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Originally
appeared in the May 2008 issue of
Network Health Dietitians Magazine
(United Kingdom)
Are
government recommendations for daily
fibre intake too low?
an evolutionary perspective
By
Jeff
D Leach
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
fibre throughout the world. Even more startling, much of our discussion on
the health benefits of fibre, at least in the U.S. and U.K., often refer
to the mechanical actions of fibre (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 fibre 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 fibre 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 fibre 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 fibre 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 fibre. 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 fibre varied spatially and temporally in the
past, our ancestors clearly evolved on a diet that included daily intake
of fibre from a huge diversity of sources that far exceed those recorded
among populations in recent intervention and prospective studies concerned
with the role of fibre in human health. These modern studies invariably
group people with fibre intakes hovering around 20 g/d as the “high
fibre” group, when in reality these high fibre 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 fibre does not yield the desired
protective role one might suspect going into the study.
The
potential protective role of dietary fibre 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 fibre 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 fibre 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 fibres 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 fibres 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 fibre in human diet and,
specifically, the role these particular fibres 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 fibre (~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 fibre 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. Fibre anyone?
Comments
welcome jeff@paleobioticslab.com
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