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:

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

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