Esential Trace Elements

The human body as part of the terrestrial ecosystem draws its elementary composition from the elements of the Earth’s crust. If a sufficiently sensitive analytical method is used, practically all the elements of Mendeleev’s periodic system can be demonstrated in the human body. However, it is very difficult, and perhaps impossible, to interpret such finding at present.

Table 10.1 shows the percentage (elemental) composition of the Earth’s crust and the human body. Given the prevalence of organic matter, the building blocks of organic compounds (O, C, H, N) dominate in the elemental com-position of the human body.

Element Percen-tage (%)

Earth’s Crust Human Body

O 47 Mg 2.1 O 62.8 Na 0.26

Si 28 Ti 0.4 C 19.4 K 0.23

Al 8 H 0.1 N 9.3 Cl 0.18

Fe 5 Al 5.1 Mg 0.04

Ca 4 Ca 1.4 Fe 0.009

Na 2.8 P 0.63 Zn 0.00003

K 2.6 S 0.60 Cu 0.000001

Table 10.1. Percentage (elemental) composition of the Earth’s crust and the human body

Trace elements are found in the body in very low, “trace” amounts not exceeding 0.01 % of body weight, with ty-pical concentrations of μmol/l (μg/l) and lower.

They can be divided into essential and non-essential trace elements. Trace elements essential for humans are: Fe, Zn, Cu, Se, Cr, Co, Mn, Mo, I; possibly essential trace elements: Al, Si, F, V, Ni; trace elements with hitherto unknown physiological function, considered toxic heavy metals: Pb, Cd, Hg, As, Bi, Sn, Ag, Au, Pt, Ti, Rb, Sr, Ba, Y (Table 2).

Table 10.2. Arrangement of the elements listed above in Mendeleev’s periodic system

10.1.1. Definition of essentiality

Essentiality is generally defined by the occurrence of a specific defect in the structure or function of a tissue or organ resulting from a deficiency of the relevant element, and the remedy or prevention of such defect when such element is replenished. The definition of essentiality is currently supplemented by the requirement for knowledge and description of the element’s action at the molecular level, including the specific physiological effect in the body.

Most essential trace elements have an important function as co-factors of enzymatic catalysis processes such as metalloenzymes and non-enzymatic metalloproteins.

Trace element deficiency may be caused by insufficient intake in food or by their reduced utilization from food, de-creased absorption (maldigestion, malabsorption), inde-creased losses (diarrhoea, bleeding in the GIT, exudative entero-pathies) or by increased requirement (stress, fever, sepsis, tumours, traumas, burns). The early years of total parenteral nutrition without the supplementation of essential trace elements can be used as a model case of induced essential trace element deficiency. Numerous interactions between trace elements and other nutrients are known which may also cause their deficiency in the organism. There are also genetic defects in the population that have a considerable effect on trace element metabolism (Zn-Acrodermatitis enteropatica, Cu-Menkes disease, Wilson’s disease, congenital thyroid disorders, haemochromatosis).

In addition to their essentiality, some trace elements show high toxicity at higher concentrations (Se); others are toxic at high doses in connection with poisoning or chronic, often occupational exposure.

10.1.2. The diagnosis of essential trace element deficiency

The diagnosis of essential trace element deficiency is currently based on the following three principles:

• Laboratory assays of the trace element content in different body fluids (plasma, serum, urine, saliva) and cells or tissues (erythrocytes, leukocytes, thrombocytes, hair, nails, bioptic material);

• Determination of the activity of trace element-dependent enzymes;

• Clinical signs of the deficiency.

Clinical signs are usually accompanied by severe whole-body deficiency conditions. The absence of a sufficiently sensitive and specific test to reveal a slight or medium deficiency of trace elements presents a continuing problem.

It is advisable to combine all three procedures alongside their comprehensive assessment and evaluation, re-specting all available knowledge about specific and non-specific signs of deficiency, including other influences on the monitored parameters. In such situation, a therapeutic test continues to have a high predicative ability. Where causal connection exists, administration of the suspected deficient micronutrient will lead to an improvement in the clinical and laboratory signs.

10.1.3. Methods for trace element determination in biological material

Flame or electrothermal atomic absorption spectrometry (AAS) is the commonest used method for determining trace element concentration (except for iodine). It is absolutely essential to respect all precautions for trace assays in the pre-analytical and analytical phase, to use special collection sets, acid-washed laboratory glassware, “suprapure”

reagents, the highest quality of water and gases (for AAS), and to maintain a clean, dust-free laboratory environment, especially for ultra-trace assays.

10.1.4. General rules for adequate and safe intake of essential trace elements from food

As the essentiality of each trace element for the human body was discovered, the essential trace elements were included in recommendations for their dietary intake from food. Issues concerning the definition and determination of recommended values for the optimum daily dietary intake of essential micronutrients (trace elements and vitamins) continue to be discussed today. Such issues have a long history. The first recommendations, Recommended Dietary Allowances (RDA), were published in the US in 1941 in connection with World War Two and the introduction of the food rationing system. They included recommended nutrient doses, including micronutrients (particularly vitamins), intended to ensure their sufficient intake for the general population. Emphasis was put on preventing the occurrence of nutritional rickets, scurvy and pellagra. RDAs for essential trace elements were only specified in later revisions of this document. Today, RDAs are being replaced with a newly defined term, Daily Recommended Intake (DRI), intended to incorporate four other terms: Estimated Average Requirement (EAR), Recommended Daily Allowance (RDA), Adequate Intake (AI) and tolerable Upper Limit. The Population Reference Intake (PRI) is defined in Europe. Definitions of the above terms differ in details and in the range of their application to differently stratified populations of healthy and diseased people.

In general, they meet the following criteria:

• Minimum micronutrient amount preventing a specific disorder from occurring;

• Amount required to maintain an optimum nutritional status;

• Daily intake with the lowest risk of potential toxic effect.

In respect of the health of the local population, it can be stated that the normal mixed diet is a sufficient source of nutrients, including trace elements, except perhaps for iodine and selenium.

10.1.5. Food chain and food production

The content of essential trace elements in food largely relates to the food chain (soil, plants, livestock and humans).

The content of trace elements in the soil, which is subject to considerable, in global terms often vast, regional differen-ces, may directly influence grazing cattle in particular. Other farm animals, for example pigs and poultry, fully rely on the supply of trace elements in prepared feed and feed mixes. Serious deficiencies afflicting entire breeds may occur and do actually exist even here. The influence of local food production is decreasing to some extent due to ongoing globalization within the worldwide trade, export and import of staple foods.

10.1.6. Utilization of trace elements from food – interactions

Trace elements are an integral part of the human diet. In particular, they come from animal products, whose or-ganically bound elements are better utilizable for the human body than their inorganic forms. The mutual interactions of micro and macronutrients and other food components should be understood and respected when considering the adequate intake of essential trace elements. The commonest interfering substances are phosphates, fibre, phytanic acid (inositol-hexaphosphate) and other chelating agents. In general, one element supplied in excess may negatively affect the absorption and metabolism of others.

The text below deals with Iodine, Iron, Zinc, Copper and Selenium in detail. The other essential trace elements, Chromium, Cobalt, Manganese and Molybdenum will be mentioned only marginally, since they have yet to find any clear practical application in clinical medicine.