1.1 Food and nutrients Flashcards
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Introduction
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This section has a focus on basic nutrition. It outlines the macro and micronutrients and highlights those that have key roles our immune systems.
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Objectives
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The objectives of this section are to: introduce basic concepts and terminology related to food, nutrition and assessment of nutritional status; and appreciate the role nutrients and nutrition play in the optimal functioning of the immune system.
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Structure of the session
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This section consists of three sessions. The first provides an introduction to nutrients in food basic and the second discussed different methods of nutritional assessment. The final session discusses the synergistic relationship between undernutrition, morbidity and mortality.
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Overview
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We know that it is important to eat well, that we need a variety of food in our diets to provide all the nutrients that our bodies need in order to grow, repair and function properly. We know that a bad diet can lead to malnutrition either via obesity & non communicable diseases (NCD’s) or undernutrition leading to immune incompetence & morbidity from disease.The nutrients are classified by how much bulk they provide in the diet: macro-nutrients - carbohydrate, fat, protein and micro-nutrients – vitamins and minerals
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Overview
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Daily nutrient requirements can be a useful tool, but we don’t typically eat ‘nutrients’ we eat ‘foods’. These days, more relatable terms, such as ‘the Mediterranean diet’ or ‘eat 5 a day’ fruit and vegetables are used and may be more suitable to the layman. This session is a brief overview of nutrition and outlines different nutrients and their important functions in the body relating to health and response to infection.
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Learning objectives
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After working through this session, you will be better able to name; and explain the roles of nutrients with known roles in immune function: describe the roles and food sources of the important macro and micronutrients in the diet; understand how different foods contribute to the nutritional value of a daily diet; and discuss the roles of both macronutrients and some micronutrients in immune function.
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Key terms Amino acids:
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Amino acids are the building blocks of proteins. Digestion breaks down proteins to amino acids before absorption. New proteins are then built from them, or they are used as an energy source.
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Key terms Anorexia:
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Loss of appetite leading to a reduction in food intake.
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Key terms Basal metabolic rate (BMR):
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Amount of energy expended in the basal state i.e., at complete rest.
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Key terms Catabolism:
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Metabolic pathways that break down body tissue and releases energy.
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Key terms Dietary Fibre:
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Indigestible carbohydrate that reaches the colon undigested and can therefore fuel the colonic microbiome. If they fuel beneficial bacteria, they are also prebiotics.
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Key terms High Density Lipoprotein (HDL):
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is a type of lipoprotein in blood that binds lipids like cholesterol and transports them away. The HDL particle is sometimes called ‘good cholesterol’.
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Key terms Lean body mass (LBM):
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Composition of the body minus fat mass.
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Key terms Low Density Lipoprotein (LDL):
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is another lipoprotein transport system but these transport cholesterol to the cardiovascular disease. Commonly known as ‘bad cholesterol’.
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Key terms Macronutrients:
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Nutrients that make up the bulk of diet: fats, carbohydrates, and proteins. They provide the basic needs of the human metabolism: energy and amino acids.
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Key terms Microbiome:
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The collection of microbes that live in and on our bodies. These number in the trillions, way in excess of our own body cell number, and can be bacteria, fungi or viruses.
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Key terms Micronutrients:
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Nutrients required by the body in small quantities. These are further subdivided into vitamins, minerals, and trace elements.
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Key terms Monounsaturated fatty acids (MUFA’s):
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Fatty acids with a single double bond in their chemical structure. High levels found in olive oil.
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Key terms Nutritional immunity:
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The process whereby micronutrients are moved into cells in the early stages of an infection, reducing their availability to pathogens.
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Key terms Polyunsaturated fatty acids (PUFA’s):
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Fatty acids with multiple double bonds in their chemical structure. Ω 3 and Ω 6 are PUFA’s.
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Key terms Prebiotics:
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Oligosaccharides that reach the colon undigested and provide a food source to colonies of potentially beneficial bacteria. Examples are inulin, fructo and galacto - oligosaccharides and beans that contain large quantities of stachyose.
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Key terms Probiotics:
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Foodstuffs, such as yoghurt, that containlivemicroorganisms. The microorganisms (colloquially known as ‘good’ bacteria) travel live into the GIT, colonise it and thereby restore an improved microbial balance. Common probiotics include lactobacilli, bifidobacteria, streptococci, and some yeasts and moulds.
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Key terms Resting metabolic rate:
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Similar to BMR but reflects measurement when the strict conditions for BMR measurement are not controlled.
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Key terms Sepsis:
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is a life-threatening organ dysfunction caused by a dysregulated host-response to infection.
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Key terms Total energy expenditure:
Total energy expended and required to maintain body weight. This includes the energy from the basal metabolic rate, the thermic effect of food and physical activity.
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Key terms Ω=6:
Omega 6 fatty acids are commonly known as fish oils and are PUFA’s. An example is linolenic acid.
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Food and nutrients
‘Food’ generally means an edible substance that satisfies hunger. However, for many people hunger is not a key driver for eating and other motivations come into play, for example. 1. pleasure. 2. habit – a biscuit with tea, snacks in the cinema. 3. social expectation – food offered by friends. 4. comfort – offering sweets to ‘take the pain away’. A diet or dietary pattern refers to the average consumption of foods over a period of time. Diets are culturally, religiously, and geographically determined, and can vary according to season or (socio-) economic constraints.
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Food and nutrients
Scientifically, food is basically a range of nutrients that should provide adequate energy and other essential requirements of the body to keep it alive and functioning in a normal and healthy manner. A foodstuff is usually made up of a range of nutrients – for example flours provide carbohydrate but also some proteins, vitamins, and minerals while meat provides protein plus fat, vitamins, and minerals. Likewise, meals typically provide all macronutrients but only a range of micronutrients and water – so increasing dietary diversity is a keyway to ensure a nutritionally complete diet. The nutrients in food are classified in Table 1.
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The functions of nutrients
The primary role of nutrients is to supply our energy needs for bodily functions and the materials for necessary for the growth, repair, replacement, and regeneration of our tissues.
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The functions of nutrients Nutrients as an energy source
The body requires both chemical and heat energy. The products of digestion are absorbed into the bloodstream and metabolised to release chemical energy. The macronutrients are the most important energy source although their relative contributions vary. The total energy contained in a food is determined by bomb calorimetry however not all this energy will be available to the host. There are two important reasons for this. 1. Not all food consumed is absorbed from the digestive tract. It is estimated that, of the macronutrients from a typical diet, 99% of ingested carbohydrate, 95% of fat and 92% of protein is absorbed. 2. Nitrogen-containing compounds (chiefly proteins) are not efficiently metabolised by the body. They are partially converted to urea and then excreted in urine; urea retains about 25% of the energy of the original protein. Energy lost through faeces (unabsorbed macronutrients) and through urine (mainly as urea) must be subtracted from the gross energy of a food to estimate the energy available to the body.
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The functions of nutrients Nutrients for growth, repair, replacement and regeneration
The body must continually synthesize new proteins in order to build new tissue or to replace tissue in damaged or senescent cells. Our food needs to provide the nutrients that are needed. Some are needed daily as they are not stored in the body, such as protein, others are stored such as iron or vitamin A. Amino acids are the basic building-blocks of proteins and proteins build new cells. Some amino acids termed ‘essential amino acids’ are provided only from the diet others can be synthesized by the body. Fats and carbohydrates are important constituents of cell membranes and of intracellular constituents. Vitamins, minerals, and trace elements are important in many varied chemical reactions in the body. Often working as enzymes or catalysts they can be involved in both catabolic and anabolic reactions.
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The macronutrients in food
Macronutrients are the bulk of the food we eat and provide the energy the body requires.
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The macronutrients in food Carbohydrates
Carbohydrates contain atoms of carbon, hydrogen, and oxygen in their molecules – hence the name. In most populations they provide the bulk of the diet and are a major source of energy. Carbohydrates provide around 4 kcal/g. Carbohydrate intake is correlated with income in many populations and can provide up to 85 % of the energy in the diet of very low-income groups. In the diets of the affluent, the carbohydrate energy content of the diet may be lower than 40% with respectively higher intakes of protein and fat.
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The macronutrients in food Carbohydrates
Carbohydrates are broadly classified as sugars (mono and disaccharides), starches (polysaccharides) and dietary fibre (non-starch polysaccharides). Sugars are small, sweet-tasting molecules which when joined into long chains no longer taste sweet. Chains of 10 or more sugar molecules are called polysaccharides. The commonest sugars in foods are glucose and fructose. Lactose and galactose occur in smaller quantities. Carbohydrates are plant based as they are synthesized by photosynthesis. The primary products are sugars, which are then polymerized to form polysaccharides. Plants contain two distinct types of polysaccharide: starch - this consists of a large number of glucose units joined together and is the energy store of plants; and non-starch polysaccharide (NSP) or dietary fibre.
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The macronutrients in food Carbohydrates
Cereals (such as rice, wheat, maize and other grains) are the principal source of carbohydrate in most communities; they can also be a source of NSP if consumed unrefined. Potatoes, cassava and other roots and tubers are also good sources of carbohydrates. Vegetables also provide carbohydrate in the diet as do legumes and fruit. Breadfruit, bananas (plantains) and sago (made from the pith of a tree) are less well known, but are the main carbohydrate source in some populations. Alcohol is a further type of carbohydrate that can make a significant contribution to energy intake; alcohol has a calorie density of 5 kcal/g. Alcohol is a toxin to the body and a key determinant of several cancers. Alcoholic beverages aren’t needed by the body and provide minimal nutrition beyond a contribution to fluid intake. Conversely, alcohol is a diuretic and increases urine output.
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The macronutrients in food Carbohydrates
During digestion, carbohydrates are broken down into glucose molecules which are then absorbed in the small intestine. Any carbohydrate remnants that enter the large bowel are broken down by the bacteria present, typically by fermentation, to short chain fatty acids (SCFA’s) and gases - carbon dioxide and methane. The SCFA are absorbed whilst the gases are excreted. Dietary fibre and prebiotics are names given to foods that enter the large bowel undigested. ‘Prebiotic’ refers to foods that promote the growth of ‘good’ bacteria.
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The macronutrients in food Proteins
Proteins differ from both carbohydrates and fats in that they contain nitrogen and sometimes sulphur in addition to carbon, hydrogen and oxygen. Because the characteristic element of protein is nitrogen, which constitutes 16% of its weight, nitrogen metabolism is often considered to be synonymous with protein metabolism in the body. Similar to carbohydrate, protein provides 4 kcal/ g. Amino acids (AA) are the building blocks of protein and humans use 20 AAs. The AA are linked together by peptide bonds to form long polypeptide chains which in turn link up to form different proteins. Essential amino acids (EAA) cannot be synthesised de novo in the body and so need to be supplied continually by the diet, see Table 2.
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The macronutrients in food Proteins
Protein forms the main functional and structural components of cells, including those of the immune system. Individual AAs may have multiple separate purposes in the body, for example glutamine is known to act as a neurotransmitter in the brain and to provide an umami taste to foods. Adequate protein in the diet is essential to maintain cellular integrity & function. Precise protein requirements vary depending on the stage of life cycle and presence of an infection/metabolic stress. On average for good health an adult needs 10-15% of their energy coming from protein. However, richer populations often choose to eat a much higher protein intake. The animal proteins (meat, fish, dairy, eggs) all provide an adequate balance of the EAA’s. However, not all plants protein sources provide all the EAA’s. Wheat, yam and cassava are low in lysine meaning that without an additional supply of lysine, from another food source, proteins requiring lysine can’t be produced. Maize, depending on the species, can be low in tryptophan. Potato, soya beans and other legumes are sufficient across the EAAs.
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The macronutrients in food Proteins
Young children have high protein requirements to allow for growth and as such require a good quality, varied diet or they are at risk of stunted growth due to inadequate AA profile in their diets. Populations that have a good dietary diversity will easily meet their protein requirements as the EAAs will be provided from different foods. Protein digestion is quite complex requiring more energy and more time than the digestion of carbohydrate. Protein is broken down into single amino acids or peptides (two or three amino acids joined together) for absorption in the small intestine. After absorption the AAs are resynthesized into the specific new proteins that are required by the body at that time for growth or repair. AAs that are not used straightaway are catabolised to provide energy and are not be stored in the body. If food is in short supply protein will be used as an energy source to maintain vital organs like the brain at the expense of growth or tissue replacement.
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The macronutrients in food Fats
Fats are made up of fatty acids and are the most energy dense macronutrient, providing 9 kcal/g. Key characteristics are that most fatty acids are not soluble in water (some short or medium fatty acids can be) and that they carry fat-soluble vitamins. The term ‘lipid’ is broader than fat and includes phospholipids, glycolipids, sterols and fat-soluble vitamins. When combined with phosphorous, lipids make phospholipids which have important structural roles in cell membranes. Chemically, fats are composed of esters of fatty acids with glycerol or cholesterol. The degree of ‘hardness’ of fats increases with the chain lengths of the constituent fatty acids and their degree of saturation. Edible fats are triacylglycerols and have a range of melting-points.
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The macronutrients in food Fats
Fatty acids are called unsaturated when there is at least one double bond between the carbon atoms in the fatty acid chain. Each double bond puts a kink in the chain making it more 3D and less dense. Mono-unsaturated fatty acids (MUFAs) contain one double bond, while poly-unsaturated fatty acids contain more than one double bond. Liquid fats such as vegetable or marine oils tend to contain high proportions of unsaturated fatty acids. Fatty acids are called saturated when each carbon atom in the chain carries all the hydrogen atoms possible; these fatty acids can be tightly packed as there are no double bond kinks in the chain. Hard fats, like meat fats, contain a relatively high proportion of saturated long-chain fatty acids.
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The macronutrients in food Fats
Fats are an important component of cell membranes and nervous tissue and all diets should therefore contain adequate amounts of fats, especially for children with their rapid growth and rapidly developing brain. Many affluent diets are high in fat but with high levels of saturated fat which is considered to contribute to cardiovascular disease. Whereas unsaturated fats in the diet seem protective from CVD and favourably affect the HDL/LDL cholesterol profile. Trans fats can occur naturally but only in very low amounts. Usually, trans fats are a by-product of hydrogenation (the addition of hydrogen to oil to make a solid fat or margarine during food processing). Artificial trans fats (in contrast to the naturally occurring trans fats) are associated negatively with cardiovascular disease.
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The macronutrients in food Fats
Visible fats are those that are apparent in the diet and include the fat on meat, and spreads such as butter or margarine. Invisible fats are hidden in the food and include those that are incorporated during preparation or cooking, for example the fat in cakes and biscuits, as well as those that are present as constituents of cell membranes of plant or animal tissues. Hence, the total fat content of the diet is not easy to measure; fat content often varies in the same food, especially in the case of meats. The important dietary sources of fat include: fats stored in the adipose tissue of meat; whole milk and dairy products; oily fish; vegetable oils from seeds and plants; and processed foods (many contain considerable amounts of fat).
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The macronutrients in food Fats
Oils and fats are liable to degrade, or turn rancid, resulting in the production of unpleasant odours and flavours. In particular, mono- and poly-unsaturated fats deteriorate more rapidly than saturated and trans fats. While marine or fish oils spoil very readily unless refined and hydrogenated. Fats in the diet need to be emulsified in the stomach before they can be absorbed into the body. The emulsification process increases the surface area of the fatty droplets allowing enzymatic digestion by lipases. The emulsified mixture passes into the duodenum where bile salts serve to make the fats water soluble which allows absorption (water micelles are formed). The mucosal cells take up and repackage the absorbed fats into chylomicrons which enter the blood stream.
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The microbiome
The microbiome is the collective name for the microbes that live on our bodies. Research relating to the relevance of the microbiome has grown enormously over the past years and shows that we host many trillion of these micro-organisms (far in excess of our own cell count) in our epithelial surfaces. The microbiome is so abundant in the intestinal tract that the bulk of human faeces is in fact dead cells from the microbes. Microbes are found throughout the digestive tract (from mouth to colon) but also in airways, genitourinary tracts and skin. The composition of the colonies varies on a day-to-day basis in response to diet, exercise, infections, medication and other factors.
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The microbiome
In the colon the microbiome feeds on the undigested remnants of our diet. These are fermented, producing short chain fatty acids (SCFA), vitamins B12 & K but also the gases leading to flatulence. The SCFA are thought to modulate the absorption of water and electrolytes in the colon; and may thus be key in diarrhoeal diseases. SCFA also feed further microbes. This area of science is young and rigorous knowledge relating to infections is in its infancy. However, a key function of a healthy microbiome is to prevent the colonisation and proliferation of infectious microbes that are damaging to our health.
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Probiotics and infections
Recently the effects of probiotics have been studied to determine their effects on nutrition and immunity and the results have been promising (Pae et al., 2012). The elderly can show a reduction in the diversity of the microbiota, with lower numbers of beneficial bacterial flora. These differences of the intestinal microbiota are not necessarily caused by aging per se, and could be associated with situations that occur frequently in the elderly, including the decline of the general state of health, malnutrition and poor diet, living conditions, and increased need for medication, such as antibiotics and nonsteroidal anti-inflammatory drugs.
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Probiotics and infections
The available knowledge suggests that the use of probiotics in the elderly is safe and could represent a useful intervention to support their health. In a large study, consumption of fermented milk containing Lactobacillus casei showed that the average duration of winter infections was significantly lower in the intervention group when compared to the control group (Turchet et al., 2003), although the incidence was not different to the placebo group.
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Probiotics and infections
In a well-designed RCT study McDonald and Bell found that specific probiotic strains in yoghurts both prevented and reduced the durations of symptoms of gut infectious diseases, including Clostridium difficile in hospitalised elderly. They argue that there is evidence that probiotics alter monocyte and natural killer cell function in the blood. Boge et al. (2009) found that a specific strain of probiotic improved antibody titres post vaccination for influenza in the over 70s. Furthermore, a systematic review (Vouloumanou, 2008) concluded that specific probiotics can reduce the severity of symptoms of respiratory tract infections but has little effect on their incidence.
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Activity 1 Turn to the Moodle Reading List and read the article by Makino et al. ‘Reducing the risk of infection in the elderly by dietary intake of yoghurt fermented with Lactobacillus delbrueckii ssp. bulgaricus OLL1073R-1’, BJN, 104, 998-1006. This study yields some interesting results and suggests that Lactobacillus delbrueckii probiotic can reduce the risk of infection from the common cold in the elderly. The subject number is low and the study faced some challenges, including being funded by a food manufacturer of yoghurt drinks. The results, however, complement those from other studies (Pae et al., 2012).
Consider these questions:
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Activity 1 What were the functional outcomes from this study and who do they apply to?
Reduced risk from the common cold, reduced symptoms from the ‘ear/nose/throat’ category in those who consumed the yoghurt over the milk group. The subjects were categorised as elderly if over 40 years of age in Funagata town or 60 years in Arita town.
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Activity 1 What immunological changes occurred during the study that could account for the outcomes?
Increased natural killer cell activity increased but not significantly in the whole group. The group was split into those with low, normal and high baseline activity. A significant increase was found in those with an original low natural killer activity, the normal group showed little change, but the activity of the high activity group was reduced to within normal parameters after intake.
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Activity 1 How can the study design be improved to provide clearer results?
An increase in sample size would hopefully clarify the results but this cannot be guaranteed. It would help if the reasons for some subjects not being allocated randomly were explained. It would also help if one larger cohort was used as opposed to two smaller ones. In Arita 112 subjects were selected but, for varying reasons, only 87 were included at the end. Whenever possible this level of dropout should be avoided. This can be a challenge however in elderly populations due to their higher levels of morbidity and mortality.
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The micronutrients in food
Vitamins, minerals and trace elements fulfil a range of functions in the body and as they are required in small quantities for normal functioning, they are collectively termed micronutrients (MN). Trace elements are minerals required in smaller, or traces, amounts by the body.
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The micronutrients in food
Adequate supplies of vitamins, minerals and trace elements are necessary for the metabolism to function optimally, and a specific or multiple deficiency may cause a dysfunction. The immune system requires a complex range of metabolic processes (e.g., protein metabolism, cell proliferation and differentiation, complex cellular processes such as phagocytosis, and maintaining the integrity of the physical barriers). Most vitamins, minerals and trace elements influence the effectiveness of the immune system, either directly or indirectly and an adequate intake is required for full functioning of the body’s defence mechanisms. The tables below present some basic information on all the micronutrients however only those considered to play a key role in infectious disease and immunity are discussed in the text.
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The micronutrients in food Vitamins
The term vitamin covers a range of different organic compounds which are needed by the body. They are required in low quantities, mg or µg/d but are considered ‘vital’ for health – as deficiency states lead to disease and ultimately death. Vitamins can function as hormones, catalysts or as co-factors in enzymatic reactions and the unique roles of specific vitamins can be found in Table 4. All vitamins are derived from the diet but we now know that vitamins D & K can be synthesised by our bodies. Vitamins are classified as those that are water-soluble and those that are fat-soluble, see Table 3.
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The micronutrients in food Fat soluble vitamins
As the name suggests these vitamins are found in fats and oils, as such a low-fat diet may not provide them in adequate amounts. Vitamins A, D and E have known roles in infectious disease and immunity. Fat soluble vitamins are stored in the body, characteristically in the liver, and do not need to be eaten on a daily basis. They don’t have active methods of excretion so toxicity can be a problem.
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The micronutrients in food Vitamin A
was the first vitamin to be discovered and was initially thought to be an anti-infective agent. Later its significant role in vision was understood when vitamin A deficiency was found to be the largest preventable cause of blindness in children. Vitamin A is a deep, orange coloured oil which brightens the colour of foods; it can be found in bright green, red and yellow fruits and vegetables. Note that white foods, such as rice, can’t contain vitamin A. As an oil, vitamin A needs to be transported in the (aqueous) blood system by plasma retinol binding protein.
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The micronutrients in food Vitamin A
Vitamin A comes in two forms – retinol or carotenoids. Retinol is the most potent form and 1 mg retinol is equivalent to 6 mg of β- carotene, the most important carotenoid. Retinol is only found in animal sources whereas the carotenoids are also found in plants. Liver is an excellent source of retinol and red palm oil is the most potent plant source. A major role of vitamin A is in the gene regulation and the control of cell differentiation – including that of immune cells. Vitamin A is also an antioxidant and plays a key role in vision.
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The micronutrients in food Vitamin A
Vitamin A deficiency (VAD) is a major public health disease worldwide. The effects of which include night blindness, xeropthalmia, growth failure, anaemia and immuno-incompetence. VAD leads to increased prevalence and severity of infectious disease and death. Even at a sub-clinical level VAD increases the risks from infectious diseases. As a consequence, vitamin A supplementation (VAS) programmes, that use a megadose of vitamin A, is a widespread programme that saves countless lives each year. VAS is accredited in reducing all-cause mortality by 12-24% in children under five. See the session Vitamin A and infection for more detail. There are a number of strategies to address VAD. These include supplementation, fortification of foods, biofortification of crops and behaviour/dietary change. The status of vitamin A is assessed by measuring plasma retinol or night blindness.
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The micronutrients in food Vitamin A
Vitamin A isn’t actively excreted from the body and can be toxic if consumed in excess. There is a wide variation in people’s susceptibility to excess vitamin A and it can cause liver damage, vomiting and can lead to death. Excess vitamin A is also be teratogenic to the foetus during pregnancy with the greatest risk during the first trimester. β-carotene doesn’t cause poisoning (in times of excess it isn’t converted to the active metabolite) but it will turn you orange!
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The micronutrients in food Vitamin D
or cholecalciferol, acts as a hormone and has multiple roles: the regulation of calcium and phosphorus homeostasis in the body; regulation of gene expression; and modulation of cell proliferation for immune responses to infection.
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The micronutrients in food Vitamin D
There are few rich sources of vitamin D is the diet, the richest are oily fish, fish liver oils and eggs. Nowadays foods like breakfast cereals, margarines and novel foods are fortified with vitamin D. However, it is unlikely that the needs of infants and the housebound elderly can be met by diet alone. The major source of vitamin D is by its synthesis in skin exposed to sunlight; thus the importance of dietary vitamin D increases in populations with low levels of sun exposure on the skin. It is of interest that the vitamin D content of mushrooms can also be increased by exposure of the gills to sunlight.
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The micronutrients in food Vitamin D
Vitamin D has a complex metabolism in the body with numerous metabolites, Human Nutrition 14th Ed gives an overview for students with interest. Vitamin D3 (cholecalciferol) is the usual dietary form and calcitriol (1, 25 dihydroxycholecalciferol) is an active metabolite involved in the regulation of gene expression. Vitamin D status is assessed by the plasma concentration of the vitamin D metabolite calcidiol (25, hydroxycholecalciferol).
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The micronutrients in food Vitamin D
Vitamin D deficiency is widespread and clinically presents as rickets in children and osteomalacia in adults. However other conditions, specifically respiratory tract infections have also been found to be associated with low vitamin D levels. (Martineau 2017, Hewison 2012). Strategies to address vitamin D deficiency include the use of megadoses, increasing the amount of UV light that reaches the skin (for example by reducing use of sunscreen) or regular supplementation with fish oils or vitamin D. Vitamin D in excess can lead to dangerously high blood pressure caused by too much calcium in the blood. Some infants can be particularly susceptible to excess vitamin D and this therefore limits the amount of fortification of artificial infant milks.
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The micronutrients in food Vitamin E
was only identified as essential to the human diet in 1983. It acts as a powerful, lipid soluble antioxidant in plasma lipoproteins and cell membranes thus preventing oxidative damage (there are many other antioxidants e.g., selenium, vitamin A).
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The micronutrients in food Vitamin E
Vitamin E is a family of tocopherols of which the most important is α-tocopherol. Oily fish, nuts, seeds and their oils are rich sources of vitamin E. A dietary deficiency of vitamin E is rare but groups most likely to become deficient are the frail elderly, some unusual cases of genetic disorders and premature infants. In premature infants, whose vitamin stores are inadequate, vitamin E deficiency causes haemolytic anaemia. Deficiency is associated with lowered antibody production and T-cell proliferation. Some supplementation studies have shown an increased resistance to infection in the moderately deficient elderly.
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The micronutrients in food Vitamin E
Known vitamin E toxicity is low. This is despite many people take large over the counter supplements of vitamin E to protect against various conditions (not often proven to be effective). The plasma concentration of α-tocopherol is taken as a measure of vitamin E status.
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The micronutrients in food Water soluble vitamins
The water-soluble vitamins are not stored in the body and are passively excreted via the urine; as such they should be consumed on a daily basis. They include the array of B vitamins, see Table 4 and vitamin C which has been long linked with the common cold.
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The micronutrients in food Vitamin C
or ascorbic acid, is an antioxidant that also functions with copper and iron containing enzymes. Its deficiency disease, scurvy, was first identified by the Egyptians back in 1500 BC who prescribed onions and vegetables as its treatment. Scurvy occurs due to impaired collagen synthesis which leads to petechial haemorrhages, fragility of blood capillaries leading to leakage, anaemia and death. About 30% of vitamin C in the blood is concentrated in white blood cells, the rest is found in plasma and erythrocytes. Vitamin C status is assessed via plasma concentration of ascorbic acid.
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The micronutrients in food Vitamin C
Vitamin C is found in fresh fruits and vegetables and particularly in citrus and blackcurrants. However, it is not very stable and is easily destroyed through exposure to air and heat. It is also lost by leaching into cooking fluids. Vitamin C increases the absorption of non-haem iron in the digestive tract by reducing the ferric iron to the more bioavailable ferrous iron.
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The micronutrients in food Vitamin C
Vitamin C deficiency is now rare but does present at times in populations entirely dependent on food aid, as in humanitarian contexts. Vitamin C containing foods are not logistically feasible to deliver in food aid and supplementation may be needed. It is the role of vitamin C as an antioxidant that suggests effects on the immune system; antioxidants protect against the oxidative stress generated during infections. Linus Pauling hypothesised that mega doses of vitamin C reduced the incidence of the common cold in 1970 and this has been contentious ever since. A Cochrane review, Hemila et al. 2008, presents the controversy and concluded that vitamin C supplementation may reduce symptoms but does not reduce the incidence of common colds in the general population.
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The micronutrients in food Dietary sources of vitamins
See table 4
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The micronutrients in food Minerals
The minerals are inorganic elements that account for 4% of our total bodyweight; of this half is the calcium found in our bones and teeth. Certain minerals are integral parts of enzymes, body cells or hormones (see Table 5) and they need to be derived from the diet. As with vitamins, only those minerals with known roles in immunity are discussed beyond the tables. The specific mineral content of foods is dependent to an extent on the soil content. The soil determines how much can be incorporated into vegetation, which in turn in turn is eaten by animals. Globalisation has decreased this effect, as foods are exported, but not removed it entirely and for example, Canadian and American wheat has much higher selenium content than European wheat.
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The micronutrients in food Iron
Iron is a necessary component of haemoglobin, the red pigment that delivers oxygen to our cells. Around 70% of body iron is in red blood cells (RBC), 25% stored as ferritin in the liver, 5% in myoglobin and the remaining iron is found in different enzymes including those involved in energy metabolism. The iron molecule has 2 forms the ferrous iron which is more absorbable (Fe 2+) and the insoluble oxidised ferric (Fe 3+). Stomach acid aids conversion of the ferric to ferrous iron ready for absorption. The amount of iron absorbed from the gastrointestinal tract is also moderated by the bodies need for iron. The dietary sources of iron can be categorised as: haem (from haemoglobin in meat) iron and non-haem iron. Haem iron has a higher bioavailability (absorption) than non-haem iron but still only 20-25% of haem iron is absorbed. The figure for plant (non-haem) sources is significantly lower.
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The micronutrients in food Iron
The rate of absorption is affected by a number of factors: Vitamin C, meat (haem iron) and copper all promote iron absorption. Phytates, tannins (in tea and coffee), zinc, calcium, eggs all inhibit iron absorption. A tip for those with iron deficiency is to not to drink tea (or coffee to a lesser extent) within an hour of a meal and to have orange juice instead. Iron deficiency can be addressed via supplementation with tablets and liquids with varying levels of tolerance. Iron sulphate, the most common supplement, can cause stomach upsets, nausea and constipation in which reduces compliance to treatment.
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The micronutrients in food Iron
Iron deficiency is a major public health disease worldwide. The effects of it include tiredness, anaemia, reduced manual labour productivity, reduced cognitive development in children, spontaneous abortion of foetus, prolonged labour and increased maternal mortality. Iron deficiency is a significant killer of pregnant women but not of other population groups. Further effects of iron deficiency are measured in economic terms with the World Bank (2017) demonstrating that iron deficiency costs 2 to 3% of GDP in some countries and ~ 11% of GDP in Bangladesh each year.
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The micronutrients in food Iron
Iron isn’t actively excreted from the body but is lost by: cells shedding into gastro and urinary tracts, skin & hair; and blood loss including menstruation, wounds and haemorrhage. Body iron content is maintained by: continuous recycling of iron from decaying RBC; regulation of iron absorption; and manipulation of intracellular ferritin stores. Iron has an oxidative capacity and free circulating iron is harmful to body tissue; as such iron is transported bound to a carrier protein (transferrin) and stored by another protein (ferritin). Hepcidin is a peptide that acts as the main regulator of iron balance. Iron status can be assessed by a number of means including measuring serum ferritin, and blood haemoglobin levels.
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The micronutrients in food Trace elements
In addition to the elements classified as minerals, the body needs some other elements in even smaller quantities. These are known as trace elements. They include cobalt, copper, magnesium, selenium, fluoride, iodine and chromium. Water, as well as food, is an important source of trace elements. See Table 6.
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The micronutrients in food Selenium
Selenium is integral part of the AA cysteine which is used in the family of seleno-protein enzymes. Of these, the glutathiones work as antioxidants, mopping up free radicals. A good selenium status has been associated with a lower risk of NCD’s, including cancer, which could be related to its antioxidant properties. There has been growing interest in the role that selenium plays in immune function and Avery and Hoffman 2018 provide a technical overview for those with interest. They argue that selenium deficiency impairs both innate and adaptive immunity, though this hasn’t yet been translated into effects on infectious disease outcomes. Selenium deficiency is rare in countries where the selenium content in the soil is high, such as Canada & US. However, much of China’s soil is deficient and has led to Keshans disease (selenium deficiency).
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The micronutrients in food Zinc
Zinc is present in every cell of the body and is an essential component of more than 100 enzymes and other proteins, including those needed for RNA and DNA synthesis. Zinc is essential for the synthesis of lean body tissue and as such a deficiency leads to stunting in children. Zinc also has specific roles in immunity – involved in the regulation of cell division - and is necessary for production of an immune response. Zinc sulphate is regularly used as a treatment for diarrhoea; and is also used preventatively to address stunting. Zinc isn’t stored in the body and needs to be provided continually by the diet. It is commonly found in meat and wholegrains but its bioavailability is decreased by phytates. Absorption rates vary but it is generally accepted that 30% of zinc is absorbed from the diet.
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The micronutrients in food Zinc
Zinc homeostasis in the body is carefully managed by absorption in the GIT and the sequestration of zinc by buffering proteins within cells. During sepsis it has been observed that zinc is moved to the liver, thus appreciably lowering serum zinc concentrations – a type of nutritional immunity (Alker and Haase 2018). Deficiency symptoms include growth retardation, increased risk from infections, delayed wound healing and loss of appetite. Zinc deficiency, alongside that of vitamin A, is a major cause of childhood mortality in low- and middle-income countries. Strategies to address zinc deficiency have so far been limited to supplementation. Zinc has low toxicity but in excess (2 g/day) can cause digestive upsets and fever.
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Water
Water is the main constituent of our bodies, around 70%, with men having slightly more than women. This water is distributed within our cells (intracellular 66%) and outside of them (extracellular 33%). Typically, we consume around 2 litres of water daily of which half is from fluids drunk and the other half from our food intake. However, turnover of water is much higher – our small intestine absorbs between 6-10 litres each day as water cycles, from secretion into and absorption from, the GIT during digestion.
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Water
The specific mechanisms of water absorption, across the lipid bilayer that forms the impermeable barrier of each cell, is still debated. However, water absorption accompanies glucose with approximately 210 water molecules and 2 Na+ ions with each molecule of glucose absorbed. Water is very dynamic throughout digestion.
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Water
A function of the colon is the final water and electrolyte absorption before the passing of the stool. However, the capacity of the colon to absorb water is limited and diarrhoea can develop if it is overloaded. Dietary fibre softens the stool by retaining water in the colon. If, the quantity of fibre reaching the colon exceeds the fermentation capacity of the microbiome then an osmotic diarrhoea can occur. In cholera, excessive water secretion into the colon causes diarrhoea, leading to dehydration and eventually death, unless treated (Schultz 2007). The basis of the action of oral rehydration salts is to provide the best glucose and salt balance to maximise water absorption and ameliorate diarrhoea.
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Summary
Food consumed daily provides nutrients needed for normal functioning of the body including the immune system. Macronutrients – carbohydrates, proteins and fats – constitute the bulk of nutrients in food. Macronutrients are an essential source of energy for the body and are also required for growth, regeneration and replacement of cells. Micronutrients – vitamins, minerals and trace elements and are needed in small quantities by the body. Many micronutrients have roles as enzymes, catalysts and hormones.
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Question 1 Fat soluble vitamins in the diet are: vitamins A & D, vitamin C, none of the above, vitamin B complex
vitamin A & D
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Question 2 Most of the energy required by the body is provided by: minerals, carbohydrates, vitamins, lipids
carbohydrates and lipids
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Question 3 Amino acids are the building-blocks for the synthesis of: DNA, polysaccharides, starch, protein
protein
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Question 4 What factors prevent the skin producing vitamin D: low levels of sunlight, clothing, all of the above, use of sun cream
all of the above
