Dietary Fiber became a part of the public’s nutritional consciousness in the mid-1970s, when the newly released US Dietary Guidelines recommended that a healthy diet include plenty of “foods with adequate starch and fiber.” With that advice, along with the simultaneous ostracism of animal-based saturated fats, dietary fiber secured its place as a sacred component of mainstream nutritional wisdom.
Today, nearly 50 years later, most nutritionists still insist that plant fiber is necessary for optimal human health. But when we look more closely at what fiber does within the body, it becomes difficult to see how it provides any health benefit. With the health success that many people find with all-animal-product diets, fiber is being shown in a new light, and in most cases, it’s proving to be more a nuisance than a nutrient for human health.
A problem with discussing dietary fiber is that, on the surface, much of the conventional wisdom makes sense. The idea that ingesting a good amount of plant roughage to help “scrub” our intestines and colon to keep us regular and free of disease seems intuitive. The thought that we must keep our gut biome happy and well fed with a mix of soluble and insoluble fiber to assure gastrointestinal tract health sounds reasonable.
What is Dietary Fiber?
Simply put, fiber is the non-digestible part of plant foods. Common fiber types include cellulose, pectins, beta-glucans, and gums. Fiber is classified as soluble or insoluble based on whether it is dissolvable in water. Beans, oat bran, avocado, and berries are sources of soluble fiber; whole grains, wheat germ, beans, flax seeds, leafy vegetables, and nuts are sources of insoluble fiber. Animal-source foods, such as meat, seafood, eggs, and dairy, contain no fiber.
At a molecular level, plant fibers are chains of sugar molecules (polysaccharides) that our digestive tract can’t break down and that don’t offer any nutritional value to humans directly. Plant fibers pass through the stomach and end up in the small intestines intact, where they are either broken down and enjoyed by bacteria or pass through us unchanged. While the fiber-eating microbes in our gut are happy, they break down a small percentage of the rough plant materials for their own benefit, with no direct use to us.
Plant Fiber Myths
Fiber has multiple alleged mechanisms of health, but the impact of each is largely unsupported. Claims for dietary fiber’s preventative and healing powers include benefits for various illnesses and diseases, including constipation, colon cancer, diverticulosis, heart disease, diabetes, obesity, and more. Unfortunately, these wisdoms are based primarily on best-guess assumptions and epidemiological studies (studies that get data from patient or subject group questionnaires). Over the decades, there have been a few controlled scientific studies regarding dietary fiber, and the results fall short of conventional nutritional advice.
Is Fiber Needed to Cure Constipation?
The notion that humans need plant fiber for proper bowel function and intestinal health is false. Research regarding fiber and constipation shows that plant fiber does not lead to better outcomes; it often worsens things. While fiber does cause those with constipation to have larger bowel movements, it does not improve any of the unpleasant symptoms of constipation. Studies have shown that removing fiber from one’s diet can improve constipation. Many carnivorous mammals (and humans) have no problem whatsoever having normal, regular bowel movements in the complete or nearly complete absence of fiber.
Does Fiber Prevent Colon Cancer?
In line with the idea of scrubbing our insides for health, many believe that high fiber intake helps reduce the risk of colon cancer, but studies have proved this to be erroneous. The Nurses’ Health Study in 1999 followed 88,757 women over 16 years and found no significant benefit in reducing colon cancer risk. In 2000, a similar randomized study of high fiber intake also failed to show any reduction in precancerous lesions known as adenomas.
Can a High-Fiber Diet Heal Diverticulosis?
Diverticulosis is the pathological process that occurs when the innermost layer of the colon (the submucosa) protrudes through the outer muscular layer, forming small pockets that pouch out from the large bowel. A high-fiber diet and increased frequency of bowel movements are associated with a greater, rather than lower, prevalence of diverticulosis. Patients with IBS, Crohn’s disease, and ulcerative colitis all seem to respond similarly well to a reduced fiber diet.
Does Dietary Fiber Prevent Heart Disease?
The Diet and Reinfarction Trial in 1989 randomized 2,033 men to three different diets after their first heart attack. A standard, low-fat diet did not seem to reduce risk at all, and a high-fiber diet showed no benefit to cardiovascular health. The high-fat Mediterranean diet was shown to be beneficial. More recent trials such as the PREDIMED confirm the benefits of eating more natural fats such as meats, nuts, and olive oil. In all of these trials, the beneficial impact of fiber was essentially nonexistent.
Can Fiber Help Control Diabetes?
The role of fiber in insulin sensitivity and diabetes is open to debate. There is some evidence that dietary fiber can help, but only in indirect ways. High-fiber foods require more chewing, which may help to reduce food intake. Soluble fiber absorbs water to form a gel, further increasing the food’s volume, helping fill the stomach, and boosting satiety. Fiber bulks up food, decreases its energy density, and slows the body’s insulin response to carbohydrates. But we must remember that plant fiber from whole fruits, starchy vegetables, and grains usually comes with more sugars and carbohydrates, which are associated with poor glycemic control.
But Fiber Works for Other Animals
The animal kingdom’s ruminants, herbivores, and omnivores have evolved many ways to extract nutrients from plants and convert them to the building blocks needed for their growth and health. A cow’s stomach has multiple compartments, each with a highly specialized microbiome to break down and digest plant materials. Gorillas and other great apes have massive intestinal tracts and colons to deal with the nearly 40 pounds of plant material they eat daily — a volume that speaks to how little nutrition is in plant materials, even for a highly adapted herbivore. Cows and great apes graze and forage almost constantly throughout their waking hours. Yet even with these plant-digesting guts, the fiber mostly just passes on through. Cows, apes, and most all other herbivores produce a lot of waste!
Humans have small colons, comparatively short intestinal tracts, a small stomach, and a gut biome that is more suited to breaking down animal proteins and fats into building blocks we can use. If the need arises, humans can digest a limited range of plant foods (fruits, berries, nuts, roots, etc.) without issue. Still, the benefit — immediate energy availability — is small and certainly not required for long-term health.
So, is dietary fiber necessary for a healthy human diet? The answer is no. The goal of human nutrition is to provide our bodies with the energy and the structural components needed to build and maintain our animal-based cells — we don’t need anything from plants to accomplish these nutritional goals. Just because humans can ingest fiber-filled food, this ability in no way indicates that fiber is a nutritional health requirement. Much of the touted benefits of plant fiber have been based on erroneous assumptions and flawed epidemiological surveys. For these reasons, plant fiber should be regarded as unnecessary at best, and perhaps even harmful for many.