Are daily dietary fibre recommendations too low? an
evolutionary perspective
By
Jeff D Leach
Co-founder, World's Healthiest Pizza
Article appeared in the May 2008 issue of
Network Health Dietitians Magazine
(United Kingdom)
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 on which we find
ourselves 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 refers 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 is 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 greater than 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 quite low from an evolutionary
perspective. Therefore, 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 peeking 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 onion, 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 the 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?
Comments
welcome jeff@paleobioticslab.com
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