Main Dietary Components

CHAPTER 4 Main Dietary Components

By the end of this chapter you should be able to:

• describe structure and associated functions of carbohydrates, fats and protein

• summarize the main features of aerobic and anaerobic metabolism of carbohydrate, fat and protein

• explain the regulation of metabolism of these major dietary components

• explain mechanisms for integration of fat, carbohydrate and protein metabolism, and response to specific dietary circumstances

• explain the glycaemic index in the context of health, with reference to specific foods

• discuss the health effects of dietary fibre

• explain what is meant by nitrogen balance

• describe methods for determining protein requirements

• describe what is meant by water balance, how this is usually maintained and the effects of imbalance

4.1 INTRODUCTION

The human body has a remarkable ability to adapt to different dietary circumstances. This is because there are biochemical mechanisms in place to ensure that flux through particular metabolic pathways is the most appropriate for the circumstances at the time. Thus, following a meal there is likely to be a drive towards the storage of fuel, as carbohydrate or as fat, whilst during the overnight fast or during the period between meals the body can mobilize these stores to ensure that all cells receive the fuel they need for normal cellular function. For these processes to occur efficiently there needs to be communication between metabolic pathways occurring in different organs but also interaction between different pathways of metabolism of the major macronutrients. For convenience this chapter examines carbohydrate, fat and protein separately, but the reader should always bear in mind that these pathways have common intermediates and there is considerable interaction between the various pathways.

4.2 CARBOHYDRATE AND ITS METABOLISM

In Western diet carbohydrate contributes about 40% to energy intake, compared with 75% or more in developing countries. Carbohydrate can be classified according to the length of the constituent polysaccharide chain. Monosaccharides are single sugar units; they exist in one of two chemical forms: aldoses, which contain an aldehyde group, as in glucose and galactose; and ketoses, containing a ketone group, as in fructose (Fig 4.1). Monosaccharides can form chemical bonds with one another, to form disaccharides. Sucrose, which is commonly known as table sugar, consists of glucose bound to fructose; lactose, which is found in milk, consists of glucose bound to galactose, and maltose, which is found in sprouting grain, consists of two glucose units. The term oligosaccharide can be used for sugars with 3–9 monosaccharide units, and this group includes maltodextrins and fructo-oligosaccharides. Polysaccharides consist of more than 9 monosaccharide units joined together. This group of sugars includes starch and non-starch polysaccharides.

Figure 4.1 Common sugar structures.

Starch is a mixture of two polymers, amylose and amylopectin, and is found in foods of plant origin including potatoes and bread. Non-starch polysaccharides include cellulose and pectins, neither of which is digestible by humans. Monosaccharides, disaccharides and maltodextrins are readily digestible in the small intestine and referred to as ‘digestible’. Non-starch polysaccharides are ‘indigestible’ or ‘unavailable’ and pass to the large intestine, providing substrate for the colonic microflora; this group of carbohydrates is also referred to as dietary fibre. There are three classes of carbohydrate in dietary fibre: non-starch polysaccharide (NSP), resistant starch (RS) and resistant oligosaccharides (ROS). The main constituent is NSP, which is found in plant cell walls. Resistant oligosaccharides include polymers of fructose and galacto-oligosaccharides from legumes.

Dietary carbohydrate can also be classified by the glycaemic index (GI), which is a measure of the extent to which blood glucose concentration is raised compared with an equivalent amount of a reference carbohydrate (glucose or white bread). Carbohydrate with a high GI generally provokes a higher secretion of insulin than carbohydrate with a low glycaemic index. The main glycaemic carbohydrates are glucose, fructose, sucrose, lactose and starch. Foods with a low GI generally have a high RS content, but some foods, such as bread and cornflakes, have quite a high GI despite having a high RS content.

Glycaemic carbohydrates function primarily as an energy source because in the fed state most tissues use glucose as their main metabolic fuel. In addition, liver and muscle synthesize the polysaccharide glycogen as a storage form of carbohydrate. Carbohydrate in excess of requirements for immediate metabolism or synthesis of glycogen can be used in the synthesis of fatty acids and triacylglycerol. Carbohydrate also contributes to the synthesis of non-essential amino acids. The metabolism of glucose is a source of pentoses, which are essential components of the nucleic acids RNA and DNA, and of glucuronic acid, which is required for the conjugation of bile salts. Carbohydrate can form complex molecules with fat or protein to form glycolipids, amino sugars such as glucosamine, and glycoproteins such as albumin and collagen.

The metabolism of glycaemic carbohydrates

Figure 4.2 gives an overview of carbohydrate metabolism integrated with fat metabolism. The first stage is glycolysis, which is the anaerobic oxidation of glucose to pyruvic acid (pyruvate), and takes place in the cytoplasm of all cells. Glycolysis yields a very modest amount of energy but because it can occur in the absence of oxygen it is important in muscle during intense exercise. Pyruvate formed from glycolysis can be oxidized further by crossing into the mitochondria for entry into the tricarboxylic acid (TCA) cycle, which generates much more energy than glycolysis. The glycolytic pathway also provides a route for the metabolism of fructose, galactose and glycerol. The metabolism of galactose and fructose occurs in the intestinal mucosa and liver, so little fructose or galactose reaches the peripheral circulation. In cells lacking mitochondria, such as red blood cells, or when conditions in a cell become anaerobic (such as in an intensely active muscle), pyruvate is reduced to lactate, which leaves the cell and goes to the liver where it can be reconverted to pyruvate. The pentose phosphate pathway (also known as the hexose monophosphate shunt) is an alternative cytoplasmic pathway for the anaerobic oxidation of glucose. This is important because it yields useful products such as pentoses for RNA and DNA synthesis, and the reduced form of the important compound nicotinamide adenine dinucleotide phosphate (NADPH), which is essential for fatty acid synthesis.

Figure 4.2 Main routes of metabolism of carbohydrate and fat following a meal. Following a meal containing carbohydrate, fat and protein there is a drive to store fuel for later use. Glycogen is stored in liver and muscle whilst fat is stored mainly in adipose tissue, as triacylglycerol (TAG). There will be net protein synthesis.

The metabolic fate of pyruvate is determined by metabolic circumstances at the time. It may be reduced to form lactate under anaerobic conditions (described above), oxidized completely via the TCA cycle and the electron transport chain, used as a substrate for glucose synthesis or used as a substrate for fatty acid synthesis. Pyruvate thus fulfils an important role in the integration of the metabolism of fat, carbohydrate and protein. For pyruvate to undergo complete oxidation it passes into the mitochondria where it is initially converted to the compound acetyl CoA in a reaction dependent upon vitamins B₁ (thiamin), B₂ (riboflavin), and niacin, which act as cofactors. Acetyl CoA then enters the TCA cycle, which involves a series of reactions leading to the production of energy. Importantly, as the TCA cycle is an oxidative process it also yields reducing power as the reduced form of the nucleotides nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). These molecules are central to the processes of energy generation from the oxidation of fuel molecules, and to the synthesis of complex molecules from simple precursors. The reduced NAD and FAD enter the electron transport chain, which involves the successive reduction and oxidation of intermediates, with the overall production of substantial amounts of energy through a process of oxidative phosphorylation. Thus, the complete oxidation of glucose, fructose or galactose yields energy and important intermediates that link carbohydrate metabolism with fat and protein metabolism.

Glycogen as a carbohydrate store

Glycogen is a branched polymer of glucose. In the fed state, glycogen is synthesized from glucose in both liver (50–150 g) and muscle (350–400 g), through the stepwise addition of glucose to an existing glycogen molecule. The branched structure of glycogen means that it binds a lot of water within the molecule. In the early stages of food restriction, such as during an overnight fast, or between meals, the body uses glycogen as a fuel source (Fig 4.3), and there is an associated excretion of this bound water. This leads to an initial high rate of weight loss but it cannot be sustained because once glycogen has been depleted the rapid loss of water (and weight) will cease. In the fasting state, glycogen is broken down in the muscle and the liver. In the liver this process leads to the formation of glucose, which can be exported to other tissues. In contrast the muscle is unable to carry out the final step in the breakdown of glycogen to glucose, meaning that muscle is unable to export glucose for use by other tissues but can use the phosphorylated glucose it generates in glycolysis.

Figure 4.3 Main routes of metabolism in the period between meals and during an overnight fast. In the absence of fuel from the gut the body mobilizes its fuel stores. There is net breakdown of glycogen in liver and muscle and of triacylglycerol (TAG) in adipose tissue. Amino acids from protein turnover will be directed towards glucose synthesis in liver and the renal medulla.

Gluconeogenesis

Cells in the liver and kidney medulla are able to synthesize glucose in a process called gluconeogenesis. This becomes particularly important during times of fasting or on very low carbohydrate diets. This process is largely a reversal of glycolysis except for three irreversible steps in glycolysis, for which alternative reactions occur. Many of the products of amino acid metabolism can be used for gluconeogenesis, since they are sources of pyruvate or intermediates in the tricarboxylic acid cycle. Substrates that give rise to acetyl CoA directly (alcohol, fatty acids, ketone bodies and ketogenic amino acids) cannot be substrates for gluconeogenesis, but glycerol, which is produced from the hydrolysis of triacylglycerol in the fasting state, can (Fig 4.3).

The control of carbohydrate metabolism

Energy expenditure is relatively constant throughout the day, but in humans most of the daily food intake typically occurs in two or three meals. There is therefore a need for metabolic regulation to ensure that there is a reasonably constant supply of metabolic fuel to tissues, regardless of the variation in intake. There is a particular need to regulate carbohydrate metabolism since the nervous system is largely reliant on glucose as its metabolic fuel, and red blood cells and the kidney cortex are entirely so. The plasma concentration of glucose is maintained in short-term fasting by the mobilization of liver glycogen, and by releasing free fatty acids from adipose tissue. Should the fasting state extend beyond about 12 hours gluconeogenesis becomes very important.

Hormonal control of carbohydrate metabolism in the fed state

Carbohydrate metabolism is regulated by the co-ordinated action of hormones, enzymes and metabolic intermediates, and in a healthy person the concentration of glucose in the peripheral circulation is maintained at about 4–6 mmol/L.

During the 3–4 hours after a meal glucose, galactose and fructose will enter the portal blood following carbohydrate digestion and amino acids will enter the portal blood following protein digestion. Under these conditions glucose is the main fuel for tissues. The increased concentration of glucose and amino acids in the portal blood stimulates the β-cells of the pancreas to secrete the hormone insulin, and suppresses the secretion of the hormone glucagon by the α-cells of the pancreas. Insulin has four main actions:

• increased uptake of glucose into muscle and adipose tissue

• stimulation of the synthesis of glycogen from glucose in both liver and muscle

• stimulation of fatty acid synthesis in adipose tissue

• stimulation of amino acid uptake into tissues, leading to an increased rate of protein synthesis.

Hormonal control of carbohydrate metabolism in the fasting state

In the fasting state (sometimes known as the post-absorptive state, since it begins about 4–5 hours after a meal, when the products of digestion have been absorbed) metabolic fuels enter the circulation from the reserves of glycogen, triacylglycerol and protein laid down in the fed state. The metabolic problem in the fasting state is that the brain is largely dependent on glucose as its metabolic fuel, and red blood cells cannot utilize any metabolic fuel other than glucose. As the concentration of glucose and amino acids in the portal blood falls, so the secretion of insulin by the pancreas decreases and the secretion of glucagon increases. Glucagon has two main actions:

• stimulation of the breakdown of liver glycogen

• stimulation of the synthesis of glucose from amino acids in liver and kidney.

At the same time, the reduced secretion of insulin results in:

• a reduced rate of glucose uptake into muscle and adipose tissue

• a reduced rate of protein synthesis, so that the amino acids arising from protein catabolism are available for gluconeogenesis

• relief of the inhibition of lipid breakdown in adipose tissue, leading to release of free (non-esterified) fatty acids into the circulation.

The net effect is an increase in the provision of glucose for the brain and RBCs and an increased availability of fatty acids for oxidation by other tissues.

Dietary fructose metabolism

High intakes of fructose cause increased plasma concentrations of triacylglycerol. This is because fructose metabolism in the liver bypasses phosphofructokinase, which is the major control point for glycolysis, so that more fructose enters the pathway than is required for energy-yielding metabolism. The resultant acetyl CoA is used for synthesizing lipid.

Functions and metabolism of non-digestible carbohydrates (dietary fibre)

Carbohydrates that are not digested in the small intestine are subject to anaerobic fermentation by the colonic microflora. Fermentation products, mainly short-chain fatty acids, are now known to play an important role in colon health. Short-chain fatty acids (SCFA) such as acetate, propionate and butyrate, and gases, notably hydrogen and methane, are the main fermentation products. Butyrate is a major source of energy for colonocytes with effects on cell differentiation and programmed cell death (apoptosis) that may be protective against cancer. Propionate inhibits liver cholesterol synthesis in experimental animals, but the importance in humans is not yet clear. Viscous types of soluble fibre that have a high water-binding capacity may inhibit gastric emptying, and also have beneficial effects on both lipid and carbohydrate metabolism, including a lowering of plasma total and LDL-cholesterol and a lowering of postprandial glucose and insulin response.

Carbohydrate malabsorption

In certain individuals malabsorption of dietary carbohydrates may cause excessive delivery of fermentable substrate to the large intestine, causing intolerance. Congenital sucrase deficiency is a rare cause of sucrose malabsorption, which can cause gastrointestinal complaints, including diarrhoea, when an increasing amount of sucrose is introduced in the diet. Similarly, congenital lactase deficiency is a rare condition causing severe diarrhoea and malnutrition in the newborn. Breast milk has a high content (7%) of lactose.

Carbohydrate and disease

Blood glucose concentration is determined by three main factors: the rate of intestinal carbohydrate absorption, the net liver uptake or output, and glucose uptake by other tissues, which in turn depends upon blood insulin concentration and the sensitivity of tissues to insulin. With a constant dietary carbohydrate load, there is a range of change in blood glucose concentration in individuals; large increases indicate impaired glucose tolerance. The change in blood glucose after a meal is determined by both the glycaemic index (GI) of the meal and the amount of carbohydrate consumed. The protein and fat content and the amount of water taken with the meal also influence the glycaemic response. Excessive postprandial glycaemia may be related to increased all-cause mortality in diabetics as well as in people with normal fasting blood glucose concentration.

There is evidence from both epidemiological studies and intervention studies that diets rich in dietary fibre are healthy, and may reduce the risk of coronary heart disease and certain cancers, as well as reducing the risk of obesity. Viscous types of dietary fibre, such as pectin, guar gum and oat β-glucans lower serum cholesterol, but it is not known to what extent the protective effects of whole grain cereals and fruits are due to dietary fibre, to other constituents of these foods, or to other diet or lifestyle-related factors associated with the consumption of whole grain foods and fruits and vegetables.

• Carbohydrates constitute the main energy source in the diet

• The main monosaccharides are all metabolized to pyruvate, or lactate under anaerobic conditions

• Pyruvate can be converted to acetyl CoA which can be oxidized further in the TCA cycle or used in the synthesis of fatty acids

• The main storage carbohydrate in the body is glycogen; its synthesis in the fed state and utilization in the fasting state are closely regulated by insulin and glucagon

• The brain has a particular requirement for glucose, and during fasting glucose can be synthesized from non-carbohydrate precursors

• The glycaemic index denotes the potential of a food to elevate blood glucose. Low glycaemic index foods may confer benefits in metabolic control of diabetes

• Carbohydrates that are not digested in the small intestine are substrates for fermentation in the colon, with probable health benefits

4.3 FAT AND ITS METABOLISM

Introduction

The main form of dietary fat is triacyglycerol (TAG) in which three fatty acids are bound to a molecule of glycerol (Fig 4.4). It is in this form that fat is stored, predominantly in adipose tissue. Phospholipids are similar in structure, but have a phosphate group and an additional base (choline, inositol, serine) in place of one of the fatty acids. Together with cholesterol, phospholipids are important constituents of cell membranes. Fatty acids consist of hydrocarbon chains with a carboxylic acid group at the head. Typically a fatty acid has between 12 and 22 carbon atoms and may contain no double bonds (saturated fatty acid) or one (monounsaturated) or more (polyunsaturated) double bonds (Fig 4.5). The hydrogen atoms at the carbon–carbon double bond may be on the same side of the double bond (cis) or on opposite sides (trans) (Fig 4.6). Trans fatty acids are absent from natural plant lipids but may be found in some animal fats such as milk fat. Trans fatty acids may also be formed during the processing of some fats for incorporation into foods. High intakes of trans fatty acids have been associated with an increased risk of cardiovascular disease.

Figure 4.4 Structure of triacylglycerol (TAG) and phospholipids. Triacylglycerol (A) consists of a glycerol backbone esterified to three fatty acids. In phospholipids (B) carbon-3 glycerol is esterified to phosphate, then to one of a number of water-soluble bases shown in (C).

Figure 4.5 Fatty acid structures.

Figure 4.6 Distribution of atoms at a carbon–carbon double bond: (A) cis and (B) trans configuration.

TAG molecules constitute an excellent storage form of fat because they are hydrophobic and can therefore be packed very densely. Following oxidation fat generates 9 kcal (36 kJ) energy per gram, which is considerably more than carbohydrate or protein and makes fat especially valuable as a fuel. Fat is stored in adipose tissue in the fed state and adipose tissue fat stores are mobilized in the fasted state.

Functions of dietary fat

Fat plays diverse roles in human nutrition. In addition to its importance as a source of energy, both for immediate utilization by the body and as a store for later utilization when food intake is reduced, dietary fat acts as a vehicle for the absorption of fat-soluble vitamins. Phospholipids and cholesterol are components of cell membranes and cholesterol is the precursor for synthesis of adrenocorticoid and sex hormones. Long chain (C20 and C22) polyunsaturated fatty acids in membrane phospholipids are the precursors of prostaglandins and other eicosanoids, compounds that have important local hormone effects.

Energy storage

The majority of fat is stored as TAG in the cells of adipose tissue. Following a meal, ingested fat which is not required by the body tissues for immediate use is transported to the adipose tissue in lipoproteins. The fatty acids are hydrolysed from the TAG in circulating lipoproteins by the enzyme lipoprotein lipase (LPL), taken up by the adipose tissue and re-esterified into triacylglycerol. Conversely, when dietary energy supply is limited, such as after an overnight fast, the fat in adipose tissue is mobilized and fatty acids are released into the circulation, bound to serum albumin. These fatty acids enter other tissues and are oxidized for energy production. This process is tightly regulated by the concentration of metabolites (glucose, fatty acids, TAG) in the blood and by hormones (insulin, glucagon, adrenaline). In addition to its role as an energy store, subcutaneous adipose tissue is important in the maintenance of body temperature, whereas internal fat (visceral fat) protects the vital organs such as the kidney and spleen. Accumulation of excessive visceral fat (abdominal obesity) is a risk factor for heart disease and diabetes and is linked with insulin resistance.

The structural role of fat in cell membranes

The basic structural unit of most biological membranes is phospholipid, although sphingolipids, which are based on a sphingosine rather than a glycerol backbone, are also widespread in membranes, and are particularly abundant in the brain and nervous system. The chain length and degree of unsaturation of the fatty acids within the membrane have a large impact on membrane physical properties. Dietary fatty acid composition has been shown to influence the composition of fatty acids in cell membranes. Cholesterol is also a component of cell membranes, in which its interactions with fatty acids are essential to maintain membrane structure and fluidity.

Membrane fats in cell signalling

Various lipids are involved in cell signalling and the conversion of extracellular signals into intracellular ones. Membrane phospholipids are important mediators of hormone and neurotransmitter action and as such are involved in regulation of cellular processes such as smooth muscle contraction, glycogen metabolism and cell proliferation and differentiation. Membrane sphingolipids are important modulators of cell growth and death, and of inflammatory responses.

Fat as precursors of the eicosanoids

The two dietary essential fatty acids, linoleic and α-linolenic acid, are precursors of long chain polyunsaturated fatty acids (PUFAs), including eicosapentaenoic acid (EPA) and arachidonic acid (AA). These two PUFAs (AA being the most important) are the precursors of a group of hormone-like compounds called eicosanoids, including prostaglandins, thromboxanes and leucotrienes, which are important for a wide range of processes including inflammatory responses, blood clotting and smooth muscle contraction.

Fat transport and metabolism

As TAG, cholesterol and phospholipids are not water-soluble, they cannot be transported free in the blood, but are carried in lipoproteins. Lipoproteins are particles which transport lipids between tissues. They have a hydrophobic core of TAG and cholesterol esters and a hydrophilic surface consisting of phospholipids and free cholesterol. Lipoproteins also contain specific proteins, apoproteins, which determine how the lipoprotein is metabolized. Apoproteins recognize and interact with specific receptors on the cell-surface, and the receptor–lipoprotein complex is taken into the cell. Apoproteins also determine the activities of a range of proteins, including hydrolysing enzymes (lipases), receptors and lipid transfer proteins, which are involved in all stages of lipoprotein metabolism.

Lipoprotein classes

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Tags: Fundamentals of Human Nutrition for Students and Practitioners

Jun 13, 2016 | Posted by admin in ENDOCRINOLOGY | Comments Off

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