Get full access to Outside Learn, our online education hub featuring in-depth nutrition, fitness and adventure courses, and more than 2,000 instructional videos when you sign up for Outside+..
Q: Is fiber really all that important in the diet? — J.D. Chafin, Pensacola, Fla.
A: When I was a kid, my grandmother was always trying to get me to eat foods you had to chew a lot. “Gives you roughage,” she’d say wisely. “Keeps you regular.”
Well, that was then, this is now. Our prune-eating grandmothers were onto something, but they had just scratched the surface. Research on fiber is exploding, and its resume of health benefits now extends to weight loss, cancer, diabetes, heart disease, and blood sugar management. Fiber is also essential to the care and feeding of a healthy microbiome.
What’s more, the old conventional wisdom about there being only two kinds of fiber (“soluble” and “insoluble”) was upended in the 1980s when two English researchers—Englyst and Cummings—discovered a third kind of fiber called resistant starch, which, as of this writing in 2016, is currently the subject of an enormous amount of research interest. (More on this in a moment.)
So what is fiber? What does it do? Why do we need it? And why should we care?
Weight Loss and Fiber
Fiber’s not expensive, it’s not exotic, and it’s certainly not sexy, but when it comes to weight loss, it works like a charm. More than a dozen clinical studies have used dietary fiber supplements for weight loss, most with positive outcomes. When you take a fiber supplement with water before meals, the water-soluble fiber binds to water in the stomach, making you feel full and less likely to overeat. Fiber supplements have also been shown to enhance blood sugar control and insulin effects and even to reduce the number of calories (adding up to about 3–18 pounds a year) that the body absorbs. And a study in the prestigious New England Journal of Medicine found that a diet with 50 grams of fiber daily lowered insulin levels in the blood. (Insulin is known to stimulate hunger and promote fat storage.)
My coauthor on the book Smart Fat—Steven Masley, MD—followed patients at his south Florida clinic for 10 years, tracking what they ate, how much exercise they did, what vitamins they took, and how much weight they lost. He found that fiber intake was one of three variables that predicted weight loss success better than anything else (the other two were minutes spent exercising and vitamin D intake).
Other Dietary Fiber Benefits
One of the most impressive studies of all followed 2,900 healthy subjects for 10 years and looked at the relationship between fiber, cardiovascular disease, weight, and insulin. The results showed that fiber
was inversely associated with insulin levels, and weight and low fiber intake turned out to be a better predictor of heart disease than saturated-fat consumption.
Fiber’s ability to lower insulin may have wide-reaching benefits. High blood sugar and high insulin have now been implicated in a baker’s dozen of unwanted degenerative disease, including heart disease. Even Alzheimer’s is now being called “Type 3 Diabetes” because of its connection to insulin resistance, which has consequences not only for your waistline but for your brain as well.
3 Types of Dietary Fiber
This is what your grandmother was talking about when she said to eat “roughage.” It doesn’t break down in the gut.
This does break down in the gut. Specifically, it’s broken down by good bacteria, which do something great with it—they convert it into short-chain fatty acids (SCFAs), the most important of which is butyric acid (also known as butyrate).
Why is this so important? Because the cells that line the gut depend on butyrate for food. “Butyrate has been around in the mammalian gut for so long that the lining of our large intestine has evolved to use it as its primary source of energy,” writes obesity researcher and neurobiologist. Stephen Guyenet, PhD. “It also has potent anti-inflammatory and anticancer effects.” (Butyrate —or butyric acid—supplements are frequently used in inflammatory bowel diseases such as ulcerative colitis and Crohn’s disease.) If you’re not getting enough soluble fiber, you’re probably not making enough butyric acid.
The third kind of fiber is resistant starch, so named because it literally resists breakdown (or digestion).Instead of being broken down by enzymes, resistant starch makes its way directly through the small intestine and winds up in the colon, where—much like soluble fiber—it becomes food for good bacteria in the gut (also called ‘probiotics’). Resistant starch is good bacteria’s favorite food. In fact, gut bacteria create more butyric acid from resistant starch than they do from any other fiber.
Did you know…
Food sources of resistant starch (a type of fiber) include chickpeas.
Butyric acid is pure joy to the cells that line your gut, keeping them healthy. Theoretically, that means less chance of leaky gut and all the myriad of problems that can accompany it. A healthy, well-fed gut lining helps make for a healthy microbiome.
That’s one reason that soluble fiber and resistant fiber are both often known as prebiotics—they’re food for the probiotics in your gut. All prebiotics are fiber, but not all fiber is prebiotic; the indigestible, insoluble kind your grandma called “roughage”—although important—is not a prebiotic fiber. Prebiotics, on the other hand, are what keep your good bugs alive and thriving.
Feeding gut cells resistant starch and soluble fiber is a good beginning. Resistant starch, in particular, even improves insulin sensitivity.
Interesting factoid: In 1981, the American Journal of Clinical Nutrition published a paper by Thomas Almy, MD, called “The Dietary Fiber Hypothesis.” The “Fiber Hypothesis,” as it’s now called, basically put forth the notion that high-fiber diets are protective against a host of diseases. But researchers recently pointed out that some of the low-risk African populations that gave rise to the fiber hypothesis actually didn’t consume high-fiber diets; they did, however, consume diets high in resistant starch.
Food Sources of Soluble Fiber
Food sources of soluble fiber include beans, oatmeal, Brussels sprouts, apples, nuts, blueberries, oranges, and flaxseeds. Food sources of insoluble fiber (“roughage”) include the seeds and skins of fruits, avocados, wheat bran, and brown rice. Food sources of resistant starch include white beans, chickpeas, lentils, rolled oats, peas, black beans, red beans, kidney beans, unripe bananas, and potato starch.
Best Fiber Supplement
Soluble prebiotic fiber can be added to a host of foods and beverages. I put a scoop in my shakes—its odorless, tasteless, and mixes well. Potato starch is a resistant starch that my friend Mark Hyman, MD, calls “the starch that makes you lean and healthy.” And potato starch—unlike potatoes—won’t raise blood sugar.
Potato starch—unlike potatoes—won’t raise blood sugar.
Ideal fiber intake?
Most Americans get a paltry 10–11 grams of fiber a day. Current recommendations range from 25 to 38 grams a day, but I believe more is better. Our caveman ancestors got between 50 and 100 grams daily, according to most research.