and the proportions in which they should be consumed. These guidelines are based on human nutritional requirements and dietary recommendations which are termed Nutrient Reference Values in Australia [3], Dietary Reference Values in the UK [4, 5], and Dietary Reference Intakes in the USA [6]. These values are general terms comprising a series of estimates of the amount of energy and nutrients required by healthy individuals of different age groups (Table 1).
Table 1. Terms of reference values for food energy and nutrients for groups of people in Australia, the UK, and the USA
Nutrients are divided into 2 categories, macronutrients and micronutrients, based on the amount required by the human body for normal metabolism, growth, and physical well-being. The impact of specific nutrients on oral health is discussed in detail in Chapters 3–6.
Macronutrients
Carbohydrates
Carbohydrates are quantitatively the most important dietary energy source for most populations, usually contributing 55–75% of total daily energy requirements [7]. They are predominantly derived from plant foods, with grains and fruits as well as dairy products as the main dietary sources.
Carbohydrates are composed of carbon, hydrogen, and oxygen in a ratio of 1:1:2, respectively. In early nutrition text books, carbohydrates were classified into 2 groups: simple carbohydrates composed of monosaccharides (glucose, fructose and galactose) or disaccharides (sucrose, lactose and maltose), which are easily and quickly utilised for energy by the body, and complex carbohydrates (oligosaccharides and polysaccharides) which take longer to digest. Carbohydrates are also often classified into 3 groups: (i) monosaccharides, (ii) disaccharides and oligosaccharides, and (iii) polysaccharides [8]. However, based on a Food and Agriculture Organization and World Health Organization joint recommendation [9], dietary carbohydrates should be classified into 3 groups based on their chemical forms (Fig. 2), as determined by the degree of polymerisation, type of linkage (a or non-a) and character of individual monomers. The precise division between these groups is not quite helpful as the physiological and health effects of carbohydrates are also determined by their physical properties, which include water solubility, gel formation, crystallisation state, association with other molecules, and aggregation into the complex structures of the plant cell wall [9]. In any case, the importance of these categorisations is insignificant for determining the nutritional quality of carbohydrates; for example, fructose (a simple carbohydrate) increases blood glucose slowly, whereas processed starches (complex carbohydrates) raise blood glucose rapidly. Glycaemic index (GI) was therefore introduced to classify different sources of carbohydrate-rich foods according to their effect on post-meal glycaemia [10]. In this categorisation, carbohydrates are ranked on a scale from 0 to 100 based on how quickly and how much they raise the levels of blood glucose after consumption: low GI (0–55), medium GI (56–69) and high GI (70–100), where low-GI foods are those being digested and absorbed slowly and high-GI foods are rapidly digested and absorbed.
Fig. 2. Major dietary carbohydrates.
The metabolism of carbohydrates starts in the mouth with mechanical and chemical digestion; mastication grinds the food into smaller fragments and salivary amylase breaks down amylose and amylopectin into smaller chains of glucose, called dextrins and maltose. Since only about 5% of starch is broken down in the mouth, starchy foods are not major risk factors, unlike simple sugars, for dental caries. Carbohydrates are mainly digested in the small intestine where monosaccharides are absorbed into the blood stream. Insulin, glucagon, and epinephrine are hormones that control blood sugar concentrations. When blood glucose concentration is too high, insulin is secreted by the pancreas, which stimulates the transfer of glucose into the cells, especially in the liver and muscles. Almost 70% of the glucose entering the body through digestion is redistributed back into the blood, by the liver, to be used by cells and tissues or, in the case of excess, converted to glycogen and stored in muscles and the liver. In humans, the main functions of carbohydrates include the production and storage of energy. Many cells have a preference for using glucose as an energy source; in particular, the brain and white and red blood cells depend on glucose as their sole energy source.
Glucose is also required to build some important macromolecules: it is converted to ribose and deoxyribose, which are essential building blocks of ribonucleic acid (RNA), deoxyribonucleic acid (DNA) and adenosine triphosphate (ATP). In addition, glucose is used to make nicotinamide adenine dinucleotide phosphate (NADPH), an important molecule for protection against oxidative stress.
In a situation where there is insufficient carbohydrate or fat in the diet, protein is broken down to make glucose needed by the body. To spare protein for tissue synthesis, carbohydrates are therefore needed to prevent such protein breakdown for glucose production. Glucose is also required to prevent the development of ketosis, a metabolic condition resulting from a rise in the ketone bodies (acetoacetate, beta-hydroxybutyrate and acetone) in the blood, which are produced by the liver from fatty acids.
Proteins
Proteins are the most common nitrogen-containing compounds in the diet. While plant structures are mainly built on carbohydrates, proteins are vital structural and functional components within every cell of the body of humans and animals. Since most foods contain either animal or plant cells, they are natural sources of protein.
Proteins are made up of long chains of amino acids, linked by peptide bonds. The proteins in the human body are made from 20 different amino acids. Based on nutritional requirements, amino acids are categorised into 3 groups as: essential, semi-essential and non-essential. Essential amino acids are those that cannot be synthesised in the human body and, therefore, must be consumed through the diet. They are: methionine, threonine, tryptophan, valine, isoleucine, leucine, phenylalanine and lysine. Semi-essential amino acids, which cannot be synthesised in adequate amounts in the body and therefore require augmentation through the diet, include histidine and arginine, which are essential for children but not adults. The remaining non-essential amino acids can be synthesised in the liver from other amino acids.
All necessary amino acids should be available during the process of protein synthesis in the body. The sequence of amino acids governs the ultimate structure and function of any given protein and is regulated by a specific genetic code stored in the associated cell nucleus as deoxyribonucleic acid.
The digestion of proteins begins in the stomach when hydrochloric acid denatures proteins within food and the pepsin enzyme breaks down proteins into smaller polypeptides and their constituent amino acids. The digestion of protein continues in the small intestine by first neutralising the food-gastric juice mixture (chyme) as a result of sodium bicarbonate released by the pancreas, which also helps to protect the lining of the intestine. The released digestive hormones, including secretin and cholecystokinin, in the small intestine, stimulate
