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 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. 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.