Vox, by Julia Belluz: When we fret about the deterioration of the American diet, we tend to focus on the excessive amounts of sugar, salt, and calories we’re now eating.
What we don’t talk about: an important ingredient that’s gone missing as we’ve been filling our plates with more chicken and cheese.
Fiber. Only 5 percent of people in the US meet the Institute of Medicine’s recommended daily target of 25 grams for women and 38 grams for men. That amounts to a population-wide deficiency — what nutritionists call the “fiber gap.”
“People are so busy avoiding carbs, they forget that these foods give [them] important dietary components,” said nutritionist Julie Jones, of St. Catherine University.
Fiber is the closest thing we have to a true superfood — or super-nutrient since it’s a part of so many different foods. Eating a fiber-rich diet is associated with better gastrointestinal health and a reduced risk of heart attacks, strokes, high cholesterol, obesity, type 2 diabetes, even some cancers. That’s because fiber is amazingly helpful in many ways: It slows the absorption of glucose — which evens out our blood sugar levels — and also lowers cholesterol and inflammation.
These benefits grow the more fiber people eat. In a recent Lancet review of 185 studies and 58 clinical trials, researchers found that if 1,000 people transitioned from a low-fiber diet (under 15 grams per day) to a high-fiber diet (25 to 29 grams per day), they’d prevent 13 deaths and six cases of heart disease. (Some researchers have described not eating high-fiber carbohydrates as “the opportunity cost” of the ultra low-carb ketogenic diet.)
If fiber were a drug, we’d be all over it. But the average American gets just 16 grams per day — half of what we should be eating.
A big reason for that has to do with what we now eat. Instead of munching on fruits, vegetables, beans, nuts, and seeds, more than half of the calories Americans consume come from ultra-processed foods. On any given day, nearly 40 percent of Americans eat fast food. These prepared and processed meals tend to be low in fiber, or even fiber free. (A cup of cooked oatmeal has 4 grams of fiber and a pear has 6 grams, while a McDonald’s hamburger has one gram and soda has none.)
This pattern of eating is not just leading to weight gain and obesity-related health issues; it’s hurting our gastrointestinal health in ways researchers are only beginning to understand. That’s because fiber’s benefits are a lot more complicated than our prune-peddling moms and grandmothers appreciated.
Fiber doesn’t just help us poop better — it also nourishes our gut microbiome. The science, while still pretty nascent, is fascinating and it points to the fact that the fiber gap may be even more damaging than we’ve realized.
There are many different types of fiber — and they do different things in our guts.
To think of fiber as just Metamucil and bran cereal is to do its complexity a disservice.
Fiber (or “fibers,” as the researchers who study it say) is a group of different kinds of plant-based carbohydrates that affect our gastrointestinal tract in myriad ways. The big difference between fiber and other carbs, like starches and sugar, is that we can’t directly digest or absorb it. And some fiber types can only be broken down by the gut microbiome, the ecology of trillions of diverse bacteria lining our intestines and colon.
Because our intestines can’t directly digest fiber, we’ve long seen fiber as beneficial for relieving constipation by adding bulk to stool and promoting regular bowel movements.
Another commonly touted fiber benefit: It can help us feel full, so we eat less and maybe even lose weight. (There’s some debate about fiber’s effect on satiety and appetite. The most recent systematic reviews of the research suggest fiber’s impact here is surprisingly modest, though others note that many studies have focused on supplements instead of whole foods, which are probably more satiating.)
Still, all this “was before people [realized] how much the non-digestible things we eat impact our gut bacteria,” said University of Michigan microbiologist Eric Martens.
Researchers now consider fiber’s role in nourishing our gut microbiome — the ecosystem of microbes in our intestines — to be one of its main health benefits. They don’t yet fully understand why fiber is so good for our gut, but they have some ideas.
Fermentable fibers — which include all soluble fibers and some insoluble fibers — are metabolized or fermented by bacteria in the gastrointestinal tract. That process produces chemicals, including short-chain fatty acids, which are important food sources for our gut bacteria.
They also carry health benefits, Martens said. Short-chain fatty acids have been shown to promote insulin production, so we can better manage the spikes of sugar (or glucose) in our blood, for example, helping to manage type 2 diabetes. In addition, they seem to have anti-inflammatory properties.
“When we don’t consume enough fiber, we are essentially starving our gut microbiome,” said Alberta’s Jens Walter, “which is likely detrimental for a variety of reasons. We also probably lose [microbiome] diversity.”
Andrew Gewirtz of Georgia State University was among the researchers who noticed that mice develop metabolic syndrome — obesity and its associated disorders, such as diabetes and high cholesterol — when they are fed a high-fat diet. But when fiber was added to the high-fat diet, most of that metabolic syndrome went away.
“We realized the fiber is very important for our metabolic parameters,” Gewirtz told Vox. So he decided to compare the microbiomes of mice on a fiber-enriched high-fat diet with mice on a low-fiber, high-fat diet, to figure out what they might reveal about why extra fiber seemed to offset the health harms of dietary fat. And he found the two sets of mice wound up having really different microbiomes: Rodents on the low-fiber diet had a marked reduction in the total numbers of bacteria in their gut and a less diverse microbiome compared to the mice on the high-fiber diet.
While it’s not yet clear how or whether these findings will translate to people, researchers know that altering the fiber in one’s diet creates changes in the human microbiome.
And for now, this science shows us that we should start thinking about fiber differently, Gewirtz said. The exclusive focus on fiber’s constipation-fighting properties misses the big picture: “It’s just one thing that fiber does” and maybe not as important as fiber’s impact on our microbiome.
Only five percent of Americans meet the recommended fiber target — and that means most miss out on fiber’s benefits. So how can you eat more fiber? Every researcher I spoke to suggested aiming to get a diversity of fiber from a varied menu of whole foods, instead of relying only on supplements or fiber-enriched processed foods, especially the sugary bars and brownies now being marketed as fiber-delivery tools.
To do that, consider snacking on whole fruits, replacing white bread with whole-grain alternatives, eating potatoes with the skins on, and tossing berries, nuts, and seeds on your yogurt, cereals, or salads, Hannah Holscher, an assistant professor of nutrition at the University of Illinois, suggested. “Lots of small changes can add up.”
If you like smoothies, throw your fruits, veggies, and nuts in a blender. Contrary to the hype about smoothies degrading fiber, some of the researchers I spoke to actually encouraged this approach. “Even baking does not destroy most fibers,” Walter said.
“[The] natural sources are probably better for both your digestive health and your microbiome. They’re more diverse from the chemical level,” Martens added. “If you can get 25 to 30 grams per day from beans, nuts, vegetables and fruits, and whole grains — that’s a good place to start.”
“Grains, fruits, nuts, and vegetables constitute the diet chosen for us by our Creator. These foods, prepared in as simple and natural a manner as possible, are the most healthful and nourishing.” The Ministry of Healing, page 296.
Nature Knows Best!