4 Minerals C. Reilly, Oxford Brookes University 4.1 Introduction Minerals are the inorganic elements, other than carbon, hydrogen, oxygen and nitrogen, which remain behind in the ash when food is incinerated. They are usually divided into two groups – macrominerals and microminerals (or trace elements). The terms are historical in origin and originated at a time when the development of analytical equipment was still in its infancy and ‘trace’ was used to refer to components whose presence could be detected, but not quantified. Modern analytical equipment that allows determination of elements at levels in the nano- and even picogram range, can show the presence of most of the minerals in almost any food. Some are present in minute amounts, but others are at significant levels. The minerals are classified as either essential or non-essential, depending on whether or not they are required for human nutrition and have metabolic roles in the body. Non-essential elements are also categorised as either toxic or non-toxic. Table 4.1 lists elements that occur in food and are important in human nutrition. In addition to the essential elements, some others, including arsenic, silicon and boron, have been shown to be required by certain animals and may also play beneficial roles in the human body. This section will present an overview of the principal essential minerals, cov- ering their chemical characteristics, basic roles in human health, dietary origins (from food and supplements), and their Reference Nutrient Intakes (RNI), includ- ing Safe Intakes (SI). This will be followed by consideration of a number of selected minerals that are of particular interest at the present time. Attention will be given particularly to their nutritional significance, including their possible roles as functional ingredients of food. The elements to be discussed in detail are calcium, iron, and zinc. Two other elements, iodine and selenium, will also be considered, though in less detail. A final section will provide suggestions for further reading. 4.2 Chemical characteristics Nearly all the minerals required by the body are elements of low atomic number, from sodium (11) to selenium (34); the exceptions are molybdenum (42) and iodine (53). In living matter, these elements are present in a number of different states: as inorganic compounds, as free ions in body fluids, or combined with organic compounds (Coultate, 1985). Approximately 99% of the body’s calcium and 85% of its phosphorus are in the hard mineral component of bone. The two elements are combined together to form a compound similar to hydroxyapatite, Ca 10 (OH) 2 (PO 4 ) 6 . Other inorganic elements, such as fluoride (F - ), magnesium sodium and potassium are also incor- porated into the bone mineral to form the partly amorphous and partly crystalline structure of bone. In contrast to calcium in the skeleton, the element iron occurs almost entirely as part of co-ordination compounds based on the porphyrin nucleus involved in the transport of oxygen. Several of the other trace elements are also mainly present in biological tissues as organic compounds, such as selenium in the met- alloenzyme glutathione peroxidase, and molybdenum in superoxide dismutase. 4.3 Impact on health, absorption and recommended intakes Minerals function mainly in three ways in the body: 1. As structural components, e.g. calcium, phosphate and magnesium in bones and teeth. 98 The nutrition handbook for food processors Table 4.1 Mineral elements in food Macrominerals (g/kg) Microminerals (mg/kg) Toxic minerals (mg/kg) Calcium (<1–12) Chromium (<0.02–0.95) Cadmium (0.001–0.07) Magnesium (1–4) Cobalt (0.008–0.32) Lead (0.01–0.25) Phosphorus (1–6) Copper (<0.2–3.3) Mercury (<0.001–0.18) Potassium (1–6) Iodine (0.04–0.66) Sodium (1–19) Iron (<0.2–92) Sulphur (<2–6) Manganese (<0.10–14.0) Molybdenum (0.004–1.29) Selenium (<0.001–0.34) Zinc (0.2–8.6) data from Reilly C (2002) Metal Contamination of Food, 3rd ed. Blackwell science: Oxford. 2. In organic combinations as physiologically important compounds, e.g. phos- phorus in nucelotides, zinc in enzymes such as carbonic anhydrase, iodine in thyroid hormone. 3. In solution in body fluids to maintain pH, help conduct nerve impulses, control muscle contraction, e.g. sodium and potassium in blood and intra- cellular fluids. The macrominerals are mainly involved in functions 1 and 3, and the micromin- erals in function 2. A normal diet, composed of a mixture of both plant and animal foodstuffs, should supply all the minerals required by the body. When such a diet is not avail- able, or in some other situations, it may be necessary to provide the missing ele- ments in the form of supplements or by fortifying the diet with additional minerals. The minerals ingested in food are absorbed after digestion from the gut into the blood stream, which transports them to the sites where they function or are stored. Not all minerals are absorbed to the same extent. Some, including sodium and potassium, are readily absorbed as ions or as simple compounds. Others, such as calcium, magnesium and phosphorus may be combined as indi- gestible or insoluble compounds in food and are less easily taken up from the gut. A few others, especially some of the trace elements such as iron, are poorly absorbed. Uptake of certain minerals from food can be affected by other components of the diet. Thus phytic acid and phytates in cereals can inhibit absorption of iron and zinc. The same effect can be caused by oxalate in certain vegetables. Iodine absorption can be limited by sulphur-containing compounds known as goitrogens, which occur in certain plants, such as some brassicae and cassava. Consumption of these vegetables can acerbate iodine deficiency and increase the likelihood of goitre. If an essential element is at a low level in the diet, a nutritional deficiency may occur, with specific symptoms. Thus an inadequate intake of iron can cause anaemia when there is insufficient haemoglobin to meet the needs of the body for oxygen transport. A deficiency of iodine can lead to goitre when the body tries to compensate for a low production of the iodine-containing thyroid hormone by increasing the size of the thyroid gland. Inadequate zinc may result in growth failure in children. Usually these conditions are corrected when intake of the missing element is increased by improving the diet or by providing supplements. An excessive intake of a mineral may also have serious consequences for health. Too much sodium in the diet may be associated with high blood pressure and increased risk of a stroke. A condition known as siderosis, in which an excess of iron is deposited in the body, can result when too much iron is absorbed. Selenosis, a sometimes fatal effect of an excessive intake of selenium is known to occur in parts of China where high levels of the element enter locally grown foods from selenium-rich soil. Less serious effects, such as nausea, can be caused by a high intake of zinc. Minerals 99 Health authorities in most countries have established recommendation for intake levels of essential minerals which both meet the nutritional requirements of consumers and at the same time prevent excessive intakes. In the UK, Refer- ence Nutrient Intakes (RNI) for 11 minerals have been published by the Depart- ment of Health to meet the requirements for the different age groups and sexes in the Community (Department of Health, 1991). The RNI is defined as ‘an amount of the nutrient that is enough, or more than enough, for about 97 per cent of people in a group’. In addition Safe Intakes (SI) have been established for another four minerals. The SI is ‘a term used to indicate intake or range of intakes of a nutrient for which there is not enough information to estimate RNI...it is an amount that is enough for almost everyone but not so large as to cause unde- sirable effects’. The RNI for minerals for adult men and women are given in Table 4.2. 4.4 Dietary sources, supplementation and fortification Because our food is almost entirely made up of components that were once parts of living organisms and since there is a broad similarity between the nutritional requirements and cellular biochemistry of most forms of animal and plant life, it is to be expected that our needs for the mineral nutrients will be met by a con- ventional mixed diet (Coultate, 1985). It is usually only in exceptional situations, where, for example, there is a reliance on locally produced food in an area where 100 The nutrition handbook for food processors Table 4.2 Reference nutrient intakes and safe intakes for minerals Mineral male (19–50 years) female (19–50 years) Calcium (RNI) mg/day 700 700 Phosphorus (RNI) mg/day 550 550 Magnesium (RNI) mg/day 300 270 Sodium (RNI) mg/day 1600 1600 Potassium (RNI) mg/day 3500 3500 Chloride (RNI) mg/day 2500 2500 Iron (RNI) mg/day 8.7 14.8+ Zinc (RNI) mg/day 9.5 7.0 Copper (RNI) mg/day 1.2 1.2 Selenium (RNI) mg/day 75 60 Iodine (RNI) mg/day 140 140 Manganese (SI) mg/day above 1.4 above 1.4 Molybdenum (SI) mg/day 50–400 50–400 Chromium (SI) mg/day above 25 above 25 Fluoride (SI) mg/kg body weight/day 0.5 0.5 + insufficient for women with high menstrual losses where the most practical way of meeting iron requirements is to take iron supplements. adapted from Department of Health (1991) Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. HMSO: London. the soil is deficient in a particular mineral, or where the diet is deliberately restricted to a limited number of food types, that problems of mineral deficien- cies occur. Some food sources are better than others as sources of minerals. Plant foods are generally poor in iron and zinc, with the exception of certain dark green vegetables such as spinach. Dairy products are generally an excellent source of calcium. Red meat and offal, such as liver, are the best dietary sources of easily absorbed iron. Many of the trace elements are found in relatively high concen- trations in fish and other seafoods. Table 4.3 lists some of the best food sources of a number of the essential minerals. As is indicated in the table, there are some unusually good sources of a number of these minerals. Milk, for example, is often an excellent source of iodine because of the presence of residual iodine- containing compounds used to sterilise dairy equipment. Tea is a major source of manganese in the UK diet. An important source of chromium in the diet of some people is canned food which picks up the metal that is one of the ingredi- ents of the alloy used to produce ‘tin’cans (Reilly, 2002). For many people supplements are an important source of minerals. It has been estimated that as many as 40% of the US population consume them, and up to 60% in the UK, either as ‘over-the-counter’ self-selected products or prescribed by a physician or other health advisor (Balluz et al, 2000). Mineral supplements are available in a number of chemical forms, either as inorganic compounds, such as ferrous sulphate and calcium carbonate, or as organic preparations such as selenium yeast and zinc gluconate. The products vary in the amounts of the different elements they contain, in their absorbability and in other qualities and while undoubtedly their use can make a definite con- tribution in some cases to nutritional health, there can also be problems such as over-dosing and interactions with other components of the diet (Huffman et al, 1999). Minerals 101 Table 4.3 Good food sources of minerals Food Mineral cereal vegetable dairy meat Fish other Ca *green * * nuts Mg * nuts Fe *fortified *green * Zn * * * Cu * * Se * * * nuts I * * iodised salt Mn * * * tea Mo * Cr * * * Brewer’s yeast The addition of minerals and other nutrients to foods to increase their nutri- tional value is widely practised. In the 1920s iodised salt was introduced in some countries to help combat endemic goitre. Iodised salt, as well as other iodised foods such as bread and monosodium glutamate, are today widely used in parts of the world where iodine deficiency diseases (IDD) are still endemic, such as India, and China, Papua New Guinea, Central Africa and the Andean region of South America. Legislation was introduced in several countries during World War II which required the addition of iron and calcium, as well as of certain water-soluble vitamins, to bread and flour in order to combat nutritional deficiencies caused by food restrictions. The success of these measures in improving health led to the extension of the legislation into peacetime. Some countries, such as the UK, still require that bread and flour be fortified with calcium and iron (Statutory Instrument, 1984). Bread and flour are the only foodstuffs required by law to be fortified with minerals in the UK. There is, in addition, legal provision for the voluntary addi- tion by food processors of other minerals to other foodstuffs, with the exception of alcoholic drinks. This has given manufacturers the opportunity to produce a variety of foods enriched with other minerals. Most ready-to-eat (RTE) breakfast cereals are enriched with iron and zinc. Some varieties will also contain added iodine and other minerals. These are normally added at levels which are well below those which might cause toxic effects (Brady, 1996). Fortified RTE cereals have been shown to make a significant contribution towards meeting the nutri- tional requirements of consumers for iron, as well as for copper, manganese and zinc (Booth et al, 1996). Currently a considerable amount of research is being carried out on methods, such as fortification of a variety of commonly used foods with minerals and other nutrients, as a way of improving nutritional status in countries where deficiency problems regularly occur (Gibson and Ferguson, 1998). In recent years there has been a growth in the production of foods, which have been deliberately selected or formulated to provide, according to their promotors, specific physiologic, health promoting and even disease-preventing benefits. They have been given a variety of names such as ‘designer food’, ‘nutraceuti- cals’, ‘functional foods’ and, officially in Japan, ‘foods for specific health use’ (FOSHU). Several of these products contain minerals such as selenium (Reilly, 1998). 4.5 Calcium Without an adequate supply of the macromineral calcium in the diet calcification of the skeleton will be adversely affected. During early growth and development the supply of calcium for this purpose is particularly critical and for this reason the amount required by a child is proportionally greater than for an adult (British Nutrition Foundation, 1989). 102 The nutrition handbook for food processors 4.5.1 Calcium absorption Uptake of calcium from food in the gut is not very efficient. Only about 30% is absorbed, with 70% lost in faeces. Absorption is a complex process, which is under the control of the cholecalciferol (vitamin D)-parathyroid hormone system. Calcium is transported across the intestinal mucosa bound to a special carrier protein. Synthesis of this protein is stimulated by an activated form of cholecal- ciferol, 1,25-dihydroxycholecalciferol (1,25-DHCC). If vitamin D levels are low, calcium absorption will be restricted and a deficiency will occur. To be absorbed, calcium must be in the soluble ionic form. Several food components can prevent this happening. These include phytic acid (inositol hexaphosphate) in cereals, and oxalate in certain dark green vegetables, such as spinach, and in rhubarb. Uronic acid in dietary fibre can have a similar effect, as can free fatty acids and certain other dietary factors, including sodium chloride and a high protein intake. 4.5.2 Functions of calcium in the body Over 99% of body calcium is in the skeleton, where it both provides structural support and serves as a reservoir for maintaining plasma levels. Calcium in plasma plays a number of roles, for example in muscle contraction, neuromus- cular function and blood coagulation. To maintain these roles, calcium levels in the plasma must be very stable. If for any reason they are altered, they are imme- diately restored to normal levels by an increased secretion of parathyroid hormone and the formation of 1,25-DHCC. In children this increase in plasma calcium means that less of the mineral goes into bones, while in adults calcium is withdrawn from the skeleton. In either case there can be significant implica- tions for bone structure. 4.5.3 Osteoporosis Osteoporosis is a condition which is characterised by loss of bone tissue from the skeleton and deterioration of bone structure with enhanced bone fragility and increased risk of fracture. It is relatively common in the elderly, especially females, but may also occur in the young. In the UK one in three women and one in twelve men over the age of 50 years can expect to have an osteoporotic frac- ture during the remainder of their lives (Prentice, 2001). The causes of osteoporosis, in spite of extensive research, remain elusive. The higher rate in women seems to be associated with a number of factors: the lower skeletal mass in women compared to men, a greater rate of calcium loss and a fall in oestrogen production with age. Lifetime history is also important. Higher intakes of calcium, especially in adolescence and early adulthood, ensure greater bone density. In addition, physical exercise can help increase calcium deposition, while high consumption of alcohol, coffee, meat, salt and cola beverages may contribute to decreased bone density (Sakamoto et al, 2001). Although there is considerable debate about the effectiveness of calcium sup- Minerals 103 plements in preventing osteoporosis, the weight of evidence points towards a role for calcium deficiency in its genesis and for calcium therapy in its prevention and management, at least in postmenopausal women (Heaney, 2001). Increases in bone mineral density (BMD) have been observed following calcium supplemen- tation in young, as well as in elderly subjects. However, although dietary calcium does play a major role in optimisation of bone mineralisation it is by no means the only factor involved (Prentice, 1997). 4.5.4 Recommended intakes of calcium There is at present no international consensus regarding calcium requirements and levels in the diet necessary to meet optimum requirements. Recommended intakes differ widely between countries, partly because different methods have been used to arrive at the recommendations. While some authorities have focused on meeting nutritional requirements, others have aimed at optimising bone density (Wynne, 1998). There is also the fact that actual intakes of calcium vary widely world-wide, without, in many cases, an apparent effect on bone develop- ment. In parts of Africa and Asia intake of dietary calcium is as low as 300– 400 mg/day, while in Northern Europe it can be 1500 mg/day or more. In the UK the Panel for Dietary Reference Values of COMA (the Committee on Medical Aspects of Food Policy) of the Department of Health, while noting the difficulty of assessing the adequacy of the dietary supply of calcium, has established RNIs for calcium for different groups in the population (Department of Health, 1991). Since the panel’s experts found that no single approach to the estimation of these values was considered to be satisfactory, these intakes are not considered to represent true basal dietary requirements, but rather to describe the apparent calcium requirements of healthy people in the UK under prevailing dietary circumstances. The UK RNI for adults aged 19–50 years is 700 mg/day, with an additional 550 mg/day for lactating women. In the US an intake of 1000 mg/day is recom- mended for the same age group, with no additional allowance for lactation (Institute of Medicine, Food and Nutrition Board, 1998). In contrast, WHO/FAO in 1974 proposed 400–500 mg/day for this group, with additional intakes for pregnant and lactating women. 4.5.5 Dietary sources of calcium Milk and dairy products are the major sources of calcium in many diets. In coun- tries such as the UK where addition of calcium to flour is required by law, bread and other cereal products also make an important contribution to intake. Sardines and other small fish, which are eaten whole, are also good sources. In countries where dairy products are not used in quantity and where fortification of flour is not required, requirements may be met by green leafy vegetables, roots, nuts and pulses. Where domestic water is ‘hard’, with a high calcium content, it can make a significant contribution to intake. 104 The nutrition handbook for food processors 4.5.6 High intakes of calcium The consumption of calcium supplements is widely practised, especially by the elderly as a precaution against the development of osteoporosis. Although there is little evidence that a high intake of calcium resulting from supplement con- sumption causes adverse health effects, in the US an Upper Intake Level (UL) has been set for the mineral at 2.5 g/day. A Safe Intake (SI) level has not been set in the UK on the grounds, according to the Department of Health, that calcium metabolism is under such close homeostatic control that an excessive accumula- tion in the blood (hypercalcaemia) or in tissues (calcification) from overcon- sumption is virtually unknown (Department of Health, 1991). 4.6 Iron In spite of the fact that iron is the second most abundant metal in the earth’s crust, iron insufficiency is probably the most common nutritional deficiency in the world. Even among the inhabitants of well-fed developed countries it continues to be common, especially in women (Looker et al, 1997). 4.6.1 Iron absorption The uptake of iron is a complex and highly regulated operation. Once the element is absorbed from the intestine into the blood, only small amounts are lost from the body, except when bleeding occurs. There is no physiological mechanism for secretion of iron, so iron homeostasis depends on its absorption. Thus the healthy individual with a good store of iron is able to maintain a balance between the small normal losses and the amounts of the element absorbed from food. Normally only a very small amount of iron, about 1 mg/day, needs to be absorbed. The metal first enters the intestinal mucosal cells where it is bound into ferritin, an iron-storage protein. This is a large molecule from which the iron can be readily mobilised when required. Some of the incoming iron may be transferred directly by a transport protein, transferrin, to bone marrow and other tissues to be used in the synthesis of haemoglobin and myoglobin. Iron absorption is apparently regulated by the existing iron status of the body. If this is low, the absorption mechanism can be stimulated to increased activity. When iron stores are high, absorption is slowed down. There is evidence that other mineral elements, such as zinc, can compete with iron for the active absorp- tion pathway. Several other dietary factors can affect absorption, including phytate and fibre, which inhibit absorption, and ascorbic acid and protein, which increase uptake. The pH of the gut also has an effect, with food iron mainly in the more readily absorbed ferrous state under acid conditions. 4.6.2 Functions of iron Iron is an essential nutrient for all living organisms, with the exception of certain bacteria. It has two major roles in human physiology. As a component of haemo- Minerals 105 globin, the red pigment of blood and myoglobin in muscle, iron atoms combine reversibly with oxygen to act as its carrier from the lungs to the tissues. In a variety of enzymes, such as the cytochromes, iron atoms, present in the ferrous and ferric states, interchange with gain or loss of an electron, as part of the elec- tron chain responsible for the redox reactions necessary for release of energy in cellular catabolism and the synthesis of large molecules (Brock et al, 1994). In addition to its major functions in oxygen transport and as a cofactor in many enzymes, iron also plays an important role in the immune system. Although the mechanisms involved are complex, there is good evidence that an abnormal iron nutritional status can lead to impaired immune function, with serious conse- quences for health (Walter et al, 1997). 4.6.3 Iron deficiency anaemia Iron deficiency ultimately results in failure of the body to produce new blood cells to replace those that are constantly being destroyed at the end of their normal 120-day life span. Gradually the number of blood cells falls and, with this, the amount of haemoglobin in the blood. The cells become paler in colour and smaller in size. These undersized cells are unable to carry sufficient oxygen to meet the needs of tissues, so energy release is hindered. This is what is known technically as microcytic hypochromic anaemia, or, simply, as iron deficiency anaemia (IDA) (Expert Scientific Working Group, 1985). Because the fall in red blood cells occurs gradually, IDA can exist for a considerable time before it is clearly detected. By then iron stores have suffered a critical fall and the person affected shows symptoms of chronic tiredness, persistent headache, and, in many cases, a rapid heart rate on exertion. There may also be other functional consequences of iron deficiency, including a decreased work capacity, a fall in intellectual per- formance, and a reduction in immune function (Brock and Mulero, 2000). There is today growing concern at the possibility that iron deficiency in infancy and childhood can have serious consequences, such as morbidity in the newborn, defects in growth and development of infants and impaired educational perfor- mance in schoolchildren (Cook, 1999). 4.6.4 Recommended intakes The UK RNI is 1.7–8.7 mg/day for both males and females from birth to 10 years of age. This rises to 14.8 mg/day for females from 11 to 50 years, when it is reduced to 8.7 for the post-child bearing years. Women with a high menstrual loss are recommended to increase their iron intake by taking a supplement. The RNI for males from 11 to 18 years is set at 11.3 mg/day, with a drop to 8.7 mg/day for later years (Department of Health, 1991). The UK recommendations are similar to those published in the US (National Research Council, 1989), but lower than those of WHO. They are also less than recommendations in many developing countries. In Indonesia, for instance, an intake of 14–26 mg/day is recommended for women of child-bearing age, with 106 The nutrition handbook for food processors an additional 30 mg/day during pregnancy (Muhilal, 1998). The reason for such differences between recommendations in the UK and in developing countries relates to the composition of the diets normally consumed by their citizens. According to the Food and Agricultural Organization (FAO), a diet typical of most segments of the population in industrialised countries includes generous amounts of meat, poultry, fish and/or foods containing high amounts of ascorbic acid (FAO, 1988). Iron absorption from such diets can be assumed to be 15%. In contrast, the less varied, high cereal diet of Indonesia and countries with a similar diet, may have a lower iron content with an absorption level of 5% or less. 4.6.5 Dietary sources of iron One of the richest sources of dietary iron is animal offal, especially liver. Other animal products, in particular red meat, are also rich in iron. This iron is organically bound haem iron, which is easily absorbed. Plant foods are generally poor sources of iron and what is present is in the inorganic form. Depending on the total composition of the diet, absorption of iron from vegetables will be as low as 2–5%. In contrast to most other vegetables, dark green species such as spinach are relatively rich in iron. While cereals in general are low in iron, breakfast cereals which are fortified, voluntarily, by manufacturers, and flour to which the addition of iron is required by law, make major contributions to intake in the UK and elsewhere. 4.6.6 High intakes of iron Iron toxicity can occur as a result of ingestion of large amounts of iron com- pounds. This is most unlikely to be caused by iron in normal foods, but by acci- dental intake of a chemical substance. A lethal dose for adults is about 100 g, while in children, among whom most cases of iron poisoning are reported to occur, it is 200–300 mg/kg body weight (Department of Health, 1991). The source of the iron in most childhood poisoning appears to be supplement pills used by their mothers which the children mistake for sweets (Barr and Fraser, 1968). A high intake of iron can be a serious problem for persons with the hereditary disorder of haemochromatosis. This condition, in which there is a gradual accu- mulation of iron in tissues and can result in liver failure, occurs in nearly one per cent of Europeans and more frequently in people of African origin. There is evi- dence that in some cases iron overload is caused by excessive intake of the metal as a result of consumption of foods and beverages prepared in iron cooking pots and containers (Bothwell et al, 1964). 4.7 Zinc While zinc was known to be an essential nutrient for plants and certain animals from early in the twentieth century, zinc deficiency in humans only began to be Minerals 107 recognised in the 1960s, when zinc-responsive dwarfism was detected in children in Egypt (Prasad, 1990). Although there was some uncertainty among researchers about the possibility of more general zinc deficiency among humans because of the element’s ubiquity in the environment (it is the 23rd most abundant element in the earth’s crust), subsequent clinical studies of children with acrodermatitis enteropathica, a hereditary error of zinc metabolism, and numerous studies of other zinc-related conditions, have confirmed the essential role played by zinc in human metabolism (Brown et al, 2001). Today zinc is known to be a key nutri- ent of world-wide significance, and has joined iodine and iron among trace ele- ments whose deficiency problems urgently need to be addressed (Ranum, 2001). 4.7.1 Zinc absorption from food An adult human contains between 1.5 and 2.5 grams of zinc, almost as much as iron and more than 200 times the amount of copper which is the third most abun- dant trace element in the body. Absorption from the diet, which occurs in the small intestine, is affected by a number of factors. Uptake has been reported to range from less than 10 to more than 90%, with an average of 20–30% (Forbes and Erdman, 1983). Various components of the diet can affect uptake. Competition for absorption occurs between zinc and other elements, especially copper, iron and cadmium. Phytate, fibre, and calcium can limit gastrointestinal uptake, whereas animal protein enhances it. A diet rich in wholemeal bread, for instance, which contains these three antagonists, has been shown to cause deficiency of the element. Zinc absorption is believed to be related to the presence of endogenous zinc binding ligands. Most of the zinc that is absorbed from the intestine is found intra- cellularly, primarily in muscle, bone, liver and other organs (Jackson, 1989). Zinc in plasma is mainly loosely bound to albumin and is also transported attached to transferrin. In the liver it is bound to the low molecular weight metal-binding protein, metallothionin. Most of the body’s zinc reserves turn over slowly and are not readily avail- able for metabolism. Only about 10% makes up a readily available pool, which is used to maintain various zinc-dependent metabolic functions. The body’s zinc content is regulated by homeostatic mechanisms, mainly through control of absorption of exogenous zinc from the gut and by regulation of excretion of endogenous zinc in pancreatic and other gastrointestinal secretions (King et al, 2000). Since only about 0.2% of the body’s total zinc is in plasma, a small change in uptake by or release from muscle or other tissues can have profound effects on levels in plasma (Hambidge et al, 1989). Plasma zinc concentrations can be affected by stress, surgery, physical exercise, infection and several other factors (Brown et al, 2001). Consequently, plasma levels do not give a reliable measure of total body zinc stores under all circumstances. 4.7.2 Functions of zinc in the body Zinc is an essential component of more than 200 enzymes in the living world, of which as many as 50 play important metabolic roles in animals. It occurs in all 108 The nutrition handbook for food processors six classes of enzymes. In addition, the metal provides structural integrity in many proteins. Zinc ligands help maintain the structure of cell membranes and of some ion channels. ‘Zinc finger protein’ is involved in processes of transcription factors that link with the double helix of DNA to initiate gene expression (Berg and Shi, 1996). The expression of certain genes is known to be regulated by the quantity of zinc absorbed from the diet. It is also believed that zinc has an intracellular role that includes regulation of cell growth and differentiation. While some zinc-dependent biological activities are organ-specific, such as the role of the metal in neuronal transmission (Frederickson et al, 2000), many of the wide range of zinc-dependent metabolic processes are required by all cells. This can explain why the consequences of zinc deficiency are so many and so varied and why they are also non-specific. This could also account for the special impor- tance of the metal during prenatal and early growth, and in systems such as the immune system, in which cells have a rapid turnover (Hambidge and Krebs, 2001). Clinical signs seen in persons suffering from marginal zinc deficiency include depressed immunity, impaired taste and smell, night blindness, impaired memory, and decreased spermatogenesis in men (Walsh et al, 1994). Severe zinc deficiency is characterised by severely depressed immune function, frequent infections, bulbous pustular dermatitis, diarrhoea, alopecia and mental distur- bances. An inadequate intake of zinc retards growth and can result in stunting, dwarfism, and failure to mature sexually. 4.7.3 Zinc requirements and dietary reference values Considerable problems have been encountered in trying to establish dietary zinc requirements. This is largely due to the difficulty of assessing zinc status and optimal zinc nutriture in humans. As has already been mentioned, measurement of plasma levels may not give a true measure of the body’s available zinc. Other biomarkers of zinc status, such as activity of zinc-dependent enzymes, are not sufficiently specific to be more than of supportive value (Hambidge and Krebs, 1995). The use of estimates of habitual intake in populations without evidence of zinc deficiency can be of some use, but lacks precision and also does not take sufficiently into account the problem of bioavailability in different types of diet. In practice, a factorial approach, which is based on the quantity of absorbed zinc required to match endogenous excretion of the element, has been used to esti- mate requirements in several countries. Although not ideal, the factorial approach is widely accepted as offering a useful strategy for estimating zinc requirements and has been adopted for this purpose in the UK and the US. Its use, however, has not produced uniform recommendations. Thus, while the UK RNI for adult males and females are 9.0 mg/day, with an additional 2.5 to 6.0 mg/day for lactating women, but with no allowance for pregnancy, the 1989 US recommen- dation was 15 mg/day for adult males and 12 mg/day for women up to the age of 50 years, with an extra 16–19 mg/day for lactating women, on top of an addi- tional 15 mg/day all through pregnancy. Minerals 109 4.7.4 Zinc levels in foods and dietary intakes In Western societies upwards of 70% of zinc consumed is provided by animal products, especially meat (Welsh and Marston, 1982). Liver and other organ meats are particularly rich in the element, as are most seafoods. Another good source is oysters which may, in some cases, contain as much as 1000 mg/kg of the metal. Other foods which contain high levels are seeds and nuts, as well as wholegrain cereals. However, these and other plant foods also contain phytate that can decrease bioavailability of the element. The average zinc intake by UK adults is about 9–12 mg/day, which means that many people are on the borderline for meeting their requirements. The zinc content of a typical adult US mixed diet is between 10 and 15 mg/day and it is estimated that as many as two thirds of the population fail to meet their recom- mended level of intake. Intakes are especially low in children, adolescent females and women during their reproductive years, and the elderly. Similar discrepan- cies between actual intakes and recommendations are seen in a number of European countries (Van Dokkum, 1995). In many Asian countries zinc intakes are particularly low because of the absence of appreciable amounts of animal products and the presence of phytate-rich plant foods in the customary diet. There is evidence that zinc deficiency is widespread, especially in children, in several countries, such as Bangladesh (Osendarp et al, 2001). Considerable efforts are currently being made by health authorities in such areas to improve zinc nutriture by provision of zinc supplements as well as by other methods, includ- ing fortification (Gibson and Ferguson, 1998). 4.7.5 High intakes of zinc It is generally assumed that zinc is non-toxic because of the strong homeostatic regulation of absorption and endogenous excretion of the metal. However, large doses can cause gastrointestinal problems and are emetic (Failla, 1999). This has been known to occur when water, which has been stored in galvanised (zinc- plated steel) containers has been consumed. Prolonged exposure to high intakes of zinc is believed to result in copper deficiency and subsequent anaemia. It may also interfere with iron metabolism. Because of evidence that similar effects may be produced by ingestion of high doses of zinc supplements, an intake of more than 15 mg/day in this way is not recommended (Department of Health, 1991). 4.8 Other minerals: iodine and selenium Three other trace minerals for which there are DRVs in the UK are copper, iodine, and selenium. Copper is considered in detail elsewhere in this volume and will not be discussed further here. In this section, the roles of iodine and selenium will be reviewed briefly. 110 The nutrition handbook for food processors 4.8.1 Iodine The non-metallic element iodine is an essential nutrient that, apparently, has a single function in the body as a component of the thyroid hormones thyroxine (T4) and triiodotyronine (T3). These hormones are necessary for a range of body processes, the most important of which are the control of metabolic rate, cellu- lar metabolism, growth and neural development. Production of T4 and T3 is con- trolled by tissue demands which are mediated by the secretions of the pituitary gland and by the supply of iodine in the diet. Deficiency of iodine can result in a number of diseases, ranging from severe cretinism with mental retardation to barely visible enlargement of the thyroid gland. Goitre is the name given to enlargement of the gland that occurs as the body attempts to compensate for a reduction of its supply of iodine by increas- ing the size of the gland. The amount of enlargement is related to the degree of iodine deficiency. The condition is widespread throughout the world, with up to a billion people affected (Hetzel and Mano, 1989). It occurs especially in poorer remote areas where the soil is depleted of iodine and the general diet is limited and lacks useful sources of the mineral. Goitre was once endemic in parts of the UK and other European countries, before the introduction of iodised salt and an improvement in the general diet. Seafood is the major natural source of iodine in the diet. Fish, crustaceans and seaweeds are rich in the element. Milk is another good, though adventitious, source of dietary iodine as a result of the use of iodine-containing chemicals to sterilise dairy equipment. This practice has now ceased in many countries, with the result that dairy products are decreasing in value as a source of the nutrient. Cereals, vegetables and meat are generally poor sources. Iodised salt (sodium chloride) was introduced in many countries in the mid-twentieth century to combat endemic goitre and its use led to a significant improvement in the iodine nutritional status. Today, a reduction in the availability of iodised salt, coupled with an overall decrease in consumption of table and cooking salt, has resulted in a fall in iodine intakes. There is some concern that as a result, goitre may return to countries where it was once endemic (Solcà et al, 1999). The RNI for iodine in the UK is 140mg/day for adults, close to the US RDI of 150mg. These levels are easily achieved by consumption of a normal diet. Higher intakes of more than 1 mg/day may cause toxicity. This can be the result of excessive use of iodine supplements or, in certain cases, even of natural iodine- rich foods (such as certain seaweeds that can contain more than 4 mg/kg of iodine). Paradoxically, high intakes of iodine depress thyroid function and produce goitre in certain individuals. Because some people, especially the elderly, may be sensitive to high iodine intakes, a Safe Limit of not more than 1 mg/day is recommended in the UK. 4.8.2 Selenium The metalloid selenium, although one of the rarest of the elements, is an essen- tial trace nutrient for humans and all animals, but not for plants. Its essentiality Minerals 111 was only recognised in the 1970s when the enzyme glutathione peroxidase was shown to be a selenoprotein (Rotruck et al, 1973). Previously the element had been known only for its toxicity (Reilly, 1996a). Selenium, in the form of the unique amino acid selenocysteine, is the co-factor in several important functional metalloproteins. At physiological pH, the sele- nium in the selenocysteine is almost totally ionised and is an extremely efficient redox catalyst. At least 30 selenoproteins have been shown to occur in mam- malian cells. Several of these have been fully characterised and their functions determined in human tissues. One group, the glutathione peroxidases, plays a role in intracellular antioxidant systems. Selenium is also an essential cofactor in the iodothyronine deiodinases, which are enzymes involved in thyroid hormone metabolism. Another important selenoenzyme is thioredoxin reductase which helps to control cell growth and division. Several other selenoproteins, including selenoprotein P and selenoprotein W, also occur in human tissues where they appear to have antioxidant and redox roles (Arthur and Beckett, 1994). Selenium deficiency is associated with several diseases of major economic importance in farm animals. In humans chronic low intake of dietary selenium is responsible for Keshan disease, a sometimes fatal cardiomyopathy which occurs especially in children and young women, as well as for Kashin-Beck disease, a chronic osteoarthropathy, which also affects mainly children. These diseases are found in parts of China and other areas of Central Asia where soil levels of sele- nium are very low. Several other selenium-responsive conditions occur in humans, including cardiomyopathies and muscular problems in patients on total parenteral nutrition (TPN) if there is inadequate selenium in the fluid. Normal function of the thyroid gland is also dependent on an adequate supply of the element (Arthur et al, 1999). There is evidence that selenium deficiency can cause a wide range of other problems including immunodeficiency (Beck, 1999), increased susceptibil- ity to various forms of cancer and to coronary arterial disease. Selenium has been added relatively recently to the dietary recommendations in some countries as evidence establishing its important role in human health has become officially accepted. The UK RNI of 60mg/day for adult females and 75mg/day for adult males, is higher than the current US Dietary Reference Intake of 55mg/day for adults (Institute of Medicine, Food and Nutrition Board, 2000). It is believed, however, by some health experts that these intakes are insufficient to meet human needs since they do not take into consideration the element’s critically important protective role against oxidative damage. Selenium is widely distributed, but normally at levels of less than 1 mg/kg, in most foods. The richest sources are organ meat, such as liver (0.05–1.33 mg/kg), muscle meat (0.06–0.42 mg/kg) and fish (0.05–0.54 mg/kg). Though cereals contain only 0.01–0.31 mg/kg, cereal products make a major contribution to intake because of the relatively large amount of such foods consumed in most diets. Another good source of the element is nuts, particularly Brazil nuts which are the richest food source of the element known (Reilly, 1999). Vegetables, fruit and dairy products are poor sources. 112 The nutrition handbook for food processors Levels of selenium in plant foods, and in animals that feed on them, reflect levels in soils on which they grow. Soil concentrations are subject to consider- able regional variations, and consequently levels in different foods can show a wide range. This has important consequences for dietary intakes in some coun- tries. Thus, in the US where much of the food-producing regions have selenium- rich soils, the average intake of the element is 62–216mg/day. In parts of China, where the soil is severely depleted, intakes are as low as 3–22mg/day. In the UK, where, as in other European countries, soil levels are relatively low, average sele- nium intake is about 40mg/day. There is concern among some nutritionists about the possible adverse health effects of low selenium intakes and steps have been taken in some countries to protect the population against them. In Finland the law requires that selenium be added to all fertilisers and, as a result, the selenium status of the population has been more than doubled in recent years. In New Zealand the law permits but does not require farmers to use selenium-enriched top dressings on grazing land, to combat selenium deficiency in farm animals. Self-medication with selenium dietary supplements is widely practised by individuals, and is actively promoted by the pharmaceutical industry and the media in many countries (Reilly, 1996b). Selenium toxicity, or selenosis, has been well documented in farm animals. It has also occurred in humans in some parts of China where very high levels occur in the soil. There have also been reports of selenosis in individuals who consume excessive amounts of selenium supplements. There is some debate about the levels of intake that will cause toxicity. Residents of some high soil areas appear to have no symptoms of selenium toxicity, although they consume as much as 700mg/day. According to the Environmental Protection Agency in the US, a daily intake of 5mg/kg body weight (350mg for a 70 kg adult) is not toxic. In the UK the recommended maximum safe selenium daily intake from all sources for adults in 6mg/kg body weight or 450mg for an adult male (Department of Health, 1991). 4.9 Sources of further information and advice beard j l, dawson h and pi?ero dj (1996), ‘Iron metabolism: a comprehensive review’, Nutr Revs, 54, 295–317 bogden j d and klevay m (2000), Clinical Nutrition of Trace Elements, Totowa NJ, Humana Press garrow j s, james w p t and ralph a (2000), Human Nutrition and Dietetics (10th ed), Chapter 10: Bone Minerals, 165–76; Chapter 11: Iron, zinc and other trace elements, 177–210. London, Churchill-Livingstone hurrell r (2001), Mineral Fortification of Food, Leatherhead, Leatherhead Food Research Association nordin b e c (1990), ‘Calcium’, in Truswell AS, Recommended Nutrient Intakes, Sydney, Australian Professional Publications, 199–219 Minerals 113 4.10 References arthur j r and beckett g j (1994), ‘New metabolic roles for selenium’, Proc Nut Soc, 53, 615–24 arthur j r, beckett g j and mitchell j h (1999), ‘The interaction between selenium and iodine deficiencies in man and animals’, Nutr Res Rev, 12, 55–73 balluz l s, kieszak s m, philen r m and mulinare j (2000), ‘Vitamin and mineral sup- plement use in the United States’, Arch Family Med, 9, 258–62 barr d b g and fraser d k b (1968), ‘Acute iron poisoning in children: role of chelating agents’, Br Med J, i, 737 beck m a (1999), ‘Selenium and host defence towards viruses’, Proc Nutr Soc, 58, 707–11 berg j m and shi y (1996), ‘The galvanising of biology: a growing appreciation for the roles of zinc’, Science, 271, 1081–5 booth c k, reilly c and farmakalides e (1996), ‘Mineral composition of Australian ready-to-eat breakfast cereals’, J Food Comp Anal, 9, 135–47 bothwell t h, seftel h c, jacobs p, torrance j d and baumslag n (1964), ‘Iron over- load in Bantu subjects. 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CF Mills, London, Springer Verlag, 1–14 king j c, shames d m and woodhouse l r (2000), ‘Zinc homeostasis in humans’, J Nutr, 130, 1360S–6S looker a c, dallman p r, carroll m d, gunter e w and johnson c l (1997), ‘Preva- lence of iron deficiency in the United States’, J Amer Med Assoc, 227, 973–6 mackey m, hill b and gund c k (1994), ‘Health claims regulations: impact on food devel- opment’, in Nutritional Toxicology, eds. 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