Dietary fibre

WHAT IS DIETARY FIBRE?

Dietary fibre used to be known as ‘roughage’. It comprises the edible parts of plants that are not broken down and absorbed in the human small intestine. Types of dietary fibre may also be categorised according to their physiological effects; helping to prevent constipation or helping to control blood cholesterol and glucose levels.

Depending on the type of fibre present in food, it is partly or completely fermented, by gut bacteria in the large intestine, producing gases (carbon dioxide, methane and hydrogen) and short chain fatty acids (butyrate, acetate and propionate). Gut bacteria are essential for the health of the large bowel. The short chain fatty acids are absorbed into the cells of the gut wall, where they can be used as fuel, or pass on into the blood stream. After increasing the amount of fibre in the diet, some people experience symptoms such as abdominal distension, discomfort and wind. However, the large intestine and gut bacteria usually quickly adapt to the increased intake and problems with wind usually decrease.

Types of dietary fibre usually include non-starch polysaccharides, oligosaccharides, lignin and associated plant substances. These are traditionally found in cereal foods, beans, lentils, fruit and vegetables. The term dietary fibre also includes a type of starch known as resistant starch, which resists digestion to its component sugar, glucose, in the human small intestine and passes unchanged into the large intestine. A number of types of resistant starch exist naturally and these are chiefly found in whole or partly-milled grains and seeds, pulses, some breakfast cereals e.g. corn flakes, and cooked and cooled (retrograded) potatoes. Commercially manufactured sources of resistant starch that are colourless, odourless and bland in flavour are now available. These may be used by food manufacturers to increase the total fibre content of a range of foods without affecting taste and texture.

Dietary fibre may also be broadly described according to its solubility. There are two broad types of fibre: insoluble and soluble. Oat, fruits, vegetables and pulses (beans, lentils, chickpeas) are good sources of soluble fibre. Wholegrain cereals and wholemeal bread are particularly good sources of insoluble fibre. Because insoluble and soluble fibre are found in different proportions in fibre-containing foods and have different properties, it is important to eat a variety of fibre-containing foods.

HEALTH BENEFITS (PHYSIOLOGICAL EFFECTS) OF EATING FIBRE

Both soluble and insoluble dietary fibre can promote a number of positive physiological effects, helping to prevent constipation, lower blood cholesterol levels and control blood glucose levels. Sources of insoluble fibre can act as bulking (laxative) agents and help prevent constipation. For fibre to have the best effect on preventing constipation, an increase in fibre intake should be accompanied by an increase in water intake. Soluble forms of fibre eaten in large amounts can help reduce blood cholesterol levels. They can also help people with diabetes to control their blood glucose levels.

On the other hand, eating a diet low in fibre is associated with diverticulitis (where the bowel wall becomes inflammed and ultimately damaged) and bowel (colorectal) cancer. Increasing evidence points to a protective effect of eating a diet rich in dietary fibre on diverticulitis and colo-rectal cancer. However, it is unclear whether this is always a direct effect of the fibre or whether it is because a higher fibre content is a characteristic of a healthier diet. In other words, it may also be that people who eat plenty of fibre follow other dietary and lifestyle advice such as to eat less fat, eat plenty of fruit and vegetables, drink plenty of fluid and do more exercise.

HOW MUCH FIBRE DO WE EAT?

Most people do not eat enough dietary fibre. The average intake in Britain is 12g per day and it has been recommended that this rises to an average of 18g per day for adults. Children need proportionally less. For pre-school children, however, introduction of fibre-rich foods should be done gradually. Too much fibre can make the diet so bulky that young children become full before they have eaten sufficient food to satisfy their need for essential vitamins, minerals and energy. The major sources of dietary fibre in the British diet are whole-grain and high-fibre breakfast cereals and bread (because they are rich in fibre and are eaten frequently).

A diet rich in fibre is usually lower in fat and contains more starchy foods, fruit and vegetables. Very high intakes of dietary fibre can reduce absorption in the small intestine (see figure) of some minerals from food as these are bound by the fibre in insoluble complexes. However, fermentation in the large intestine can release some of the bound minerals so they can eventually be absorbed into the blood stream. The amount of vitamins and minerals lost through eating a diet rich in fibre is not likely to be significant unless an individual’s diet is already poor.

Source: British Nutrition Foundation