Copper and zinc contents of Nigerian foods and estimates of the adult dietary intakes

Abstract

Concentrations of copper and zinc were determined in 80 Nigerian food items of various classes which were obtained from the markets of Ibadan city. These consisted of leafy and fruity vegetables, tubers, fruits, meats, fish, legumes, cereals, spices, dairy products, sweeteners, fats and oils, confectioneries, canned foods and alcoholic beverages. Copper levels ranged widely from 0.06 to 13.3 mg/kg, while zinc levels ranged from 0.06 to 56.9 mg/kg in various foods. The differences in mean metal levels were found to be significant (P=0.05) among the various food classes. Highest levels of both metals were found to occur in legumes (Cu, 8.3±3.7 mg/kg; Zn, 29±12 mg/kg). The levels of the metals compare well with those which have been obtained in foods from studies elsewhere in the world. The dietary intakes of both metals for the adult population was found to vary with the income groups, with higher intakes being estimated for higher income groups. The estimated weighted average dietary intakes for the entire adult population were calculated to be 2.64 mg Cu/day and 15.8 mg Zn/day.

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.