What is “fibre”?

“Fibre” is the term used to describe any dietary substance that is not able to be digested in the small bowel. Fibre proceeds into the large bowel and is either broken down by bacteria into smaller molecules, some of which can be absorbed or it passes out unchanged in our faeces. Plants deliver many different types and quality of fibre (pectins, gums, hemicelluloses, mucilages, lignin, and cellulose). Some starches, so called “resistant starches” also cannot be digested in the small bowel.

What use is “fibre”?

We are all aware that fibre increases the weight and volume of our stool, allowing the bowel muscles to propel bowel contents along the length of our bowel more easily. Diverticular disease, haemorrhoids, bowel spasm, constipation are all more likely to occur if dietary fibre is low. Less well recognized is that the bacterial fermentation of some fibres produces useful by-products in addition to increased gas (not so welcome) and water (which again aids passage of the stool). Bacteria use energy derived from the fermentation process to stimulate their growth. They proliferate and increase stool bulk (50% of our stool is comprised of bacteria). The fermentation process also produces volatile fatty acids which in turn are absorbed by the large bowel mucosal lining and used as an energy source by us to maintain mucosal health. Increasing fibre content in the diet is not useful for people with “mega-colon”.

 

How much? 20-30gm daily is recommended for adults. A simple of rule of thumb for children is their age in years plus 5 equals the grams of fibre recommended daily.

 

Soluble vs Insoluble?

Most “fibre” increases stool weight not only with its own weight but by binding water during its transit through the gut.
Fibre that is fermented by bacteria creates even more water (and gas, bacteria and other products).
Some types of fibre are soluble in water, and in general these are more likely to be able to be digested by colonic bacteria.
The soluble fibres in general produce more “gas” which can be the most troubling side effect of increased fibre intake.
A normal varied diet contains approximately one third soluble and two thirds insoluble fibre.
If symptoms such as bloating and increased flatulence are problems with a high fibre diet try different types of fibre. Fibre derived from fruit, vegetables or oat bran is often better tolerated than that from wheat bran.
Wheat bran also contains phytic acid which can interfere with the absorption of iron.

 

Soluble Fibre	Type of foods	Insoluble Fibre	Type of foods
Pectins	Fruits, seeds	Lignin	Wheat bran, Vegetables Fruits, legumes
Hemicelluloses	Fruits, nuts, cereal
Mucilages	Oats, barley, seeds Bulking agents	Cellulose	Vegetables, fruits Legumes, Cereals, Nuts
Gums	Seeds, cereals Food additive