Some 2,500 years ago, Hippocrates
declared that: “Wholemeal bread cleans out the gut and passes through as
excrement. White bread is more nutritious as it makes less feces”. For a long time fibre was regarded as a
non-nutrient and of no importance. Thomas Richard Allinson[1] was a medical doctor who advocated
vegetarianism and wholemeal bread and in 1892 he was struck off the medical
register in the UK for his non-establishment views. In 1936 the American
Medical Association formally condemned the use of bran, a view which would
dominate for the next three decades.
Fibre was of course of enormous
importance in animal nutrition, a science that was far more developed than
human nutrition. To understand why, its best to first consider fibre from the
botanical viewpoint. When a seedling finally gets to the sunshine of boundless
photosynthetic energy, it is green and very leafy. As it grows, its stem moves
from a predominantly photosynthesizing function to the dual function of
physical support and the transport of minerals to the growing leafy top. Its
chemistry changes and it develops strong cell walls for support and these cell
walls are very fibrous growing less green as the harvest nears. The leafy top
give rise to the seeds which will also be wrapped in a fibrous outer husk. For
animal nutritionists, the higher the fibre content of forage, the lower was its
energy value. The most abundant carbohydrate on the planet is cellulose which
like starch, is a polymer of glucose. The polymer of cellulose is organised slightly
differently from starch such that our starch- digesting enzymes in our gut
cannot break down cellulose. For those animals that depend on a cellulose rich
diet, they have developed a very sophisticated association with gut bacteria
that can break down this cellulose such that the bacteria use some of the
cellulose as energy and the leftovers are used by the host animal. Its a win-win situation. In most of the
grazing animals, the bacteria are found at the start of the digestive tract,
the rumen, in cows and sheep while in humans, who don’t have to forage on
cellulose, our gut bacteria are are the end of the gut, the colon.
Some 30 years after the AMA had
downgraded bran to junk status, a small number of distinguished medical doctors, challenged conventional
wisdom and fibre entered the lexicon of human nutrition. The most notable was
Denis Burkitt, a County Fermanagh man educated at Trinity College Dublin. He
raised huge interest in fecal matters in relation to diseases of the digestive
tract, ranging from constipation to colon cancer. Higher fibre diets lead to
higher fecal outputs and this can be achieved in either of two ways. Some
fibres are pretty resistant to any form of microbial degradation but have a
huge capacity to absorb water. Thus if taken in sufficient quantity, the
promote a high fecal output of undigested fiber, bran in particular, which has
a very high water content leading to a soft stool. Indeed, Burkitt used to
discuss two types of fecal stools, floaters and sinkers, the former on low
fibre diets and the latter on high fibre diets. The second route to increased
stool output is to provide the colonic microflora with a very fermentable fibre
such that the biomass increases leading to more frequent excretion of softer
stools. However, if the fibre is very water soluble and fermented very rapidly,
as happens for example with the small sugar-like fibre in beans, flatulence is
promoted leading to the ditty: “Beans, beans good for your heart. The more you
eat, the more you fart. The more you fart the better you feel so beans, beans
at every meal”!
The evidence that dietary fibre reduces
the risk of colon cancer is strong and a recent meta analysis of all studies in
this field, has shown the colon cancer risk is 17% lower with three servings of wholegrain per day and that
increasing fibre intake from 5g/d to 30g/d led to a decline of about 30% in the
risk of colon cancer. Cereal fibre is way ahead of fruit and vegetable fibre in
reducing this risk[2].
This cancer accounts for about 10% of all cancers and is of course the
most preventable, not simply by diet, but also by endoscopic screening.
Fibre has taken its rightful place in
human nutrition but now, there is a slightly different twist in that the
colonic bacteria have raced to the top of biomedical fashion. We started with
the genome ( the collective noun for all genes) and then we moved to the next
level, the proteins which we called the proteome and of course soon after we
had the metabolome. Now we have the microbiome. In my recent book, I devote an
entire chapter to this topic because it is a very interesting and potentially
important topic. However, I fear that it will go down the route of many great
biomedical fashions and deliver very little. Many of the studies in this field
are built on bizarre animal models which involve the breeding of germ-free
animals without any gut bacteria. Many others are built on association studies
which reveal that some pattern of colonic microbial flora is biologically
superior to some other pattern. However, few human intervention studies have
been conducted and within that limited literature, the data are not unanimous.
However, the hype rolls on. To some extent, and I am in this business a long
time, it reminds me of the anti-oxidant theory, a veritable bio-Klondike. It
rose, dominated and fizzled out. Beware of theories that explain everything,
well almost everything.
An adequate intake of dietary fibre is
important. I remain a healthy sceptic about the promise of the microbiome.
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