Dietary Fibre

Grains and pulses are useful sources of dietary fibre and resistant starch. It is well established that dietary fibre plays an important role in maintaining healthy bowel function and hence may play an important role in preventing colon cancer. More recent research suggests that there are different types of dietary fibre with a greater diversity of health benefits.

What is dietary fibre?

The definition for dietary fibre adopted by the ANZFA: “dietary fibre means that fraction of the edible part of plants or their extracts, or synthetic analogues that:

A. Are resistant to the digestion and absorption in the small intestine, usually with complete or partial fermentation in the large intestine; and

B. Promote one or more of the following beneficial physiological effects

1. laxation
2. reduction in blood cholesterol
3. modulation of blood glucose and, includes polysaccharides, oligosaccharides (degree of polymerisation > 2) and lignins.

The following components are generally considered to constitute dietary fibre:

1. Non-starch polysaccharides (NSP) include cellulose, hemicellulose, b-glucans, gums and pectin – NSP has been categorised into soluble and insoluble fibre.
2. Lignin – a non-carbohydrate component of the plant cell wall.
3. Resistant starch is defined as “the sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals”.
4. Non digestible oligosaccharides are small carbohydrates (with less than 10 monomeric units), e.g. raffinose, stachyose, oligofructose and inulin.

There is general agreement that NSP and lignin are the principal components of dietary fibre. Resistant starch is increasingly regarded as dietary fibre. More recently, it has been suggested that non digestible oligosaccharides also function as dietary fibre. Dietary fibre is therefore usually defined as food components that resist digestion by human enzymes in the small intestine and that pass into the large intestine where they may or may not be fermented by gut bacteria. Dietary fibre which is not fermented is excreted in the faeces. The degree of fermentation influences the role of dietary fibre in the body.

The different types of dietary fibre are generally classified according to their main physiological effect in the body:

1. Soluble fibre (mainly (-glucans found in oats and barley and soluble pentoses found in rye)
2. Insoluble fibre (cellulose, hemicellulose and lignin found in wheat and rice)
3. Resistant starch (includes insoluble and fermentable fibre)
4. Oligosaccharides (found in pulses, artichokes, onion and garlic)

Dietary fibre and bowel function

Insoluble fibre and stool weight

Dietary fibre, particularly insoluble fibre, increases stool weight and, decreases gut transit time and in so doing, helps to relieve constipation.

The laxative effect observed with dietary fibre seems to be achieved via two different mechanisms:

1. Insoluble fibre, found in wheat and rice bran, is resistant to fermentation in the gut and is excreted in the faeces. It increases stool weight by holding water.
2. Soluble fibre, found in oat and barley bran, is almost completely fermented in the colon and provides energy for bacterial growth. Although virtually no soluble fibre is excreted in the faeces, stool weight is increased due to an increase in bacterial cells which also hold water. However, insoluble fibre tends to have a greater faecal bulking effect.

Benefits of insoluble fibre

1. By improving bowel function, dietary fibre can reduce the risk of diseases and disorders such as diverticular disease, haemorrhoids and constipation.
2. Increased faecal bulk and decreased transit time allows less opportunity for carcinogens to interact with the walls of the intestine. Dietary fibre may also bind or dilute secondary bile acids which are potentially carcinogenic. Similarly, dietary fibre may reduce the toxic effect of heavy metals and pesticides. Wheat bran is the type of fibre most consistently shown to inhibit carcinogenesis.

Dietary fibre and fermentation

NSP, resistant starch and oligosaccharides, which resist digestion by human enzymes, are fermented by bacteria in the gut, producing short chain fatty acids and gases. Short chain fatty acids include acetate, propionate and butyrate. Acetate and propionate are metabolised by the body, mainly in the liver. Butyrate is an important source of energy for the cells of the colon which helps to keep them healthy. Wheat bran and oat bran produce higher proportions of butyrate. It is thought that resistant starch produces more butyrate than NSP.

Benefits of short chain fatty acids

1. Production of short chain fatty acids (SCFA) lowers the pH of the gut. In this acidic environment, the conversion of primary bile acids into carcinogenic secondary bile acids is inhibited and the solubility of free bile acids is reduced making them less carcinogenic.
2. Butyrate not only keeps cells in the bowel wall healthy, it has also been shown to inhibit the growth and proliferation of tumour cells in vitro (i.e. the experiment is conducted outside the living body, traditionally in the test tube).

Factors which influence the extent of fermentation

1. The chemical composition of the dietary fibre (i.e. the proportion of soluble to insoluble NSP).
2. The physical form of the grain or seed determines whether dietary fibre or starch is fermented. Undisrupted cell walls are more likely to pass intact through the gastrointestinal tract.
3. Lignin and phenolic acids in the cell walls of the grain kernel restrict access to bacteria and hence limit fermentation of dietary fibre or starch. When the cell walls are disrupted, microflora have access to starch, oligosaccharides and NSP allowing fermentation to occur.

Dietary fibre as pre-biotics

Probiotics
Probiotics are food supplements, such as some yoghurts and products like Yakult, which contain beneficial bacteria. These beneficial bacteria live in the gut of humans and contribute to good health.

Pre-biotics
Pre-biotics are non-digestible food ingredients, such as resistant starch and oligosaccharides, which are used as a source of fuel for bacteria in the gut. Without food, these bacteria cannot survive and perform their beneficial effects in the large bowel.

Role of pre-biotics

1. Resistant starch and oligosaccharides have been shown to stimulate the growth and/or activity of beneficial bacteria, such as bifidobacteria, and to reduce the concentration of pathogenic bacteria, such as Escherichia coli, Clostridia, and bacteroides. Hence pre-biotics contribute to the overall health of the bowel.
2. Resistant starch has been shown to reduce the severity of bacterially induced diarrhoea.
3. Resistant starch has been used to protect the probiotic bacteria during processing and consumption. For instance, by adding Hi-maize(r) to yogurt, the survival of probiotic bacteria is improved.

Soluble dietary fibre and blood cholesterol

Several studies have reported a cholesterol-lowering effect with consumption of pulses, oat bran, barley as well as purified sources of soluble dietary fibre, including guar and pectin. Oats and barley are high in b-glucan, a soluble fibre.

When considering the effect of soluble fibre on blood cholesterol, care must be taken to account for other factors which may have caused the reduction in blood cholesterol. For instance, increased dietary fibre intake may reduce the proportion of saturated fat, known to increase blood cholesterol, in the diet. Differences in baseline blood cholesterol concentrations of subjects will affect the results with greater changes expected from those with high blood cholesterol than those with normal levels.

The effect of other grains and pulses on blood cholesterol

Large amounts of rice bran (100 g/day), which contains a different type of soluble fibre in the form of hemicellulose, has also been shown to lower total and LDL-cholesterol, slightly raise HDL-cholesterol and lower triglycerides. The oil contained in rice, which contains a sterol called b-oryzanol, may also contribute to its cholesterol-lowering effect.

Several mechanisms have been hypothesised including:

1. Soluble fibre may bind bile acids or cholesterol in the intestine, preventing their reabsorption into the body. The liver responds by taking up more LDL-cholesterol from the blood stream thereby lowering the concentration of LDL-cholesterol in the blood.
2. Short chain fatty acids, products of fermentation from soluble fibre in the gut, may inhibit synthesis of cholesterol by the liver, reducing the concentration of blood cholesterol.
3. The high viscosity of soluble fibre may slow the rate of digestion and absorption of carbohydrates, affecting insulin activity, which is implicated in the removal of LDL-cholesterol in the blood.
4. Other aspects of grains and pulses, which have also been shown to contribute to a cholesterol-lowering effect may also be effective. For example, the low saturated, high polyunsaturated fatty acid profile; vitamin E content; phytochemicals such as plant sterols, saponins, phytic acid and tannins. A study in rats found that whole oats lower blood cholesterol more than defatted oats or oats without fibre, suggesting that factors other than dietary fibre may also be operative.

Dietary fibre and blood glucose

There is some evidence that dietary fibre, particularly soluble dietary fibre (mainly b-glucans) found in barley and oats, may slow digestion and absorption of carbohydrates and hence lower blood glucose and insulin responses.

Resistant starch may also reduce or delay the rise in blood glucose and insulin following a meal by slowing the rate and extent of digestion and absorption of carbohydrates.

Phytochemicals in grains and pulses, such as phytic acid, saponins and tannins have also been shown to lower glucose and insulin responses after a meal.

The type and amount of dietary fibre is a factor which seems to impact on the Glycemic index of carbohydrates and in this way contributes to determining the overall rate of absorption of carbohydrate and hence their impact on blood glucose levels.

Other factors influencing the Glycemic index include particle size, fat content and the type of the starch (i.e. proportion of amylose to amylopectin).

Dietary fibre and weight control

A diet which is high in dietary fibre is generally bulky, requires more chewing and is less likely to cause overeating and the risk of weight gain.

Dietary fibre and satiety

The University of Sydney’s Human Nutrition Unit has developed the world’s first satiety index of foods. It tells you which foods fill you up most for the same number of kilojoules.

People seemed to eat less after eating high satiety foods than after eating low satiety foods. High satiety foods include low-fat, high-fibre foods, such as porridge, grain bread and wholemeal pasta, whereas low satiety foods included high-fat, low-fibre foods, such as croissant, doughnuts and chocolate. Other factors, such as the form of the food, the fat content and the amount of protein also seem to influence the satiety of a food.

Dietary fibre and energy density

High-fibre foods are generally considered low energy dense foods because they provide few calories per gram of food. In other words, they allow you to eat more food for the same amount of calories than high energy dense foods such as chocolate or cake. Energy density is also affected by the fat and water content of the food.

How much dietary fibre is recommended?

Adults should consume at least 30 g of dietary fibre from a variety of different food sources.
Children aged 3-18 years should have their age plus 5 g of dietary fibre a day, i.e. 8 g/day at 3 years, 15 g/day at 10 years and 25 g/day at 20 years.

Dietary fibre in grains and pulses

Dietary fibre – good, better, best
According to ANZFA’s “Code of Practice for Nutrient Claims in Food Labels and Advertisements”

* An “excellent source of fibre” contains more than 6 g of dietary fibre per serving.
* A “good source of fibre” provides 3 g of dietary fibre per serve.
* A “source of fibre” has more than 1.5 g per serve.

References