Dietary Fibre and Resistant Starch

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:

• Non-starch polysaccharides (NSP) include cellulose, hemicellulose, (-glucans, gums and pectin - NSP has been categorised into soluble and insoluble fibre.
• Lignin - a non-carbohydrate component of the plant cell wall.
• Resistant starch is defined as "the sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals".
• 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.

There are many methods for measuring dietary fibre. Some of these measure all of the above components, whilst others only measure NSP. This has implications when reviewing the evidence regarding the health benefits of different types of dietary fibre.

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

• Soluble fibre (mainly (-glucans found in oats and barley and soluble pentoses found in rye)
• Insoluble fibre (cellulose, hemicellulose and lignin found in wheat and rice)
• Resistant starch (includes insoluble and fermentable fibre)
• 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.

Factors that influence stool weight

1. The chemical composition of dietary fibre, i.e. insoluble versus soluble fibre.
2. The larger the particle size of the grain or seed, the larger the stool weight and lower the transit time. For instance, coarse wheat bran has been shown to have a greater faecal bulking effect than finely ground wheat bran when consumed at the same dosage.

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 cholesterol1

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 (-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 b-glucan in oats on blood cholesterol

Meta-analyses are studies which statistically evaluate the results from a number of similar, well controlled studies. Using this type of analyses, it has been reported that the effect of soluble fibre on blood cholesterol levels is small, but significant1.

• Three grams of soluble fibre from oats reduces total and LDL-cholesterol by about 0.13 mmol/L.
• The effect is independent of any other dietary changes - two servings of oats can reduce blood cholesterol by an additional 2-3% beyond that of modifying the type of dietary fat in the diet.
• Soluble fibre from oats and barley (mainly (-glucan), psyllium or pectin have similar cholesterol-lowering effects on total and LDL-cholesterol, but do not affect HDL-cholesterol or triglycerides.
• The USA Food and Drug Administration (FDA) believes there is sufficient evidence to approve the following health claim on food labels in the USA: "Beta-glucan from oats, when consumed as part of a diet low in saturated fat, reduces the risk of CHD".

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 (-oryzanol, may also contribute to its cholesterol-lowering effect.

Dietary fibre from wheat and other sources of insoluble dietary fibre (e.g. cellulose, wheat bran, maize bran) have no effect on blood cholesterol levels.

Pulses contain soluble fibre and have also been shown to reduce blood cholesterol.

How does soluble fibre reduce blood cholesterol?
It is as yet unclear how soluble fibre lowers blood cholesterol.

Several mechanisms have been hypothesised including:

• 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.
• 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.
• 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.
• 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 (-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 glycaemic index of carbohydrates and in this way contributes to determining the overall rate of absorption of carbohdyrate and hence their impact on blood glucose levels.

Other factors influencing the glycaemic 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"2:

• 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.

What is a serve?
There is considerable inconsistency in the definition of a "serve size" for many cereal foods. The Australian Guide to Healthy Eating3 recommends the following serve sizes:

• Bread = 2 slices of bread or 1 piece of Lebanese bread/roll/bagel
• Breakfast cereal = 30-45 g (about 1 1/3 cups of flaked breakfast cereal)
• Boiled pasta, rice and noodles = 1 cup (180 g);
• Dried beans, peas or lentils (cooked or canned) = 1/2 cup (75 g).

Dietary fibre content of grains and pulse-based foods

Resistant starch
The chemical composition of starch
Starch is made up of amylose and amylopectin. Amylose has a linear molecular structure whereas amylopectin has a branched structure. Most plants contain about 20-25% amylose. But some, like pea starch have 60% amylose and certain species of maize starch have 80% amylose (e.g. Hi-Maize(r)). Waxy varieties of grains, such as rice, are low in amylose.

Factors that affect the formation of resistant starch
The physical and chemical composition of starch determines whether starch is digested in the small intestine or whether it ferments in the colon.

There are several reasons why starch may not be digested:

• The physical structure of the grain protects the starch from digestion (e.g. partly milled grains and pulses). The larger the particle size, the higher the amount of resistant starch.
• The natural chemical composition of the starch in foods influences the amount of resistant starch. The higher the amylose content of starch, the greater its resistance to digestion. Raw potato, green bananas, pulses and high amylose maize starch have a high amylose content.
• When starch is heated, starch granules swell and are disrupted. This process, known as gelatinisation, makes the starch much more accessible to digestive enzymes. Starch with a high amylose content and starch which is inaccessible due to the physical structure in which it is located, are less susceptible to gelatinisation and hence are more resistant to digestion.
• When starch that has been heated, is cooled, retrogradation occurs converting the starch to a crystalline form which is resistant to digestion. Foods, such as bread, cornflakes, cold cooked potato, rice and pasta, contain retrograded starch which is resistant to digestion.

How much resistant starch is required for good health?
Some resistant starch is measured when total dietary fibre is measured. However, there is currently no official analytical method for measuring the resistant content of foods. It has been estimated that resistant starch intake in Australia is around
5-7 grams/person/day. Approximately 20 grams a day is recommended to obtain the beneficial health benefits of resistant starch.

References
1. Truswell AS. Cereal grains and coronary heart disease. A review of the literature commissioned by "Go Grains" in 1999 (gograins.com.au).
2. National Food Authority. Code of Practice. Nutrient claims in food labels and in advertisements. Canberra: AGPS; 1995.
3. The Australian Guide to Healthy Eating. Background information for nutrition educators. Canberra: Commonwealth of Australia; 1998.

For Further Research