Dietary fibre is found in cereals, fruits and vegetables. Fibre is made up of the indigestible parts or compounds of plants, which pass relatively unchanged through our stomach and intestines. The main role of fibre is to keep the digestive system healthy. Other terms for dietary fibre include ‘bulk’ and ‘roughage’, which can be misleading since some forms of fibre are water-soluble and aren’t bulky or rough at all.
Fibre is a carbohydrate
Fibre is largely a carbohydrate. The building blocks of all carbohydrates are different types of sugars, and they can be classified according to how many sugar molecules are combined in the carbohydrate. For instance:
- Simple sugars – consist of one to two sugar molecules, for example, glucose, fructose, sucrose, maltose and lactose
- Oligosaccharides – consist of three to 10 glucose molecules joined together, for example, raffinose
- Starch polysaccharides – have more than 10 glucose molecules joined together, for example, starch (consisting of amylose and amylopectin) and glycogen
- Non-starch polysaccharides – have more than 10 sugar molecules, for example, dietary fibre, xylose and mannose.
Fibre keeps the digestive system healthy
Dietary fibre is mainly needed to keep the digestive system healthy. It also contributes to other processes, such as stabilising glucose and cholesterol levels. In countries with traditionally high-fibre diets, diseases such as bowel cancer, diabetes and coronary heart disease are much less common than in Western countries.
Most Australians do not consume enough fibre. On average, most Australians consume 18–25 g of fibre daily. The Heart Foundation recommends that adults should aim to consume approximately 25–30 g daily.
Children aged between four and eight need a minimum of 18 g of fibre each day. Girls aged nine to 13 and 14 to 18 years need between 20 g and 22 g per day respectively. Boys aged nine to 13 and 14 to 18 years need 24 g and 28 g per day respectively.
Disorders that can arise from a low-fibre diet include:
- Irritable bowel syndrome
- Heart disease
- Some cancers.
Two types of fibre
There are two categories of fibre and we need to eat both in our daily diets:
- Soluble fibre – includes pectins, gums and mucilage, which are found mainly in plant cells. One of its major roles is to lower LDL (bad) cholesterol levels. Good sources of soluble fibre include fruits, vegetables, oat bran, barley, seed husks, flaxseed, psyllium, dried beans, lentils, peas, soy milk and soy products. Soluble fibre can also help with constipation
- Insoluble fibre – includes cellulose, hemicelluloses and lignin, which make up the structural parts of plant cell walls. A major role of insoluble fibre is to add bulk to faeces and to prevent constipation and associated problems such as haemorrhoids. Good sources include wheat bran, corn bran, rice bran, the skins of fruits and vegetables, nuts, seeds, dried beans and wholegrain foods.
Both types of fibre are beneficial to the body and most plant foods contain a mixture of both types.
Resistant starch, while not traditionally thought of as fibre, acts in a similar way. Resistant starch is the part of starchy food (approximately 10 per cent) that resists normal digestion in the small intestine. It is found in many unprocessed cereals and grains, unripe bananas, potatoes and lentils, and is added to bread and breakfast cereals as Hi-Maize. It can also be formed by cooking and manufacturing processes such as snap freezing.
Resistant starch is also important in bowel health. Bacteria in the large bowel ferment and change the resistant starch into short-chain fatty acids, which are important to bowel health and may protect against cancer. These fatty acids are also absorbed into the bloodstream and may play a role in lowering blood cholesterol levels.
Fibre keeps the digestive tract healthy
The principal advantage of a diet high in fibre is in improving the health of the digestive system. The digestive system is lined with muscles that massage food along the tract from the moment a mouthful is swallowed until the eventual waste is passed out of the bowel (a process called peristalsis). Since fibre is relatively indigestible, it adds bulk to the faeces.
Soluble fibre soaks up water like a sponge, which helps to bulk out the faeces and allows it to pass through the gut more easily. It acts to slow down the rate of digestion. This slowing down effect is usually overridden by insoluble fibre, which does not absorb water and speeds up the time that food passes through the gut.
You must drink lots of fluid
A high-fibre diet may not prevent or cure constipation unless you drink enough water every day. Some very high-fibre breakfast cereals may have around 10 g of fibre per serve, and if this cereal is not accompanied by enough fluid, it may cause abdominal discomfort or constipation.
Fibre and ageing
Fibre is even more important for older people. The digestive system slows down with age, so a high-fibre diet becomes even more important.
Lowering blood cholesterol
Recently, there has been a great deal of interest in oat bran, since some studies showed that regular intake of foods high in soluble fibre – such as oat bran, baked beans and soybeans – reduced blood cholesterol levels. When blood cholesterol levels are high, fatty streaks and plaques are deposited along the walls of arteries. This can make them dangerously narrow and lead to an increased risk of coronary heart disease.
It is thought that soluble fibre lowers blood cholesterol by binding bile acids (which are made from cholesterol to digest dietary fats) and then excreting them. Cereal fibre seems to be more protective against coronary heart disease than the fibre from fruit and vegetables.
Fibre and weight control
In many cases, people who are overweight or obese have been shown to lose significant amounts of excess body fat simply by increasing the amount of dietary fibre, especially soluble fibre, in their daily diet.
Fibrous foods are often bulky and, therefore, filling. They also tend to be low in fat. Soluble fibre forms a gel that slows down the emptying of the stomach and the transit time of food through the digestive system. This extends the time a person feels satisfied or ‘full’. It also delays the absorption of sugars from the intestines. This helps to maintain lower blood sugar levels and prevent a rapid rise in blood insulin levels, which has been linked with obesity and an increased risk of diabetes.
The extra chewing time often required of high-fibre foods also helps contribute to feeling satisfied. As a result, a person on a high-fibre diet is likely to eat less food and so consume fewer kilojoules (calories).
Fibre and diabetes
For people with diabetes, eating a diet high in fibre slows glucose absorption from the small intestine into the blood. This reduces the possibility of a surge of insulin, the hormone produced by the pancreas to stabilise blood glucose levels.
Conditions linked to low-fibre diets
Eating a diet low in fibre can contribute to many disorders, including:
- Constipation – small, hard and dry faecal matter that is difficult to pass
- Haemorrhoids – varicose veins of the anus
- Diverticulitis – small hernias of the digestive tract caused by long-term constipation
- Irritable bowel syndrome – pain, flatulence and bloating of the abdomen
- Overweight and obesity – carrying too much body fat
- Coronary heart disease – a narrowing of the arteries due to fatty deposits
- Diabetes – a condition characterised by too much glucose in the blood
- Colon cancer – cancer of the large intestine.
Diet, cancer and heart disease
Studies that show a reduced risk of some cancers and coronary heart disease have received much attention. Increasing dietary fibre and wholegrain intake is likely to reduce the risk of cardiovascular disease, type 2 diabetes, overweight and obesity, and possible overall mortality.
It is also very likely that these observed health benefits occur indirectly, through the protective effects of ‘phytochemicals’ (such as antioxidants) that are closely associated with the fibre components of fruits, vegetables and cereal foods.
Studies have shown that dietary fibre, cereal fibre and wholegrains are protective against colorectal cancer. Fibre is thought to decrease the risk of colorectal cancer by increasing stool bulk, diluting possible carcinogens present in the diet and decreasing transit time through the colon.
In addition, bacterial fermentation of fibre results in the production of short-chain fatty acids, which are thought to have protective effects against colorectal cancer. A recent review found that each 10 g per day intake of total dietary fibre equates to a 10 per cent reduction in risk of colorectal cancer.
Ways to increase your fibre intake
Simple suggestions for increasing your daily fibre intake include:
- Eat breakfast cereals that contain barley, wheat or oats.
- Switch to wholemeal or multigrain breads and brown rice.
- Add an extra vegetable to every evening meal.
- Snack on fruit, dried fruit, nuts or wholemeal crackers.
A daily intake of more than 30 g can be easily achieved if you eat wholegrain cereal products, more fruit, vegetables and legumes and, instead of low-fibre cakes and biscuits, have nuts or seeds as a snack or use in meals.
You do not need to eat many more kilojoules to increase your fibre intake. You can easily double your fibre intake without increasing your kilojoule intake by being more selective.
A sudden increase in dietary fibre
It should be noted that a sudden switch from a low-fibre diet to a high-fibre diet can create some abdominal pain and increased flatulence (wind). Also, very high-fibre diets (more than 40 g daily) are linked with decreased absorption of some important minerals such as iron, zinc and calcium. This occurs when fibre binds these minerals and forms insoluble salts, which are then excreted.
This could increase the risk of developing deficiencies of these minerals in susceptible people. Adults should aim for a diet that contains 25 g to 30 g of fibre per day and should introduce fibre into the diet gradually to avoid any negative outcomes.
It is better to add fibre to the diet from food sources rather than from fibre supplements, as these can aggravate constipation, especially if you do not increase the amount of water you drink daily.[Health Facebook]