Nutrition, trace element analysis

Present Status and Future Development of Trace Element Analysis in Nutrition... 

This discussion examines the recent progress of nutritional trace element research and its implications for trace element analysis. Elements recently identified as essential are present in low concentrations for which analytical methods are not yet reliable. Biological availability of trace elements depends on chemical form and on interactions with other inorganic and organic constituents of the diet. Therefore, information on elemental species is required, in addition to quantitative data. Finally, the demonstration of essential functions of trace elements previously known only for their toxicity necessitates establishing safe ranges of intake, free from danger of chronic toxicity but sufficient to meet human needs. 

This is documented by the discovery of essential functions for the "new trace elements in the past seven years (I) and by the increasing knowledge of marginal or pronoimced deficiencies in humans of such elements as iron (2), zinc (3), chromium (4), and perhaps copper (5) and selenium (6), Although some of these deficiencies occur only in special age and sex groups or under unusual conditions, their existence has aroused public and scientific concern for the exact definition of human trace element needs and for the assessment of trace element nutritional status. These two major challenges for human trace element research cannot be met without proper analytical support. I will attempt to describe the present status of trace element analysis as it relates to nutrition research and to point out some new concepts that might well influence the future direction of trace element analysis. 

Four new concepts that have emerged from recent trace element nutrition research could influence the future development of trace element analysis. 

Although the identification and analysis of such modifying factors go beyond the scope of trace element analysis, the importance of these factors should be recognized. The complexity of the process that leads from the first step of trace element analysis to the final statement of biological implication necessitates the close collaboration between the analytical chemist and the life scientist. The chemist should not be considered the provider of data, nor the biologist the interpreter of results rather, both scientists must be aware of the whole process—its complexity and its diflBculties (25). Only through this collaboration can the enormous amount of trace element analytical data be put in order and be interpreted properly. On this whole process depends the progress of trace element nutrition research and the improvement of the nutritional status of man. 

Mertz, W., Trace Element Analysis in Nutrition Research, in Abstracts... 

Keith, L.H. et al. (1983). Principles of environmental analysis, Anal. Chem. 55, 2210 Klich, H. and Walker, R, (1993). COMAR - The international database for certified reference materials, Fresenius J. Anal. Chem. 24S, 104 Kurfurst, U., Pauwels, J., Grobecker, K.-H., Stoeppler, M., Muntau, H. (1993). Micro-heterogeneity of trace elements in reference materials - determination and statistical evaluation, Fresenius J. Anal. Chem. 345.112 Parr, R.M. (1984). Quality assurance of trace element analysis, in Health Effects and Interactions of Essential and Toxic Elements (Proc. Symp. Lund, Sweden, June 1983) Nutrition Research, special supplement... 

Speciation analysis comes into its own mainly in environmental, nutritional, and biomedical research. The sample matrices are generally highly complex and the requirements for reliable (trace) element determinations are stringent (even for total amounts). The most important challenges in this context involve... 

There is still much ongoing debate on the validity of the different approaches to perform dietary analysis (single food analysis, duplicate diet analysis, total diet, and total mixed diet analysis) to provide an adequate answer to nutritional questions. The approach followed in this study has proven useful for the assessment of the actual dietary intake of trace metals that may be lost and/or added during the preparation of meals. The measurements done on cooked samples prepared according to the Italian local traditions provides an assessment of the actual concentrations of trace elements uptaken by the population through the diet. 

Inductively Coupled Plasma Atomic Emission Spectrometry ICP-AES is a technique half-way between FAAS and ET-AAS in terms of detection power. Among all ICP-AES features its robustness against matrix effects and its ability to carry out multielemental analysis predominate as the most advantageous [76-80], Multielemental analysis has also been successfully used to establish reference values [6, 76, 81-84] for many major and trace essential elements in different matrices of biological and nutritional interest, particularly in milk samples [81-83], Reference values for minor and trace element in human milk are collected in Table 13.8. 

An overview of the application of atomic spectrometric techniques to the elemental analysis of milk samples has been given. Elemental composition of milk, its nutritional role, sample preparation methods for analysis and measurement techniques have been described in detail. It appears that ICP-MS and ICP-AES are the most reliable techniques for the multielemental analysis of major, minor, and trace elements in milk samples. 

Elemental analysis of body tissues and fluids by atomic absorption spectrometry with electrothermal atomisation has advanced significantly the understanding of the role of trace elements in clinical biochemistry. All of those aspects of metabolic processes that are affected by changes in the concentrations of accessible trace elements have been studied. These include deficiencies of essential trace elements as a result of inherited or acquired metabolic disorders, or from nutritional inadequacy and excesses of trace elements producing toxicity states as a result of inherited metabolic disorders involving essential trace elements or from the inappropriate exposure to, or ingestion of, non-essential trace elements. 

In contrast, the remaining trace elements of nutritional interest are silicon, vanadium, chromium, manganese, nickel, arsenic, selenium, molybdenum, tin, and perhaps cadmium. These elements present serious problems of analysis in the concentration range that is of interest to the nutritionist. Only a few specialized laboratories have developed expertise... 

Work on the speciation of iron and zinc in vegetable products is just beginning (23), and some progress is being made on the speciation of chromium (24) but not enough data have been accumulated to be of interest to the nutritionist. Speciation analysis has produced impressive advances in the toxicology of heavy metals. Similar advances can be expected in trace element nutrition if the diflBculties of methodology can be overcome. 

Tissue concentrations of vitamins or trace elements are rarely measured in nutritional assessments because of the lack of availability of suitable tissue however, where such tissue is available, measurement may be helpful (e.g., copper analysis on liver biopsy of patients with suspected Whson s disease). 

In this connection one must point out the importance of multi-element analysis in serum for diagnostic purposes. The determination of a singie trace eiement gives no evidence about the relative abundance of this element, because many factors influence the "normal" actual trace element status stress situations, nutrition etc. The trace elements are mostly bound to proteins and therefore it should be standardized to the total protein content (Fig. 16). In practice, the phosphorus concentration in serum is a sufficient... 

There Is an advantage in using a readily obtained sample of head and/or body hair for trace metal analysis. This is a recognised technique when investigating suspected toxic metal exposure, and has been advocated as an index of nutritional status for the essential trace elements (Valcovic, 1977). 

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