Читать книгу The Impact of Nutrition and Diet on Oral Health - Группа авторов - Страница 12

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 digestive processes to break down the proteins further. The pancreas releases most of the digestive enzymes, including the proteases trypsin, chymotrypsin and elastase, which break complex proteins into smaller individual amino acids. The amino acids are then transported across the intestinal mucosa to different tissues of the body where they are either used in replacing damaged tissues or in the synthesis of proteins. Excess amino acids may be converted by liver enzymes into keto acids, which are used as sources of energy via the citric acid cycle, or converted into glucose or fat for storage, and urea which is excreted in urine and sweat.

All cells in the body contain proteins; certain hormones such as insulin and glucagon, as well as antibodies and almost all enzymes, are proteins. Proteins transport nutrients and oxygen in the blood and also help maintain the acid-base balance of blood and tissue fluids.

Proteins are essential for growth and repair and maintenance of health. However, a number of health concerns are associated with protein originating primarily from animal sources including: cardiovascular disease, due to the high saturated fat and cholesterol associated with animal proteins, and bone health, from bone resorption due to sulphur-containing amino acids associated with animal protein [11]. Generally, the quality of proteins in the diet depends on their constituent amino acids. Compared with plant proteins, the nutritional value of animal proteins is higher because the distribution of amino acids in animal cells is similar to that in human cells [11]. Low intake of animal-sourced proteins during late pregnancy is believed to be associated with low birth weight [12]. Meat-based diets have also been shown to cause a significantly greater net protein synthesis and greater gains in lean body mass compared to vegetarian diets, which could be a function of reduced breakdown of protein with the former [13].

Fats and Lipids

In biology, lipids have been loosely defined as a group of organic compounds that are insoluble in water but soluble in non-polar solvents. Contrasting with carbohydrates, lipids are not polymers but smaller molecules extracted from the tissues of plants and animals [8]. Dietary fat includes all the lipids in plant and animal tissues that are eaten as food. Meats and dairy foods are the most obvious sources of fat, but most foods contain some fat. Vegetable sources rich in dietary fat are nuts and seeds, olives, peanuts and avocados.

Although lipids cover an extremely heterogeneous collection of molecules from a structural and functional perspective, all lipids have a polar, hydrophilic “head” and a non-polar, hydrophobic, hydrocarbon “tail.” According to their insolubility in water, lipids are commonly categorised into 3 major groups of simple, compound and miscellaneous lipids in some nutrition text books [8] (Fig. 3). However, in the LipidBank database [14], lipids are classified based on their response to hydrolysis as: “simple lipids”, those yielding at most 2 distinct types of compound upon hydrolysis (e.g., acylglycerols: fatty acids and glycerol), “complex lipids” yielding 3 or more products upon hydrolysis (e.g., glycerophospholipids: fatty acids, glycerol and headgroup), and “derived lipids”, alcohols and fatty acids resulting from hydrolysis of simple lipids. However, the LIPID MAPS classification system has been created based on the concept of 2 fundamental “building blocks”: ketoacyl groups and isoprene groups [15]. Consequently, lipids are defined as small hydrophobic or amphipathic (or amphiphilic) molecules that may originate entirely or in part through condensations of thioesters and/or isoprene units [15]. Based on this classification system, lipids have been divided into 8 categories: fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids and polyketides (derived from the condensation of ketoacyl subunits); and sterol- and prenol-lipids (derived from the condensation of isoprene) [15].

Fatty acids are the key constituents of lipids in food and the body and are categorised into 3 types: saturated, monounsaturated and polyunsaturated, according to the number of carbons, the number of double bonds and the position of double bonds in the molecular chain. Based on the nutritional need, fatty acids are also categorised as essential and non-essential. The essential fatty acids are a-linolenic (a type of omega-3) and linoleic (a type of omega-6), which cannot be synthesised in the body and, therefore, must be obtained through the diet. The most prevalent form of dietary fat are the triglycerides, which are composed of 3 fatty acids and a glycerol molecule.

Fig. 3. Classification of lipids [8].

The digestion of dietary fat starts in the stomach as its churning action helps to form an emulsion. After entering the intestine, the partially emulsified fat is mixed with bile and is further emulsified. The emulsion is hydrolysed by lipases secreted by the pancreas, converting triglyceride to monoglycerides and free fatty acids which are then absorbed by the enterocytes of the intestinal wall. Fatty acids with a chain length of <14 carbons enter directly into the portal vein system and are transported to the liver; whereas fatty acids with 14 or more carbons are re-esterified within the enterocyte and enter the circulation via the lymphatic route as chylomicrons. Fat-soluble vitamins (vitamins A, D, E and K) and cholesterol are delivered directly to the liver as part of the chylomicron remnants [16]. Fatty acids are transported in the blood as complexes with albumin or as esterified lipids in lipoproteins.

From the endogenous fat, liver produces very-low-density lipoproteins (VLDL) which are the main carriers of triglycerides, supplying fatty acids to adipose and muscle tissues. The end-products of VLDL metabolism are low-density lipoproteins, which carry approximately 60–80% of cholesterol in plasma. High-density lipoproteins remove fat molecules (phospholipids, cholesterol, triglycerides, etc.) from the cells and tissues and transport them back to the liver [16].

Fats are an important component of diet, being the most energy-dense macronutrient. They are an essential component of cell membranes and internal fatty tissues that protect the vital organs from trauma and temperature change by providing padding and insulation. In recent years, lipid nutrition research has been focused on the role of specific fatty acids in the metabolism of cholesterol, lipoprotein and glucose. The type and amount of fatty acids in the diet have been shown to affect the plasma concentrations of low-density lipoprotein, VLDL, high-density lipoprotein, cholesterol and triglyceride [17], as well as insulin sensitivity and glucose metabolism [18]. In respect of human health, essential fatty acids are precursors to the formation of prostanoids, thromboxanes, leukotrienes and neuroprotectins, which in turn regulate key physiologic functions such as blood pressure, vessel stiffness/relaxation, thrombocyte aggregation, fibrinolytic activity, inflammatory responses and leukocyte migration [19].

Although early studies suggested that dietary saturated fats increase the risk of coronary artery disease, several recent analyses have shown that saturated fatty acids, particularly in dairy products, can improve health [20]; whereas the evidence of omega-6 polyunsaturated fatty acids (PUFAs) promoting inflammation and some diseases is growing. Oxidation of PUFAs, as well as sugars, produces various aldehydes which are known to initiate or intensify several diseases, such as cancer, asthma, type 2 diabetes, atherosclerosis and endothelial dysfunction [20].