Are government recommendations for daily fiber intake too low? an evolutionary perspective

Modern humans are the latest in a diverse line of species within the genus Homo that evolved on a nutritional landscape very different from the one we find ourselves on today. During the ~ 2.5 million years since the first member of our genus made an appearance in the fossil record, humans subsisted on an extraordinary diversity of wild plants and animals from a dynamic environment that literally changed at a glacial pace. It is only within the last 5,000 to 10,000 years that our food supply has begun to include domesticated plants and animals. For more than 99 % of human history, our genome and its nutritional and physiological parameters were selected during our non-domesticated foraging life-way conditioned, in no small way, by a diet that included large amounts of dietary fiber from a significant diversity of sources.

Even though this important reality underlies the basic evolutionary biological principles of modern human nutrient requirements, it is all but missing from policy and research discussions on recommended intake of dietary fiber throughout the world. Even more startling, much of our discussion on the health benefits of fiber, at least in the U.S. and U.K., often refer to the mechanical actions of fiber (stool bulking, for example) and nearly ignores the critical role of dietary fiber as a nutrient base of sorts for the trillions of microbes living throughout the human gut.

It’s safe to say that our current chronic low-intake of dietary fiber in the western world (~12 to 15g/d) – coupled with our overuse of antibiotics and the increase in multiple antibiotic resistance in pathogens – has started a large-scale genetic “re-engineering” experiment on the slowly evolved and critical symbiotic relationship between humans and our little evolutionary hitchhiking friends, with limited discussion of its outcome on public health.

As you read this, there are millions of tiny microbes swimming around in the fluid surrounding your eyeballs. But you can’t see them. There are millions more under your fingernails, on your hands, arms, legs and just about every imaginable section of your fleshy real estate. There are millions more lining your moist nasal passage, many more maneuvering about your liver, heart, lungs, pancreas and trillions more have been living throughout your continuous gastrointestinal tract – from mouth to anus – from the moment you enter this world. But this is good news.

The bad news is as we fill our shopping carts and pantries with the latest neatly boxed and wrapped goodies of industry, we continue down a path that began some ten thousand years ago with the emergence of agriculture – an event that eventually, along with steel roller mills in the 1880s, farm subsidies in the 1970s, and the divergent interests of food sellers and public health, may be leading us on a path to one of the greatest unintended consequences in human history by tinkering with the health of our intestinal microbes. Current dietary advice would be well served by an appreciation that the average human is a complex super-organism, rather than a single individual.

The archaeological and ethnographic record serves as an interesting reminder of the magnitude of the shift in the diversity and quantity of fiber in human diet.

Along the shores of the Sea of Galilee in modern-day Israel, a remarkably well-preserved collection of plant remains recovered from the 23,000-year-old archaeological site of Ohalo II  provides an extraordinary window into a broad-spectrum diet that yielded a collection of >90,000 plant remains representing small grass seeds, cereals (emmer wheat, barley), acorns, almonds, raspberries, grapes, wild fig, pistachios, and various other fruits and berries. Owing to excellent preservation, a stunning 142 different species of plants was identified, revealing the rich diversity of fiber sources that was consumed by the site inhabitants.

In Australia, Aborigines are known to have eaten some 300 different species of fruit, 150 varieties of roots and tubers, and a dizzying number of nuts, seeds, and vegetables. Recent analysis of over 800 of these plant foods suggest the fiber intake was estimated between 80 to 130 g/d – possibly more – depending on the contribution of plants to daily energy needs.

In semi-arid west Texas, a nearly continuous 10,000-year record of ancient foraging reveals a plant-based diet that conservatively provided between 100 to 250 g/d of dietary fiber. Analysis of hundreds of preserved human feces (coprolites) recovered throughout the 10,000-year archaeological sequence reveal a significant diversity of plants were consumed.

While the diversity and quantity of fiber varied spatially and temporally in the past, our ancestors clearly evolved on a diet that included daily intake of fiber from a huge diversity of sources that far exceed those recorded among populations in recent intervention and prospective studies concerned with the role of fiber in human health. These modern studies invariably group people with fiber intakes hovering around 20 g/d as the “high fiber” group, when in reality these high fiber or upper quintile groups are in fact low from an evolutionary perspective. Therefore, from an evolutionary perspective we should not be surprised when analytical hair splitting of these minute amounts of fiber does not yield the desired protective role one might suspect going into the study.

The potential protective role of dietary fiber among these modern studies may further be complicated by the lack of diversity as much as the quantity. According to data compiled by the Economic Research Service, United States Department of Agriculture in 2007, 57% of all vegetables consumed by Americans are limited to five sources (potatoes, tomatoes, leafy greens, lettuce, and onions). Unfortunately, the most consumed vegetable in America, the potato, is often in the form of oil-soaked french fries or potato chips. For fruit, five sources (apples, bananas, grapes, strawberries, and oranges) account for 71% of the total intake. From an evolutionary perspective, this minimal diversity, even when coupled with the handful of whole grains and beans/legumes consumed, translates into a striking shortfall in the physical and chemical diversity of fiber once consumed by humans and subsequently utilized by the hundreds of bacterial species that inhabit the human gut. We have changed the rules of the game between “us and them” in such a way as to possibly disrupt the organic harmony that evolved in this symbiotic relationship to a nutritional tipping point.

The emergence of prebiotics as a “super fiber” of sorts is just one example of the importance of diversity of fiber in the human diet. The steady clip of scientific papers demonstrating the health benefits of prebiotics is fascinating as we are literally peaking under the evolutionary curtain of our nutritional past.

Unlike probiotics, which are live microbial organisms that are naturally present in the human body, prebiotics are literally food for probiotics. While many fibers claim to be prebiotics, true prebiotics selectively stimulate the growth of certain probiotics known to be beneficial to humans, such as bifidobacterium and lactobacillus, while not promoting the growth of less useful or even harmful strains, such as clostridium.

Even though prebiotic fibers are present in more than 30,000 edible plants throughout the world, American and European diets only include 1 to 3 g/d – sometimes a little more, sometimes a little less. When we look into the archaeological record, like the west Texas example discussed above, we see daily consumption (though variable seasonally) of 10, 15 and often more than 20 g/d from desert plants such as agave, prickly pear, sotol, wild onions and so forth. Dozens of peer-reviewed studies have shown that test subjects who consumed between 5 to 20 g/d of prebiotic fiber, mainly in the form of inulin and fructo-oligosaccharides derived from chicory roots, were able to stimulate the growth of “good” bacteria and increase calcium absorption, blunt hunger, relieve symptoms of irritable bowel syndrome, reduce biomarkers of some cancers, reduce inflammation through various mechanisms, improve immunity, and fortify our natural defenses against many food-borne pathogens. And the list goes on.

It would be a mistake to look at the science and health benefits emerging from clinical benefits of prebiotics as a new discovery of some magic bullet. More correctly we are simply witnessing a rediscovery of the importance of the diversity of fiber in human diet and, specifically, the role these particular fibers play in the health and well-being of gut bugs.

The exciting science behind prebiotics coupled with the underlying biological reality that humans are still designed to ferment a large and diverse quantity of fiber (~50 to 90 g/d, minimum), and that much of our health is tied to the maintenance of a healthy population of gut bacteria, should serve as a wake up call for new therapeutic approaches to health. We don’t need yet another diet for us, but desperately need a diet for our entire “super-organism’ – both us and them.

Even though humans evolved from nothing more than a run-of-the-mill large mammal on an open savannah of other large mammals, to something of a geological force in an evolutionary blink of an eye, we owe much of our current success as a species to these tiny microorganisms. They require little more than a safe place to live and a steady flow of the quantity and diversity of fiber that they and their microbial ancestors evolved on.

Continuing to ignore our shared nutritional past with our tiny friends and adhering to the very human-like notion that we are somehow separate from nature will only result in progression of many human diseases to levels that will require the medical community to seek new vernacular to describe the public health hardships that potentially lie ahead. Fiber anyone?

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