Dietary intake and bioavailability of trace elements

�9 1989 by the Humana Press, Inc. All rights of any nature, whatsoever, reserved. 0163-4984/89/2113--0173 $02.00

Dietary Intake and Bioavailability of Trace Elements

MOHAMED ABDULLA, *'l ABDULLA BEHBEHANI, 2

AND HUSSEIN DASHT! 2

'Faculty of Medicine, Baqai Medical College, Karachi, Pakistan; and Department of Surgery, Faculty of Medicine,

Kuwait University, Kuwait

Received October, 1988; Accepted December, 1988

ABSTRACT

In order to assess the nutritional importance of trace elements, it is relevant to consider the factors regulating their metabolism. One of the most important factors is the true intake level. Conventional techniques such as diet history and interview studies in conjunction with standard food tables do not provide the true intake levels from prepared meals. Employing the duplicate portion technique, we have investigated the dietary intake of trace elements in prepared meals consumed by children, adults, and elderly in Sweden. The results in- dicate that the intake of potassium, magnesium, zinc, copper, and selenium is low when compared with the present recommended dietary allowance (RDA) values. It appears that a marginal deficiency of a number of trace elements may exist in the general population of affluent countries. When the dietary intakes are known, it is neces- sary to consider the bioavailability. This depends on the chemical form as well as the concentration of other dietary constituents such as fiber, phytate, carbohydrates, macrominerals, and vitamins in the diet. Knowledge of these interactions are important to improve the overall nutritional status of the population in general and patients in particular.

Index Entries: Dietary intake; trace elements; bioavailability.

*Author to whom all correspondence and reprint requests should be addressed.

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174 Abdulla, BehbehanL and Dashti

INTRODUCTION

The nutritional importance of trace elements has grown rapidly during the last few decades, mainly because of a better understanding of their biological roles. In deficiency states, most essential trace elements create health problems. Iron-deficiency anemia, iodine-deficiency goiter, and growth retardation caused by zinc deficiency are good examples illustrating the importance of trace elements in human nutrition. Several other pathological conditions in humans and animals are implied to be associated with deficiencies of essential trace elements. Although starva- tion and malnutrition are restricted to certain poverty-stricken areas of the globe, it has become increasingly evident that a marginal deficiency of a number of trace elements, including zinc, copper, and selenium, is fairly common even in affluent countries (1-3).

In order to assess the nutritional importance of trace elements, it is relevant to consider the factors regulating their metabolism. These factors include: intake levels and bioavailability; transport from the gastrointestinal tract; the ultimate manifestation of a specific biological activity and storage and excretion. Once the essentiality of a trace element has been established, the first important factor regulating its metabolism is the concentration found in the diet. Barring occupational exposure, the food chain remains the major pathway through which the trace elements enter the human body. From a public health point of view, it is important to assure the general population that the intake of all essential trace elements is adequate to meet the requirements. Requirements established by controlled balance studies exist only for a few elements. Information concerning the intake of trace elements from prepared meals is limited at present. Moreover, the data that is available currently is often unsatis- factory since it is not based on the actual analysis of the diets consumed by the general population. The conventional techniques such as diet history and interview techniques that are commonly used for the assess- ment of dietary constituents have limitations when used for the estimation of trace elements. Food tables documenting the concentration of various nutrients in individual food items do not provide satisfactory information on the contribution of trace elements by prepared meals. Only direct analysis of the actual food consumed during 24 h can give the accurate information concerning the true dietary intake. The present paper will describe the results concerning the dietary intake of minerals and trace elements obtained from the chemical analysis of 24-h diets obtained from various populations in Sweden, employing the duplicate portion technique. This study has extended to Kuwait to relate the trace element status and some of the very common diseases, such as diabetes mellitus and hypertension, which are very common in that country.

Biological Trace Element Research VoL 21, 1989

Dietary Intake of Trace Elements

MATERIALS AND METHODS

175

The material consisted of 900 daily dietary samples from a group of children (11-14 y), adults (20-55 y), and elderly (more than 65 y) living in various parts of Sweden. The food samples were collected employing the duplicate portion technique (1,4). The duplicate portion was copy collected while eating normally to represent the food and fluids consumed during 24 h. The material collected during the day was pooled and later homogenized. The homogenized material was then lyophilized and the dry powder was used for the estimation of minerals and trace elements. For their analysis, approximately 0.5 g of the powder was wet-ashed using concentrated sulfuric, perchloric, and nitric acids. The temperature was kept under 200~ Following the wet ashing, the samples were diluted with ion-free water and stored in the cold room until analysis. For the analysis of calcium and magnesium, the samples were diluted with 2% w/v lanthanum chloride. Sodium and potassium were analyzed by flame photometry and the others by atomic absorption spectrophotometry. Iodine was estimated by a spectrophotometric method (5). A number of the dietary samples were also subjected to neutron activation for comparison of the results obtained by other techniques.

RESULTS

The results concerning the intake of Na, K, Mg, and Ca are shown in Table 1. The intake of potassium and magnesium is very low when compared with the RDA levels. There were no significant differences in the intake of the electrolytes between the groups, except sodium, which was significantly high in the diet of children. Table 2 shows the intake of Zn, Cu, Fe, and Se. Once again, the intake levels of Zn, Cu, and Se is low when compared with the RDA values. Table 3 shows the concentration of Ni, Cr, I, Hg, Cd, and Pb in a randomly selected number of 24-h diets. The concentration of the toxic metals, Hg, Cd, and Pb is much lower than the permitted levels. The intake of I, Cr, and Ni, on the other hand, is much more than that is considered to be adequate. The iodine levels are, in fact, 3-4 times higher than the recommended levels. An intake of up to 1000 i~g/d is considered safe for adults. The highest level found in the present study was 600 ~g. Some of the elements analyzed by both neutron activation and atomic absorption spectrophotometry showed a dif- ference of up to 15%. The I and Se analyzed by neutron activation, for example, was 15% lower than that obtained by atomic absorption spectrophotometry. We have also analyzed a number of minor elements such as Mn, As, Mo, V, and Si in a limited number of samples. The results are not included in this paper.


176 Abdulla, Behbehani, and Dashti

Table 1 Intake of Sodium, Potassium, Calcium, and Magnesium,

mmoVDay, Mean, and Range

Element

Groups

Children Adults, Elderly 11-14 y 20-55 y >65 y

Sodium 140 (25-320) 110 (20-280) 100 (18-270) Potassium 58 (16-100) 55 (17-90) 52 (17-85) Calcium 30 (15-120) 20 (6-70) 18 (6-68) Magnesium 11 (4-21) 10 (4-20) 9 (4-19)

Table 2 Intake of Iron, Zinc, Copper, and Selenium,

(~mol/d, Mean, and SD)

Groups

Element Children Adults Elderly

Iron 300 + 48 216 - 54 200 -+ 50 Zinc 125 +- 48 120 • 45 110 • 42 Copper 23 + 7 21 • 6 20 • 5 Selenium 41 • 5 40 • 4 40 • 4

Table 3 Intake of Chromium, Nickel, Iodine, Lead, Mercury,

and Cadmium, ~g/d, Mean, and Range

Element Number of Portions Concentration

Chromium 60 160 (50-580) Nickel 70 410 (90-1200) Iodine 100 290 (80-600) Lead 120 45 (8-90) Cadmium 75 18 (5-45) Mercury 80 7 (1-24)

DISCUSSION

In mos t of the nutr i t ion surveys conduc ted in various parts of the world, food tables documen t ing the individual food i tems have exten- sively been used to assess the dietary composi t ion. Such tables are generally based on the analysis of raw foodstuffs, and, as such, may not be accurate for est imating the intake of minerals and trace metals f rom prepared meals ready for consumpt ion . Besides, m a n y food tables have no data at all concerning the concentrat ion of trace e lements such as Cr, Se, Mn, and As. We have el iminated all these disadvantages by making use of the s tandardized duplicate port ion technique. The validity of the technique has been invest igated in detail by measur ing the urinary o u t p u t of

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Dietary Intake of Trace Elements 177

Na, K, and N2 and the subsequent comparison of the intake levels of the electrolytes and protein, taking into consideration the losses in sweat and feces (6, 7). From our results, it appears that the food collection technique did not significantly affect the daily dietary pattern.

The bioavailability of minerals and trace elements is usually ex- pressed as the percent absorbed from the gastrointestinal tract and uti- lized by the cells. This may depend on the chemical form of the element present in the diet as well as the presence of other dietary components such as phytate, fiber, proteins, macrominerals, carbohydrates, and certain vitamins. Besides inorganic species of trace elements, organometallic compounds may constitute the major fraction of the trace elements found in the diets. The bioavailability of organic and inorganic species may differ significantly. In this context, the absorption of iron from dif- ferent sources may illustrate this. Whereas an anemic person can absorb 35% of the iron from meat sources (heme-iron, organic), a healthy individual might absorb as low as 1% of the inorganic iron from foodstuffs, in the mixed diet (8). Generally speaking, salts of the element that are easily soluble in w/~ter and in the acidic envi ronment of the stomach are more available for absorption than the insoluble ones. Once in the duodenum, the solution chemistry of the elements determines their biologic fate. Many o f them are precipitated as insoluble compounds in the duodenum, bound to molecules such as phytate, fiber, and phosphate. These are thus made unavailable for absorption and excreted in the feces. The inhibition of trace element and mineral absorption by high fiber diets may be related more to the phytate contents than the fiber itself. Differ- ent forms of fiber may act differently on the uptake of trace elements from the gastrointestinal tract. Pectin, which is usually high in many fruits, does not affect mineral and trace element absorption. Other fiber forms, such as cellulose and hemicellulose, which are rich in vegetables, may affect some trace elements (9). Generally speaking, oversupple- mentation of diets with fiber may have an overall negative effect on trace element and mineral absorption.

From Table 1, it can be observed that the intake of calcium is fairly high when compared with some of the other minerals and trace elements investigated in the present study. The intake of calcium is particularly high in the diet of children. The high consumption of milk and milk products by the children is the main reason for the high calcium concentration in their diets. It is well known that the ingestion of very high levels of calcium can inhibit the absorption of other divalent cations such as iron, zinc, and magnesium (10,11). The use of mineral supplements, especially that of calcium, has increased significantly during the last few decades. In the US, a recent estimate indicates that the calcium supplements showed a sevenfold increase between 1980-1985 (12). Min- eral-trace element interactions may, thus, be of great importance when discussing the bioavailability of trace elements, especially when the intake is far below the recommended levels.

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178 Abdulla, Behbehani, and Dasht i

The bioavailability of trace elements may also be affected by the concentration of proteins, peptides and certain vitamins in the diet. Some proteins and peptides, such as hemoglobin and lactoferrin, may facilitate the absorption of minerals and trace elements such as iron and calcium. Lactose in the milk and milk products is k n o w n to enhance the absorption of calcium. Among the vitamins, folic acid, vitamins C and D are the most interesting ones with regard to trace e lement and mineral absorption. Al though the interactions be tween some of the trace elements are far from complete, some of them are practically very important. Inges- tion of very high amounts of iron may influence the absorption of zinc, whereas a high dietary zinc will inhibit copper absorption, resulting in anemia. It is, thus, important to consider these interactions when strate- gies for fortification and supplementat ion of diets with trace elements are planned.

REFERENCES

1. M. Abdulla, Nutritional Adequacy, Nutrient Availability and Needs, J. Mauron, ed., Birkh~iuser Verlag, Basel, Boston, Stuttgart, 338 (1983).

2. W. Mertz, Science 213, 1332 (1981). 3. W. Mertz, Phil. Trans. R. Soc. Lond. B.294, 9 (1981). 4. M. Abdulla, I. Andersson, P. Belfrage, I. Dencjer, M. J~igerstad, A.

Melander, /~. Nord6n, B. Scherst6n, T. Thulin, and B. Akesson, Scand. ]. Gastroenterol. 14, suppl. 52, 28 (1979).

5. M. Abdulla, M. J~gerstad, A. Melander, &. Nord6n, S. Svensson, and E. W~hlin, Scand. J. Gastroenterol. 14, suppl. 52, 185 (1979).

6. M. Abdulla, University of Lund, Sweden, Ph.D. Dissertation, 15 (1986). 7. M. Abdulla, M. J~igerstad, K. Kolar, /~. Nord6n, A. Schutz, and S.

Svensson, Trace Element Analytical Chemistry in Biology and Medicine, P. Br~itter and P. Schrammel, eds., de Gruyter, Berlin, New York, 75, (1983).

8. E. R. Monsen, L. Hatlberg, M. Layrisse, D. M. Hegsted, J. D. Cook, W. Mertz, and C. A. Finch, Am. J. Clin. Nutr. 31, 134 (1978).

9. J. L. Kelsay, Dietary Fibei" in Health and Disease, G. V. Vahouny and D. Kritchevsky, eds., Plenum, New York (1982).

10. J. L. Gregor, Nutrition Today 22, 4, 4 (1987). 11. C. M. Weaver, G. H. Evans, Food Technol. 40, (12), 99 (1986). 12. M. D. Uehling, K. Springen, Newsweek, Jan. 27, 52 (1986).


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Dietary intake and bioavailability of trace elements