Contemporary food issues - nutrition

Fibre

Fibre consists of groups of complex carbohydrates which resist the enzymes of your digestive system but contribute to intestinal health, so it is sometimes classified as an active non-nutrient.

Types of fibre

Each type of fibre has a different molecular structure. Some are insoluble and some are called soluble, although they don’t completely dissolve but form a gel in water.

The solubility factor generally determines the role of fibre in your digestive system.

Insoluble fibre

Insoluble fibres are:

  • cellulose – the main component of plant cell walls; consists of longchain glucose molecules; found in all plants especially cereals, wholegrain breads, and vegetables

  • hemicelluloses – the main fibre in cereals; consists of branching chains of various monosaccharides; some are insoluble, for example in wheat and corn, others are soluble

  • lignin – the tough woody parts of small seeds and older vegetables such as carrots; consists of a complex non-polysaccharide molecule.

Soluble fibre

Soluble fibres include:

  • pectins common in fruits and vegetables, and most abundant in citrus fruit and apples; contain carbohydrate derivatives and various monosaccharides; pectins readily form gels in water

  • hemicelluloses with less complex molecules are soluble, for example in bran from oats, psyllium and barley

  • gums and mucilages are common in plants, gums secreted from injury sites; composed of various monosaccharides and related compounds; rich sources are legumes (including peanuts) and fruits; used as stabilizers and thickeners in food manufacture

  • resistant starch, which resists enzymatic digestion, either because:

    • it is too compacted to be broken down by digestive enzymes, such as in raw potatoes or in specially bred strains of maize

    • it is trapped inside a cellulose coating which enzymes can't penetrate, as in whole or coarsely-ground seeds and cereals

    • it has become less digestible after cooking and cooling, as in bread and cold cooked potatoes and cereals.

Role of fibre in the diet

Insoluble dietary fibres generally act to:

  • accelerate the passage of food through the gastro-intestinal tract

  • promote bowel movements

  • slow down starch digestion and glucose absorption.

Soluble fibres generally act to:

  • delay the passage of food through the intestines

  • delay glucose absorption

  • bind with bile acids in the intestines

  • lower blood cholesterol.

The exceptions include: insoluble rice bran, which also lowers blood cholesterol; soluble fibre, which includes resistant starch, does not lower cholesterol and; soluble hemicelluloses and mucilages of psyllium, which act like insoluble fibre in promoting bowel movements, but also reduce cholesterol like soluble fibre.

The positive effects of a high-fibre intake include a contribution to:

Fibre in the gastro-intestinal tract absorbs large amounts of water. This produces soft, bulky faeces in the large intestine and combined with the accelerating action of insoluble fibres and psyllium, results in easy bowel movements.

Constipation is prevented or alleviated, so there is much less risk of diverticular disease, haemorrhoids and appendicitis. The intestinal lining remains healthy, and able to prevent harmful compounds from entering the bloodstream.

The contribution of soluble fibres and rice bran to heart health is well established. Part of the effect could be because foods high in fibre don't naturally contain much fat or any animal fat, but research has shown a beneficial effect of fibre regardless of fat intake.

The benefits are mainly due to the way soluble fibres bind to bile acids.

Normally, bile enters the intestine to assist in fat digestion, and is then reabsorbed, taken to the liver and changed to cholesterol. When bile binds to fibre, it’s excreted so cholesterol production is reduced, and also less fat is digested and absorbed. The result is seen in lower levels of blood cholesterol, and a healthier cardiovascular system.

Fibre-rich foods help to maintain normal weight because they are bulky, satisfy the appetite, take longer to eat, delay hunger and add little to the energy intake because they are low in fat and generally free from added sugars. The fatty acids produced by fermentation of fibre in the colon are only partly absorbed and have a low energy value.

All fibres retard glucose absorption, which moderates the rapid increase in blood sugar levels associated with the onset of type-2 diabetes. The role of fibre-rich foods in weight control adds to diabetes protection.

Fibre and food manufacture

Food processing/manufacturing has traditionally resulted in fibre depletion as grains are refined, fruit is peeled and insoluble fibre is softened and degraded by high heating processes such as canning.

The recent consumer demand for healthy food has acted to reverse this trend. The range of food products containing fibre continues to grow with more wholegrain and multi-grain cereal products available to provide insoluble fibre and plant gums, mucilages, pectins and resistant starch functioning as stabilisers and thickeners in many foods, making soluble fibres available in some animal food products.

List at least five examples of foods which could feature in a fibre-rich diet.

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Exercise 2

  1. Outline the main health benefits of a wide range of fibre-rich foods in your diet.

  2. Refer to your three-day food diary and list the foods which supplied you with good sources of:

    1. soluble fibre

    2. insoluble fibre.

  3. Describe four changes you could make to your regular diet to increase natural sources of both types of fibre.