Copper and zinc contents of Nigerian foods and estimates of the adult dietary intakes
Introduction
Accurate and adequate food composition data are invaluable for estimating the adequacy of intakes of essential nutrients and assessing exposure risks from intake of toxic non-essential elements. In many less-developed countries such data are not readily available (Bruce and Bergstrom, 1983, Burk and Pao, 1980, Hoover and Pelican, 1984). Trace heavy metals are significant in nutrition, either for their essential nature or their toxicity. Copper and zinc are known to be essential and may enter the food materials from soil through mineralisation by crops, food processing or environmental contamination, as in the application of agricultural inputs, such as copper-based pesticides which are in common use in farms in some countries. The adult human body contains about 1.5–2.0 ppm of Cu (Kies, 1989) which is essential as a constituent of some metalloenzymes and is required in haemoglobin synthesis and in the catalysis of metabolic oxidation (Schroeder, 1973, Underwood, 1977). Symptoms of copper deficiency in humans include bone demineralisation, depressed growth, depigmentation, and gastro-intestinal disturbances, among others, while toxicity due to excessive intake has been reported to cause liver cirrhosis, dermatitis and neurological disorders (Graham and Cordano, 1976, Lucas, 1974, Somer, 1974). Zinc constitutes about 33 ppm of adult body weight and is essential as a constituent of many enzymes involved in a number of physiological functions, such as protein synthesis and energy metabolism. Zinc deficiency, resulting from poor diet, alcoholism and malabsorption, causes dwarfism, hypogonadism and dermatitis, while toxicity of zinc, due to excessive intake, may lead to electrolyte imbalance, nausea, anaemia and lethargy (Fairweather-Tait, 1988, Prasad, 1976, Prasad, 1984, Valee et al., 1957). Considerable interest has thus been focused on the determination of the copper and zinc contents of foods, and the estimation of their dietary intakes among different groups of people.
Summaries of the composition of Nigerian foods (Oguntona and Akinyele, 1995, US Department of Health, Education and Welfare, 1967) generally show a sparsity of data on trace heavy metals contents. A survey of the literature reveals only a few studies of Nigerian foods for Cu and Zn contents, and these are very limited in the scope of foods surveyed (Akpanabiatu et al., 1998, Mbofung and Atinmo, 1980, Odukoya and Ajayi, 1987, Onianwa et al., 1999, Udoessien and Aremu, 1991). The study reported in this paper seeks to obtain more comprehensive data on the zinc and copper contents of a wider variety of Nigerian foods, for use in estimating the dietary intake of these elements in the average adult Nigerian. In doing this, about 80 different food items of various food groups which make up the daily diets of the ordinary Nigerian from the southern parts of the country were analysed. Using the results of these and other analyses, the dietary intakes of both metals have been estimated from a survey of the food consumption patterns of the various income classes of the adult population.
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Materials and methods
Eighty different food items of the following food groups were sampled and analysed: leafy and fruity vegetables, fruits, meats and related items, alcoholic beverages, fish, cereals, legumes, tuber/tuber products, spices/seasonings, dairy products, canned processed foods, additives and sweeteners, fats and oils, and confectioneries. The samples were purchased from markets in the city of Ibadan, SW Nigeria, and were handled carefully to avoid contamination. Samples were generally analysed as
Results and discussion
The concentrations of copper and zinc in the individual food items are given in Table 1. Table 2 gives the mean concentrations (± S.D.) of the metals for the various food groups. The Cu levels ranged widely from a low 0.06 mg/kg in wine to a relatively high 13.3 mg/kg in locust bean (a food seasoning). Analysis of variance of the concentrations of copper in the food items indicates that the differences observed in the Cu concentrations among the groups is statistically significant (F
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