the least of its attributes. “When soluble fiber gets down into the lower intestine, the bacteria there can convert it into short-chain fatty acids that are anti-inflammatory,” Lustig explains. This conversion produces short-chain fatty acids (SCFAs) which lower inflammation not only in the GI tract but also in the lungs, the brain, and elsewhere. Research has linked the consumption of fiber and the resulting uptick in SCFAs to reduced lung inflammation and a lower risk for airway disease and also to a reduction in the types of brain inflammation that contribute to age-related cognitive decline. These same short-chain fatty acids also have anti-cancer effects, according to a 2016 study in the journal Pharmacology and Therapeutics.

Insoluble fibers — the ones that water can’t break down and that maintain their string-like structure throughout digestion — bind to water molecules and also to stray food particles. This binding helps shuttle water and waste through the upper GI tract and into the colon, promoting the formation of bulkier, softer poop. (That’s a good thing.) Insoluble fiber also binds to bile acids, carcinogens, and other harmful substances, thereby ensuring they’re harmlessly dumped out of the body. Lustig says insoluble fibers also act like “little scrubbies” on the walls of the colon, helping to clear away dead cells and leftover globs of partially digested food.

Last but not least, the trillions of bacteria that comprise the gut microbiome and that live primarily in the colon feed on fiber. Fiber-rich foods not only serve as sustenance for these bacteria, but they have prebiotic effects — meaning they promote the health of the gut’s microflora.

“Fiber has been shown to produce a diverse microbiome,” says Dr. Berkeley Limketkai, director of clinical research at the UCLA Center for Inflammatory Bowel Diseases. He mentions a 2016 study published in the journal Nature that found feeding mice a “western” diet low in fiber changed their offspring’s gut bacteria in unhealthy ways. “That study showed that if you deprived mice fiber, subsequent generations were born with depleted microbiome diversity and richness,” he explains. A healthy and diverse microbiome plays a pivotal role in proper immune function, and microbiome dysfunction is associated with diseases of the gut, heart, brain, and lungs.

Over and above all of these fiber-specific effects, fiber-rich foods also tend to also be good sources of vitamins, minerals, phytochemicals, and other healthy constituents, says Dr. Walter Willett, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health.

Willett says that whole foods — including some that may surprise people — are the healthiest sources of dietary fiber.

How to get more fiber in your diet

Most whole plant foods — fruits, vegetables, whole grains, and legumes — are good sources of fiber. “Everything that comes out of the ground and is not processed,” summarizes Lustig.

But legumes in particular are fiber superstars. A cup of peas, beans, lentils, peanuts, and other legumes packs at least twice as much fiber as a like amount of most fruits or vegetables. If the prospect of loading your diet with beans and lentils has you worried about gas, those worries are well-founded: Gas is one of the byproducts of the kind of healthy soluble-fiber fermentation that produces beneficial SCFAs. Consider flatulence a sign that you’re doing it right.

Willett says that whole grain foods are a source of fiber that research has consistently linked with a reduced risk for obesity and disease. But not all whole grains are created equal. “It is best to consume them mainly intact, such as brown rice, wheat berries, or steel-cut oats rather than as fine flour,” he says.

The same is true of fruits and vegetables. Lustig says that any form of processing — juicing, blending, etc. — can degrade the fiber in these foods and reduce or eliminate their healthy effects. For this reason, he recommends eating whole fruits and vegetables instead of slurping smoothies. Berries, apples, and pears are all loaded with fiber. When it comes to common vegetables, broccoli, brussels sprouts, artichokes, pumpkin, and skin-on potatoes are fiber standouts.

It may be tempting to load your diet with one or two fiber-rich foods, but Willett says eating a variety is best. He also says the popularity of low-carb diets may be leading American eaters in the wrong direction, nutritionally speaking. “The elimination of dietary fiber [due to] the very low carbohydrate diets being promoted is a concern,” he says.

Fiber may not be new or exciting. But the science to date suggests that few elements of the human diet are as critical to overall health.

I write about health and science. I live in Detroit with my wife and kids. I’m trying to learn German, but my progress so far is nicht gut.

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