With all the noise about superfoods and fad diets (have you met a breatharian yet?) it’s easy to forget about the fundamentals of healthy eating.
These include maintaining a well-balanced diet made up of mostly whole foods, and seeking out essential nutrients that cannot be synthesized by the body—vitamins, minerals, fatty acids, and amino acids, to name a few. They also include getting enough of a macronutrient that evades classification, and, too often, public awareness: fiber (1).
A 2015 survey found that just 4% of adult males and 13% of adult females were meeting their daily recommended fiber intake. This doesn’t just create problems in the bathroom. Studies have shown that fiber can reduce your risk for heart disease and stroke, combat obesity and type 2 diabetes, and even help prevent certain cancers (2, 3, 4, 5, 6). That shouldn’t come as a surprise, considering the influence the digestive system has on the rest of the body.
Fiber isn’t just a true-to-life superfood; it’s the original one (7). According to researchers at the Dietary Guidelines of America, it’s also “an underconsumed nutrient of public health concern” (8). But before you go raiding your local cereal aisle, it’s important to learn what fiber is, and how it’s used by our bodies. It’s also important to learn the differences between various types of fiber, because they are not all created equal.
Let’s dig in.
What Is Fiber?
Dietary fiber is actually a fairly broad category. It refers to the part of plant-based carbohydrates that cannot be directly digested or absorbed (unlike, say, sugar and starches). Fiber can be found in fruits and vegetables, legumes, nuts and seeds, and whole grains, and comes in numerous forms: soluble (easily dissolved in water) and insoluble, which includes viscose (gel-forming) and fermentable (digestible only by bacteria) fibers. Those fermentable fibers—a relatively recent discovery—are particularly fascinating, due to the myriad of ways in which they interact with our gut microbiome.
How Do Our Bodies Use It?
If we think of fiber at all, it is likely in the context of prune-pushing grandmothers and their stomach-turning digestive tales.
Fiber, we’re taught, is necessary for bowel movements. And it’s true—the roughage produced by indigestible fiber passes relatively intact through your digestive system and helps both solidify and soften stool, which can lead to more regular and, well, pleasant bowel movements, as well as helping to prevent hemorrhoids and diverticulitis. Fiber also had a brief moment in our mixed-up diet culture: foods rich in fiber may help you feel full longer, leading to weight loss (9, 10).
But the benefits of a high-fiber diet go far beyond the time you spend on the toilet. Recent studies have shown that increasing your intake of soluble fiber can lower the amount of low-density lipoprotein (or “bad”) cholesterol in your body; fiber has also been linked to a decrease in blood pressure (11, 12). These effects indicate that fiber intake plays a role in preventing heart disease and heart attacks.
In fact, a recent Lancet review of fiber studies concluded that for every 1,000 people that transitioned to a high-fiber diet, six cases of heart disease would be prevented (13). The Lancet review also demonstrated consistently lower rates of type-2 diabetes in individuals with higher fiber intake, and fewer incidents of bowel cancer.
Other studies have demonstrated a lower risk of colorectal cancer, and researchers are now looking into whether fiber intake can prevent other types of cancers, including cancers of the mouth, pharynx, larynx, esophagus, colon, rectum and stomach (14, 15). The bottom line? Eating more fiber can lower your mortality rate (16). Not even the miracle acai berry can promise that.
And What About The Microbiome?
It’s only recently that fiber’s role in the health of our microbiome has become a focus of study.
We know that for our ancestors, fiber was essential — and essentially unavoidable, as whole foods (mainly fruits, vegetables, and legumes) dominated the majority of human diets. Only in the last 100 years did fiber cease to be a cornerstone of our nutritional intake (17). That means that for hundreds of thousands of years, our diverse ancestral microbiomes adapted to a steady dose of fiber.
Hunter-gatherers relied on the microbiota-accessible carbohydrates (MACs) found in dietary fiber to shape their microbial ecosystems. Recently, research has discovered that low-fiber diets can cause microbial extinction in just a few generations. That means when we don’t get enough fiber from diverse sources, microbes disappear, lowering gut microbial diversity. And when they’re gone, they don’t come back (18).
Studies have found that immigrants to the U.S. experience a rapid loss in gut microbiome diversity as they start consuming a low-fiber Western diet, and that over generations, that loss of diversity increases the risk of obesity in these populations (19). Bottom line? The American diet, with its focus on processed foods that lack a healthy diversity of plant fibers, is slowly destroying the rich community of beneficial microbes inside of us—undoing thousands of years of co-evolution with our ancestors.
What Kind of Fiber Do My Microbes Really Need?
Recent studies that focus on the interaction of fiber and our microbiome have discovered what may be the key to understanding just how important fiber is to our overall health.
Remember those microbiota-accessible carbohydrates (MACs) found in fiber? Without MACs, our gut bacteria are unable to generate short-chain fatty acids (SCFAs) like acetate and butyrate. SCFAs are gut miracle workers, stimulating blood flow and increasing mucus production throughout our digestive systems, and have even been shown to prevent and treat diseases such as ulcerative colitis, diabetes, and cancer (20, 21). And it’s those SCFAs that are first to vanish when our bodies are starved of dietary fiber.
As we learn more about our miraculous microbiomes, one thing becomes clear: we want to treat them well, which means feeding them what they want to eat. And while dietary fiber isn’t as buzzy (or as marketable) as kombucha, it’s what our microbiome is clamoring for, and what it evolved to crave. Starving our microbiomes of fiber means they need to find other sources of food: namely, the mucus linings of our digestive organs, leading to a whole host of inflammatory issues.
You’ve Convinced Me! Now, Where Do I Find More Fiber?
The National Institute of Medicine recommends a daily fiber intake of 25 grams for women and 38 grams for men. The average American gets only 16 grams a day. If you’re part of the 95% not meeting that target, you’re missing out on a health panacea. So what can you do about it?
First, understand that not all fiber is created equal. Fructo-oligosaccharides, for example, is a type of fiber found in fruit and vegetables that has been shown to reduce inflammation. Oligosaccharides are also prebiotics: they resist digestion in the small intestine in order to be fermented by microflora in the colon.
Inulin, another type of fiber that is more difficult to obtain from food alone (it is found in limited sources such as sunchokes and chicory root) has been found to encourage the growth of bifidobacterium and lactobacterium, two broad bacterial genera especially beloved by the microbiome (22).
A wide-ranging study by the American Gut Project found that people who ate more than 30 different plant types each week have significantly higher gut microbial diversity (23). The best way to close your own fiber gap? Work on eating a diverse diet of whole foods. Beans, nuts, whole grains, fruits and vegetables—all of these are fabulous sources of fiber that are easy to incorporate into a daily diet.