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European Journal of Clinical
Nutrition (2007) 61, 140–142 
see also
[PubMed]
Evolutionary perspective on dietary intake of fibre and colorectal cancer
By
Jeff
D Leach
Paleobiotics Lab
Abstract
From an evolutionary perspective, the ongoing discussion of fibres role in
colorectal cancer is possibly limited by the overall low intake of fibre
across study groups. Our ancestral diet consistently included a diverse
range of plants that regularly contributed >100 g/d of dietary fiber.
Importantly, this diversity assured that, due to a range of physical and
chemical structures, a steady flow of fermentable substrates promoted
metabolic activity into the distal regions of the colon.
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 today. During the two million years since the first member
of our genus made an appearance in the fossil record, humans subsisted on
foraged 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 did that food supply include agricultural crops, domesticated
animals, and their byproducts. Therefore, the modern human genome and its
nutritional and physiological parameters were selected during our
non-domesticated foraging lifeway conditioned, in no small way, by a diet
of diverse fibre- and
nutrient-rich plants and lean meats.
Even though this important
reality underlies the basic evolutionary biological principles of modern
human nutrient requirements, it is all but missing from current
discussions of dietary fibre intake and our attempts to understand its
role in the etiology of colorectal cancer. As the steady stream of
European and US-based studies demonstrate (e.g., Fuchs et
al. 1999; Bingham et al.
2003; Park et al. 2005), the
protective role of dietary fibre is inconsistent, often frustrating, and
made more allusive with a growing list of confounding risk factors (e.g.,
smoking, alcohol, red meat). This is further complicated when well-known
fibre sources, such as resistant starch and inulin-type fructans, are not
consistently considered when calculating total fibre intake amongst many
studies.
Part of the dilemma in our ability to derive clear answers to the
possible role of dietary fibre in colorectal cancer may be, from an
evolutionary perspective, the remarkably low intake of fibre among various
populations and study groups (Park et
al. 2005) – even for those that fall within the upper quintiles. It
is well known that dietary habits among westernized societies are
characterized by increasing caloric intake from added sugars, fats, and
highly-processed nutrient- and fibre-poor grains. This caloric shift is in
discordance with our evolutionary past (Eaton et al. 2002) and continues to be at the expense of dietary diversity
and consumption of fibre- and nutrient-rich plants. A few examples from
the archaeological and ethnographic record demonstrate the magnitude of
this shift as it pertains to the diversity and quantity of fibre in human
diet.
Located along the shores of
the Sea of Galilee in modern-day
Israel
, a remarkably well-preserved collection of plant remains were recovered
from the 23,000-year-old archaeological site of Ohalo II (Weiss 2002).
Ohalo II has provided 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 were
identified, revealing that a rich diversity of fibre sources 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 (Brand-Miller and Holt 1998). Based on the analysis of over
800 of these plant foods, the fibre intake was estimated between 80 to 130
g/d, depending on the contribution of plants to daily energy needs
(Brand-Miller and Holt 1998). This daily intake is most likely higher when
you consider that fibre in the form of resistant starch and
oligosaccharides were not measured by the researchers among the
economically important roots and tubers.
In the semi-arid Trans-Pecos region of west
Texas
, a nearly continuous 10,000-year record of a foraging lifestyle has been
documented in dry cave deposits. Considered one of the most complete
records of foraging lifestyle in North America, nearly three decades of
excavation and extensive analysis of well-preserved macrobotanical remains
and human coprolites (feces) from a number of cave sites (Sobolik 1994)
reveal a plant-based diet that conservatively providing between 150 to 250
g/d of dietary fibre from dozens of plant species. The fiber-rich diet is
well-illustrated by the visual presence (Figure 1) of undigested fiber
(cellulose) in nearly 100% of the human coprolites studied throughout the
entire 10,000-year sequence (Sobolik 1994).
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 diversity of sources that far exceed those recorded among
populations in recent intervention and prospective studies concerned with
the protective role of fibre against colorectal cancer. While stool
bulking, dilution of colonic contents, and reduced transit time are
clearly positive mechanisms of this ancestral intake of fibre, of
particular interest may be the increased opportunities for consumption of
long chain molecules (e.g., inulin), in combination with insoluble fibres
(e.g., cellulose, hemicellulose), that are known to slowly and selectively
stimulate anaerobic bacterial fermentation into more distal areas of the
colon. The slow, sustained
effect of metabolic activity and production of SCFA (specifically
butyrate), and corresponding reduction in pH and conversion of bile acids,
into more distal regions, has been shown to have a strong physiological
impact in biomarkers (Van Loo 2004).
With
modern palates that trend towards less lignified portions of plants, in
combination with a food industry that is likely to select added fibres
more for their technical or economical characteristics than physiological
ones (Redgwell and Fischer 2005), modern populations ‘most likely’
consume more rapidly fermented fibres over more slowly fermented ones than
at any point in our evolutionary past. Said differently, rapid
technological advances within food industry and a decreasing variety and
quantity of fibre sources throughout much of western civilization, has
resulted in decreased metabolic and physiological activity in the distal
colon, thus opening the pathogenic door to cancer in this region.
Future studies on the
protective role of dietary fibre against colorectal cancer may benefit
from a research agenda that includes an overarching understanding of the
evolutionary landscape in which our current nutrient requirements were
selected. In the case of even the best-designed intervention or
prospective study, clear and optimal results may never be achieved as the
diet and lifestyle of participants may differ significantly from their
evolution-based and thus genetically determined optimal intake of fibre
and other nutrients.
Until we have better understanding of the diversity and quantity of
fermentable substrates that entered our ancestral bowels, and thus
conditioned our current nutritional parameters and physiological
responses, accompanied with modern analytical tools and techniques that
allow us to compare the range of chemical and physical fibres present in
the modern food supply, the possible and important protective role of
fibre in etiology of colorectal cancer may not be forthcoming.
References
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(2003): Dietary fibre in food and protection against colorectal cancer in
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Figure
1. 5,500-year-old
human coprolites recovered from dry cave deposits in west Texas.
Undigested fiber, which is visible in all three samples, is characteristic
of nearly all coprolites analyzed from the region. Photograph, K. Sobolik.
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