Assessment of Select Carcinogenic Heavy Metals in commonly consumed Foods in Nyambene, Meru County Kenya

Heavy metals are naturally occurring elements with high molecular weight and density greater than that of water ^[1]. Heavy metals are broadly classified into essential and non-essential metals. Essential heavy metals are required for normal body metabolic processes at low levels such as zinc (Zn), magnesium (Mg), cobalt (Co) and nickel (Ni), whereas non-essential metals are generally toxic to living organisms even at small concentration ^[2]. Non-essential heavy metals of public health importance according to World Health Organization include mercury (Hg), arsenic (As), cadmium (Cd), chromium (Cr) and lead (Pb) ^[3].

Heavy metals environmental contamination can originate from both natural sources and human activities. Natural sources include leaching and volcanic eruptions while anthropogenic activities include agrochemicals and occupational exposure amongst others ^[4]. Since most heavy metals are not degradable, they tend to accumulate and their overall concentrations remain high for a long period after being deposited into the environment ^[5].

Even in trace concentrations, heavy metals can build up in some edible crops. These heavy metals endanger human health through causing diseases like colorectal cancer when they get into our food chain ^[6]. To protect the public from heavy metal toxicity, the World Health Organization (WHO) and Food and Agriculture Organization have set up permissible limits for these heavy metals in foods ^[7].

To determine the levels of heavy metals in foods a number of analytical techniques are available. The earliest analytical method was basic gravimetric and titrimetric techniques and later on advanced instrumental approaches such as spectroscopic methods such as atomic absorption/emission spectroscopy (AAS and AES), inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectroscopy (ICP-MS) ^[8]. The choice of analytical technique is largely influenced by level of sensitivity, selectivity and analytical accuracy required with the more selective, accurate and analytical sensitive instrument/technique being costly.

In Kiegoi location of Nyambene, the commonly consumed foods include maize, beans, potatoes and arrowroot.

MATERIALS AND METHODS

Study design

This study employed a cross-sectional design to collect and analyze food samples from Kiegoi location of Nyambene.

Study setting

The research was conducted at the Kenya Plant Health Inspectorate (KEPHIS) headquarters in Karen, Nairobi Kenya.

Sampling technique and sample size determination

Food samples (maize, beans potatoes and arrowroot) were collected via a stratified random sampling across Kiegoi’s five agro-ecological zones. Yamane’s formula was used in determination of the sample size of the foods to be analyzed ^[9].

n=N/[1+N*e*e]

Where n=sample size

N= Population size

e= Margin of error

Taking an error margin of 5 % and using the five agro-ecological zones as the population size

n=5/[1+5*0.05*0.05]

We get a sample size of 5. Five samples in triplicates were therefore collected.

This involved randomly collecting one sample of the four selected food crops from different locations within each of the five zones.

Elemental analysis

Elemental analysis was performed with an agilent 7900a ICP-OES coupled with mass hunter (version 4.3) software. The agilent was equipped with standard nickel sampling and skimmer cones having orifice diameter of 1.0 mm and 0.4 mm respectively, along with a standard gas nebulizer.

Sample acid digestion

Acid digestion was carried out according to the procedure adopted from US environmental protection agency ^[10]. A gram of peeled dried and crushed arrowroot sample was weighed and placed into a 500 mL kjeldahl flask. 10 mL of concentrated nitric acid was added and the mixture heated in a digestion block at 180 degrees until the vigorous reaction subsided. 10 mL of hydrochloric acid was then added and the mixture heated until it was light colored and dense white fumes disappeared. 10 mL of perchloric acid was finally added and the mixture heated at 180 degrees until dense white fumes appeared. The mixture was then allowed to cool and filtered using Whatman’s number 1 filter paper.  The filtrate was transferred to a 50 mL volumetric flask and filled to volume with distilled deionized water. The procedure above was repeated for the other 3 foods without the addition of perchloric acid.

The diluted filtrate samples were then introduced to the ICP-OES for elemental analysis.

Data analysis and interpretation

Data was analyzed by two main ways one sample t test and Analysis of Variance (ANOVA). Differences were determined at P<0.05 and data presented using mean. IBM version 26 was used in analysis.

RESULTS AND DISCUSSION

Heavy metal contents in analyzed samples 

Four foods were sampled that is arrowroot (CA250425-CA250429), beans (CA250430-CA250434), maize (CA250435-CA250439) and potatoes (CA250440-CA250444) (Table 1). The mean concentration of each heavy metal across the samples provides an initial overview of the contamination intensity. Mercury range was 0.0695-0.280 mg/kg and its mean concentration was 0.1725 mg/kg which was well above the permissible limit of 0.02mg/kg. Lead had the ranges 0.063-0.2577 mg/kg with a mean of 0.1932 mg/kg that was marginally below the permissible limit of 0.2 mg/kg. Cadmium had the ranges 0.0096-0.035 mg/kg with a mean of 0.0256mg/kg which was way below the permissible limit. Arsenic had the ranges of 0.3030-0.7683 mg/kg with a mean of 0.4886 mg/kg which was slightly under the borderline health risk. Chromium had the ranges 13.68-48.35 mg/kg with a mean of 24.80 mg/kg which was way above the regulatory limit of 1mg/kg posing numerous health risks to the consumers. Findings from this study are similar to those by Oyugi et al., (2024) who analyzed heavy metals in Khat samples from Meru County and found elevated levels of heavy metals in these samples ^[11]. Overall, mercury and chromium exhibited the highest contamination rates with all 20 samples exceeding permissible limits, lead and arsenic each had 10 samples exceeding the permissible limits while cadmium levels in all samples were below permissible limits. Many of the samples therefore pose numerous health risks if consumed.

Table 1 Levels of heavy metals in foods

Comparison of food heavy metal levels with regulatory limits

Heavy metal levels in the analyzed foods was compared to WHO/FAO food safety thresholds using one sample t test (Table 2) to determine whether the observed means differ significantly. The results revealed significant deviation for mercury and chromium at 0.05 significance levels in every food sampled posing serious concerns about the potential health implications. The p value of cadmium was close to 0.0001 in all foods sampled indicating very strong evidence of cadmium levels being significantly below regulatory thresholds. Arsenic levels did not significantly exceed the regulatory threshold though several samples were close. Lead showed borderline excess in beans and arrowroots suggesting potential excesses but not statistically significant.

Table 2 One sample t test p values