Carbohydrate is a generic term used to describe starches and sugars. These are sources of energy in humans and play an important role in physical performance. Starches are produced by plants and comprise of long chains of sugar units, actually glucose, that can be either branched (amylopectin starch) or straight (amylose starch) in their nature. Fibre is also produced by plants and is composed of sugar units that includes glucose, joined in long chains. However, while dietary starch can be digested to its individual glucose units and used as a source of energy, the digestion of fibre is not possible with endogenously produced enzymes in humans. This makes dietary fibre a group of carbohydrates that are non-digestible by humans (although fibre can be degraded for energy in the colon through the action of colonic bacteria). Although not used directly for energy, fibre is increasingly being shown to be an essential dietary component to health because it may be required for the correct digestion of starch.
Refined carbohydrates, including white flour and refined crystalline sugars, are devoid of their original dietary fibre that accompany them in the parent plant material. Evidence suggests that consuming such refined carbohydrates may be a cause of Western lifestyle disease. Low fibre diets cause blood sugar changes because the absence of fibre alters the postprandial digestion and absorption rates of the carbohydrates and this negatively affects insulin sensitivity. High intakes of refined carbohydrates have been shown to be associated with an increased risk of cardiovascular disease, and this may be driven by their ability to increase insulin resistance and facilitate the development of the metabolic syndrome. For example, in one study1 carbohydrates and glycaemic load were found to be associated with ischaemic stroke and total stroke incidence in women. These association was highest in those with the highest body weights. In contrast, cereal fibre was inversely associated with haemorrhagic and total stroke.
Therefore fibre may protect from the detrimental effects of refined carbohydrates. In other words it is the removal of the fibre from the plant material that makes carbohydrates disease causing agents. While low carbohydrates diets are clearly beneficial at improving insulin resistance, blood lipid profiles and causing weight loss, it should be remembered that many of the positive effects of such diets may come about because they limit, by definition, refined carbohydrates. High carbohydrates can be healthy, however, the carbohydrates that make up healthy high carbohydrates diets must possess the original fibre that accompanied them in the plant material to limit any deleterious effects through changes in postprandial glycaemia. Refining the fibre from fruit to create fruits juice is a classic example of such detrimental effects. Most natural sources of sugars and starches are packaged with fibre, the exception being honey, and this is protected by angry bees, which limits its availability considerably.
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