Concentrations of Mn, Fe, Cu, Zn, Cr, Cd, Pb, Ni in selected Nigerian tubers, legumes and cereals and estimates of the adult daily intakes

Highlights

• We analysed the raw forms of some commonly consumed staple foods in Nigeria.

• The levels of metals in the samples analysed were within range previously reported.

• Intake of average amount of these foods poses no health risk to the consumers.

• This report is useful in risk assessment of consumer exposure to these metals.

Abstract

This study was designed to determine the levels of microminerals (manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), chromium (Cr)) and heavy metals (cadmium (Cd), lead (Pb) and nickel (Ni)) in some tubers, legumes and cereals obtained from the markets in Abeokuta city, South-West Nigeria. The food samples were digested by dry ashing procedure and their minerals were determined by atomic absorption spectrophotometer. The results show mean values of 1.67–32.00, 7.25–61.58, 1.59–10.56, 6.65–46.99, 0.02–0.58, <0.01–0.09, <0.08, and 0.06–0.14 mg/kg for Mn, Fe, Cu, Zn, Cr, Cd, Pb and Ni respectively. The levels of these metals in all the samples analysed were within the ranges reported for similar tubers, legumes and cereals from various parts of the world. The daily intakes of the metals through tubers, legumes and cereals were found to be lower than the provisional tolerable daily intakes proposed by Joint FAO/WHO Expert Committee on Food Additives.

Introduction

Food safety is a major public health concern worldwide and food consumption has been identified as the major pathway for human exposure to certain environmental contaminants; accounting for over 90% of intake compared to inhalation or dermal routes of exposure (Fries, 1995). About 30% of human cancers are caused by low exposure to initiating carcinogenic contaminants in the diet (Tricker & Preussmann, 1990). Such contaminants include microminerals (when higher than normal) and heavy metals. Microminerals, also known as trace elements or trace minerals, include those nutrients that are required in milligram or microgram quantities by organisms on a daily basis (Berdanier & Zempleni, 2009). The chief function of microminerals, except boron and chloride, is to serve as constituents of prosthetic groups in metalloproteins and as activators of enzyme reactions. Without trace elements as a “spark plug”, the enzyme system in organisms would simply be an inert mass of proteins (Gupta, Kening, & Siyuan, 2008).

Heavy metals are among the major contaminants of food supply and may be considered the most important problem to the environment. Such a problem is becoming more serious all over the world especially in developing countries. Heavy metal contamination of agricultural products may occur due to contamination with irrigation water, the addition of fertilizers and metal-based pesticides, industrial emissions, transportation, harvesting processes, storage and/or sale (Radwan & Salama, 2006).

The ingestion of heavy metals (Cd, Ni, Pb, etc.) can cause depletion of some essential nutrients in the body, which in turn causes a decrease in immunological defences, intrauterine growth retardation, psychosocial dysfunctions, disabilities associated with malnutrition and a high prevalence of upper gastrointestinal cancer (Turkdogan, Fevzi, Kazim, Ilyas, & Ismail, 2003).

Regular survey and monitoring programmes of micromineral and heavy metal contents in foodstuffs have been carried out for decades in most developed countries (Barlas, 1999, Jorhem and Sundstroem, 1993, Milacic and Kralj, 2003, Pennington et al., 1995a, Pennington et al., 1995b, Saracoglu et al., 2007). However, in developing countries such as Nigeria, limited data are available on microminerals and heavy metals, in some hospital diets (Akinyele & Osibanjo, 1982), in beverages and food drinks (Onianwa, Adetola, Iwegbue, Ojo, & Tella, 1999) and in several Nigerian foods of various groups (Onianwa, Adeyemo, Idowu, & Ogabiela, 2001). Maduabuchi et al. (2007) reported high levels of arsenic and chromium in canned and non-canned beverages in Nigeria. Williams, Ayejuyo, and Ogunyale (2009) also determined the trace metal levels in fruit juices and carbonated beverages in Nigeria.

All the above highlighted studies were limited to the determination of few metals in these foodstuffs. More so, major reports on tubers, legumes and cereals analyses are not recent (dated 10–15 years ago). It is therefore necessary to widen the scope of the microminerals and heavy metal analysis to include more foods and provide adequate information at present day. Thus the aim of this study was to determine the levels of microminerals (manganese (Mn), iron (Fe), copper (Cu) and zinc (Zn)) and heavy metals (chromium (Cr), cadmium (Cd), lead (Pb) and nickel (Ni)) in some Nigerian tubers, legumes and cereals. Furthermore, this study was designed to estimate the contribution of these foods to the daily metal intakes of Nigerians and compare the estimated intakes with international standards.