Trace minerals and atherosclerosis

Although there is no evidence for a direct cause-effect relationship between mineral and trace element status and atherosclerosis in humans, many elements exert a strong influence on individual risk factors for cardiovascular disease, such as disorders of blood lipids, blood pressure, coagulation, glucose tolerance, and circulating insulin. Studies in humans and animals have shown that optimal intakes of elements such as sodium, magnesium, calcium, chromium, copper, zinc, and iodine can reduce individual risk factors; some of these studies are consistent with the results of epidemiologic correlations. Influences of local geochemical environment and of dietary practices can result in mineral and trace element imbalances; deficiencies of chromium, iron, copper, zinc, selenium, and iodine are well defined. Detection and correction of such imbalances in populations, through diminishing individual risk factors, might ultimately reduce the incidence of atherosclerotic heart disease.

Epochal trace elements and evolution

The use of some trace elements by plants and animals during the evolutionary process has resulted in epochal changes. Noteworthy is the fact that plants (but not animals) needed boron in order to grow stems and roots as they left the seas and became anchored on land. Iodine is plentiful in sea water but rare on land. Therefore, the iodination of tyrosine provided an iodine transport mechanism which allowed for the metamorphosis and the development of warm bloodedness--a great evolutionary advantage. Zinc from clay was needed for the formation of the first primitive nucleic acids and, later, the presence of zinc in the retina provide the enhanced night vision of the nocturnal predators--a natural advantage. Hence, boron, iodine and zinc can be termed epochal trace elements. Inquiry should be directed towards the possible roles of other trace elements, which may have been epochal in evolution.

The essential trace elements

Essential trace elements are required by man in amounts ranging from 50 micrograms to 18 milligrams per day. Acting as catalytic or structural components of larger molecules, they have specific functions and are indispensable for life. Research during the past quarter of a century has identified as essential six trace elements whose functions were previously unknown. In addition to the long-known deficiencies of iron and iodine, signs of deficiency for chromium, copper, zinc, and selenium have been identified in free-living populations. Four trace elements were proved to be essential for two or more animal species during the past decade alone. Marginal or severe trace element imbalances can be considered risk factors for several diseases of public health importance, but proof of cause and effect relationships will depend on a more complete understanding of basic mechanisms of action and on better analytical procedures and functional tests to determine marginal trace element status in man.

Oxygen, oxidases, and the essential trace metals

The dominant function of dioxygen as the terminal electron acceptor in aerobic systems is well established; the roles of iron and copper in the terminal oxidases are less well understood. The minor, but crucial, part that dioxygen plays in other biological processes has recently attracted much attention. The chemistry of the reduction products of dioxygen is described and the possible relation of these products to the toxic properties of dioxygen is discussed. It is suggested that the uncontrolled reaction of dioxygen with reduced species, to give the superoxide ion, hydrogen peroxide, the hydroxyl radical and perhaps other entities derived from these, is potentially hazardous to the organism. Defences exist against these species, not least in the dismutases dependent on copper-zinc, manganese and iron, in catalase and in the selenium-dependent peroxidase. The effectiveness of these defences is examined and their reduction products of dioxygen during phagocytosis is discussed.

Metabolic and functional defects in selenium deficiency

This paper is concerned with present-day knowledge of the biological role of selenium, of its interaction with other nutrients including trace elements, and with the importance of selenium in human nutrition and health. Selenium has been shown to be an integral part of glutathione peroxidase, which catalyses the reduction of a large range of lipid hydroperoxides and hydrogen peroxide. The interrelation between vitamin E, selenium and polyunsaturated fatty acids is complex. First, selenium in glutathione peroxidase may control intracellular levels of hydrogen peroxide, which affect the formation of active oxygen metabolites that may serve as initiators of lipid peroxidation; this role of selenium is closely related to that of superoxide dismutases, which control intracellular levels of the superoxide anion. Secondly, vitamin E may control the formation of lipid hydroperoxides through its antioxidant function, as well as possibly entering into a structural relation with membrane phospholipids. Thirdly, glutathione peroxidase may catalyse the reduction of lipid hydroperoxides, formed from membrane lipids, to hydroxyacids without detriment to the cellular economy. In the field of human nutrition, the lack of selenium has been shown to be the cause of a cardiomyopathy known as Keshan disease, occurring in the People's Republic of China. Blood selenium levels in patients from this area are compared with blood selenium levels in three other parts of the world and the conclusion is reached that the blood selenium level of populations in Keshan disease regions are exceptionally low and that Keshan disease is the first demonstration that selenium is an essential trace element for man.

The roles of trace elements in foetal and neonatal development

Manganese, zinc and copper are essential for normal prenatal and neonatal development. Manganese deficiency causes skeletal abnormalities, congenital ataxia due to abnormal inner ear development, and abnormal brain function. Depression of mucopolysaccharide synthesis and manganese superoxide dismutase activity may be fundamental to ultrastructural and other defects. In copper deficiency, neurological and skeletal abnormalities are due to impairment of phospholipid synthesis and collagen crosslinking, and possibly to low activity of copper metalloenzymes. The fundamental defect leading to the extremely teratogenic effects of zinc deficiency is related to depressed synthesis of DNA. In the neonatal period, poor survival and growth and depressed function of the immune system are salient features. Developmental patterns of trace element concentrations in various tissues suggest that important changes in metabolic regulation of trace elements may occur during the neonatal period. This hypothesis is being investigated by studies of molecular localization of trace elements in certain neonatal tissues, in conjunction with similar observations in milk.

An appraisal of the newer trace elements

For an element to be considered essential it should satisfy three criteria: (1) it must be present in living matter; (2) it must be able to interact with living systems; (3) a dietary deficiency must consistently result in a reduction of a biological function, preventable or reversible by physiological amounts of the element. Ideally, essentiality should be established in more than one species and confirmed in more than one laboratory. Since 1970, vanadium, fluorine, silicon, nickel and arsenic have been shown to meet all the criteria listed above, and evidence from one laboratory has indicated that tin may have an essential biological role in the laboratory rat. A review is presented of the evidence on which the essentiality of these elements has been established and, when known, an indication of their biochemical functions. The possible significance of these 'newer' trace elements to the health of man and animals is discussed.

The scientific and practical importance of trace elements

E. J. Underwood's discovery of the essentially of cobalt for ruminant animals is the classic example of the vast benefits to agricultural production of research into the nutritional significance of trace elements. The extension of this discovery, culminating in the identification of vitamin B12, resulted in similar benefits for human health, notably the conquest of pernicious anaemia. Since then, additional essential trace elements have been discovered. Deficiency or imbalance, whether occurring naturally or from human activities, has been shown to present significant problems for the health of man and animals. Essentiality has been proved for a rapidly growing range of 'new' trace elements, whose biochemical mechanisms of action and implications for human health are unknown. In spite of an increasing knowledge of significant changes in the exposure of man and animals to trace elements from diet and environment, the concern of nutrition policy planners for inorganic micro-nutrients remains overshadowed by that for the bulk components of the diet. The application of existing knowledge of trace element nutrition to problems of human and animal health will depend on a clear understanding of events that link molecular, biochemical mechanisms to the clinical manifestation of deficiencies.

Trace elements in man

It is likely that most, if not all, of the elements found to be essential in animals will be shown to be so for man, and the clinical picture produced by deficiency of the elements in the human patient will differ little from that seen in the animal, although this has been established for only five elements (I, Fe, Cu, Co and Zn). However, the link between lack of a given element in the soil and a human patient is far less direct and much more complex than that met with in the animal grazing on deficient pastures, except in isolated primitive communitis. Zn is the most protean of the trace elements and has been chosen to illustrate this in human practice. Excesses of essential elements (both trace and major) give rise to toxic effects and the importance of a proper balance especially of the transitional elements in the human diet is discussed with special reference to Cu, Zn and Fe. Certain non-essential trace elements are individual and community hazards: Cd, Pb and Hg are the principal offenders for humans. Mankind is now largely dependent on grassland products, cereals and livestock with increasing dominance of the former in human nutrition. This has reduced the bioavailability of trace elements so that study of trace metals, especially Zn and Cu, in skeletal and dental remains at human burial and occupation sites should prove useful in assessing the consequences of this striking change in dietary habits.

Trace elements and health: an overview

Trace element deficiencies, toxicities and imbalances in man are more difficult to relate to geochemical factors than they are in farm animals. The reasons for this are discussed and examples of such differences between man and grazing animals presented. The most convincing evidence of a geochemical causal link with human disease comes from the incidence and distribution of endemic goitre. The influence of technological developments upon this relation is discussed. Other associations between the physical environment, including the air and drinking water, and to health are given and critically examined in relation to the criteria necessary to distinguish between association and causation. The nature and extent of man-made modifications of the natural geochemical environment through technological change are discussed in relation to intakes of Fe, I, Zn, Pb and Se and their relation, in turn, to human health and disease. The currently proposed permissible limits or maximum tolerances of potentially toxic elements are presented, and the importance to these tolerances of the chemical and physical forms of the element and their metabolic interactions with other elements is emphasized.

Trace elements in animals

Trace element deficiency and toxicity in animals induces a wide variety of clinical effects although few are sufficiently specific to permit diagnosis without supporting investigation of changes in tissue trace element content or of the activity of metabolic processes influenced by trace element supply. Study of such trace element dependent processes has shown that extensive changes often arise before overt signs of disease appear. Some of these subclinical effects have pathological consequences and thus cannot be ignored when seeking correlations between geochemical anomalies and disease incidence. Many past estimates of the quantitative requirements of animals for the essential trace elements are imprecise. Although recent work is providing clearer definition of requirements, many common dietary components have a marked influence upon the efficiency with which such elements can be utilized from the diet. Recent evidence indicates that such antagonists influence both the absorption and the subsequent fate of essential and toxic elements in body tissues and these processes have to be taken into account when investigating the aetiology of disorders believed to be attributable to anomalies in trace element supply. Their existence is not always detectable if attention is confined to the trace element analysis of body tissues or to the nature of clinical lesions. Provided the complexity of soil-plant-animal relations with respect to trace element supply is fully recognized in the interpretation of data, the geochemical approach to the initial recognition of areas associated with a high risk of anomalies in trace element supply to animals and man has considerable potential value. This is already apparent from investigations upon the incidence of trace element problems in animals. As yet, its validity for similar purposes in man is less fully established.

Geochemistry and cardiovascular diseases

Deficiencies or excesses in the content or availability of trace elements in rocks and soils, or in water flowing through them, is hypothesized as a possible cause of certain chronic diseases, including cardiovascular diseases. Geographic distribution of cardiovascular diseases is often associated with geochemical differences. This trend is particularly evident in the United States and in Europe, with higher rates for cardiovascular mortalities being present in areas uunderlain by soils that are poor in most essential trace elements. Confirmation of this trend is found in connection with the degree of mineralization of local water supplies. Areas that are served by soft waters usually show higher rates of cardiovascular mortality and other forms of cardiovascular pathology, compared with the areas that are served by hard waters. Such a negative association between water hardness and cardiovascular pathology is evident in many countries, both industrialized and developing.

Selenium and total parenteral nutrition

Despite the increasing recognition of selenium (Se) as an essential trace element in man, little is known about its metabolism during total parenteral nutrition (TPN) and the possible development of Se deficiency in high risk patients. From a general population known by its geographical location to have low Se blood levels, we studied a group of 23 surgical patients receiving TPN for at least one week. Whole blood Se levels were less than in the normal general population and, being some of the lowest observed in adult man, approached levels observed in animals with Se-responsive syndromes. Se continued to be lost predominantly in the urine although the Se content of the TPN fluids was very low (less than 0.6 micrograms/24 hr). Patients with excessive volumes of gastrointestinal excretion lost more Se. Se supplementation may be required in some patients receiving TPN.

[Determination of the limits of the physiological action of mineral elements in hydrogen bacteria]

The growth of the hydrogen bacterium Hydrogenomonas eutropha Z-1 was studied in the conditions of continuous cultivation at different content of mineral elements in the medium. The physiological action of phosphorus, sulfur, potassium and magnesium in the bacterium was found to be displayed within a wide range of concentrations (mg/ml): P, 18-1400; S, 5.5-350; K, 4-550; Mg, 4.8-430. If the control of mineral elements was above or below these values, the specific growth rate of the cells decreased and their intracellular chemical composition changed.

Trace element analysis: a diagnostic tool

Human body continuously assimilates a variety of elements from the environment and the concentration of these elements in blood is regulated by means of various homeostatic mechanisms. Some of the elements, though present in very small amounts perform highly specialized functions in initiating many biochemical reactions. These elements, known as essential trace elements, are closely related to human diseases as their deficiency or excess induces physiological changes. Many diseases such as hypertension, atherosclerosis, diabetes etc are related to trace element imbalance. Therefore the measurement of trace elements in body fluids and tissues can be effectively employed for diagnostic tests.

Biochemistry of zinc

The biochemistry of zinc has come under intensive investigation at the molecular level during the past 20 years. More than 70 zinc metalloenzymes are now known, and they span a broad range of biologic activities. Substitution of zinc by cobalt, for example, serves to locate a paramagnetic probe at the active site of the enzyme which can then provide information regarding the coordination properties of the metal and the active site environment.