A carbohydrate is an organic compound comprised of carbon, hydrogen and oxygen molecules. Carbohydrates are saccharides, easily divisible into four groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. These groups vary in degree of complexity. The monosaccharides and disaccharides form simple carbohydrates (refined sugars) (Flitsch & Ulijn, 2003), whereas the oligosaccharides and polysaccharides are more complex (Barasi, 2007). Monosaccharides are the simplest type of carbohydrate; examples include glucose, fructose, lactose and ribose. Monosaccharides are the precursors for the disaccharides, such as sucrose, and polysaccharides such as starch. Polysaccharides function as energy stores (e.g. starch and glycogen) and as structural component of cells (e.g. cellulose and chitin). Oligosaccharides contain a small number of simple sugars and often remain undigested until they reach the colon where they undergo fermentation serving as a food source for intestinal microflora (Barasi, 2007).
Saccharides and their associated derivatives include many important biological molecules such as the monosaccharide ribose, which is a component of RNA, and deoxyribose, a component of DNA (Flitsch & Ulijn, 2003). Some carbohydrates resist digestion and comprise the non-starch polysaccharides which are part of the dietary fibre which aids in the function of the gastrointestinal tract (Barasi, 2007). In terms of food consumption, complex carbohydrates such as
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cereals, bread, and pasta contain starch, whereas the simple carbohydrates, found in sweets and jams, contain refined sugars.
1.2.2.1 Carbohydrate Metabolism
The most utilised of the carbohydrates is the monosaccharide glucose, which can be successfully metabolised by all organisms. Simple carbohydrates, such as refined sugars, can be broken down in cells. More complex carbohydrates, such as the disaccharide sucrose, are broken down in the small intestine by enzymes specific to the sugar which split the chain releasing simple sugars. In the instance of sucrose, this is broken down by sucrase. If sucrase is not secreted, then intolerance will develop which can lead to malabsorption and diarrhoea. Another carbohydrate example is the polysaccharide starch, which is metabolised by the enzyme amylase, present in saliva of the mouth. This enzyme breaks the starch into amylose which is then further digested in the duodenum into maltose and glucose (Barasi, 2007).
There are other carbohydrates, such as cellulose, that humans cannot digest as they do not produce the enzymes necessary for the process. Carbohydrates are an instant fuel source because they are easier to metabolise than fats and protein. For all mammals, glucose is the most important carbohydrate, and the brain is fuelled primarily by this sugar. The concentration of circulating plasma glucose is involved in the control of insulin concentrations (Barasi, 2007).
1.2.2.2 Glucose Regulation
When carbohydrates are metabolised, glucose is transported from the small intestine across the apical membrane and into the bloodstream by a twostep process. Firstly, glucose transporter proteins, present in cell membranes, facilitate the movement of glucose down its concentration gradient from the lumen of the small intestine into the apical cells. Secondly, glucose moves from the apical cells into the bloodstream via facilitated diffusion (Barasi, 2007). This process allows for the maintenance of steady concentrations of glucose in the body, but this can also be helped by choosing foods with certain glycaemic index. The glycaemic index provides an indication of how blood glucose concentrations change after the consumption of different carbohydrates. Essentially, glycaemic index is a measure of how quickly food glucose is absorbed. Lower glycaemic index foods are preferable in situations where
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an extended period of glucose release is necessary, for example to regulate diabetic control, or for athletes in training for sporting events (Barasi, 2007).
1.2.2.3 Dietary Treatment of Type 2 Diabetes
As mentioned previously, type 2 diabetes is a disease increasing in prevalence due to the rise in the number of obese individuals. This disease results from a dysfunction
in carbohydrate metabolism as characterised by hyperglycemia (McGeoch et al.,
2011). The importance of glycaemic control in the prevention of chronic complications associated with diabetes was demonstrated by the UK Prospective
Diabetes Study (Stratton et al., 2000). The consumption of carbohydrates is one of
the main factors affecting glycaemic control and both the quantity, and quality (as indicated by the glycaemic index), of the ingested carbohydrate has an effect on diabetic control (Jenkins et al., 1981). The regular consumption of foods with a high glycaemic index has been linked with the development of several conditions such as diabetes, obesity and cardiovascular disease, whereas low glycaemic index foods are recommended as preventative measures and even treatments (McGeoch et al., 2011). These diets, low in glycaemic index, tend to have a higher amount of fibre which aids in the absorption of dietary cholesterol, contributing to a greater release of satiety signals and reducing the likelihood of feeling hungry, thus helping to maintain body weight control. In patients with type 2 diabetes, this results in a lower insulin discharge which helps to maintain a normalised, steady, blood glucose
concentration (Jenkins et al., 2002 Willett et al., 2002; Colombani, 2004).
Additional macronutrients which effect body weight regulation are described in the following sections.
1.2.3 Fatty Acids
The consumption and composition of dietary fatty acids has been the subject of extensive research because of the involvement of fats in the development of diseases such as obesity, diabetes and cardiovascular disease. Fats are essential in the diet to provide a source of energy, structural components within the body and as a constituent of metabolic pathways. Most dietary fats take the form of triglycerides (TGs), which contain three fatty acids (FAs) attached to a glycerol backbone. These fatty acids are the main components of dietary lipids. Their basic structure comprises a carbon backbone, with a carboxyl (-COOH) group at one end and a methyl group (-
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CH3) at the other end. The FAs differ from one another in their chain length and number of double bonds (degree of saturation). These variations determine how they are metabolised and therefore, their effects on health. Specific FAs are important for cell membrane structure and function. There are several general categories of these fatty acids; these include the saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) (Moussavi et al., 2008).
A FA nomenclature system exists as follows: number of carbon atoms, number of double bonds, followed by location of first double bond (carbon atom counting from the methyl group end); e.g. 18:1 (n-9) (oleic acid) is an 18-carbon chain with one double bond located at the 9th carbon from the CH3 end; n is
sometimes written as omega (ω); e.g. n-3 = ω-3. Additional examples can be seen in
Table 1.1.