HUMAN DIETARY EXPOSURE TO HEAVY METALS THROUGH CONSUMPTION OF FISH IN NAKURU TOWN, KENYA

ABSTRACT

Heavy metal pollution in the environment is a problem experienced all over the world. Whereas heavy metals exist naturally in the environment, heavy metal pollution is associated with anthropogenic activities. Pollution studies reveal that the aquatic ecosystems in Kenya and the fish that inhabit these ecosystems are contaminated with heavy metals. Literature also reveals that exposure to levels of heavy metals above the recommended doses may possibly cause various adverse health effects. This study assessed human exposure to lead, cadmium and copper through consumption of fish in Nakuru town, Kenya. Three hundred and eighty five (385) fish consumers who buy and consume fish from the markets in Nakuru town were randomly selected to participate in a cross-sectional survey, to establish the consumption patterns of the widely consumed fish species in Nakuru town. Composite samples of the widely consumed fish species were bought and transferred to Egerton university laboratory for heavy metal analysis using flame atomic absorption spectrophotometer. Data was analyzed using both descriptive and inferential statistical tools. In addition, estimated weekly intake (EWI), target hazard quotient (THQ) and hazard index (HI) of lead, copper and cadmium were computed in order to assess if human exposure to these three heavy metals in fish is above the recommended standards. The findings revealed that Tilapia, Silver sardine, Nile perch and Lungfish were the widely consumed fish species in Nakuru town. The pooled averages of lead, copper and cadmium in the widely consumed fish species were 7.0898±2.6889, 1.9977±1.0577 and 0.4498±0.3394 mg/kg respectively. Computed estimated weekly intake (EWI) of lead, copper and cadmium for the consumers of the widely consumed fish species were within FAO/WHO recommended safe limits. Computed THQ values of cadmium for high consumers of large Tilapia from Ponda Mali market and small Tilapia from the Main municipal market indicated possible health risk for the consumers of these fish. Fish fillets were the safest forms of fish in the market as far as exposure to the three heavy metals is concerned. The findings of this study can be used by food safety and quality control entities in Kenya as a premise upon which to prescribe the amount of fish that is safe for consumption with minimal health risks over a life time.

CHAPTER ONE

INTRODUCTION

Background Information

Heavy metal pollution in the environment is a problem intensified by increasing anthropogenic activities. Rapid urbanization and industrialization generate extremely large amounts of waste which contain heavy metals (Singh et al., 2010). Other anthropogenic activities including agriculture, transportation, quarrying, mining and combustion of fossil fuels also contribute to heavy metal pollution in the environment. These heavy metals are eventually transported into the aquatic ecosystems (Zhang & Shao, 2013) where they become assimilated by aquatic fauna and flora (Tirkey et al., 2012). Fish specifically are exposed to heavy metals in their habitats through their diets, water ingestion, the ionic exchange of dissolved metals in the gills, as well as adsorption of these metals on their skin surfaces (Tirkey et al., 2012). These toxic substances bio-accumulate in different organs of the fish and through the process of bio- magnification may ultimately reach levels that are a couple of hundreds or thousands of times higher than the concentration in the water, sediments and their diets (Babatunde et al., 2012).

Bio-magnification also known as biological magnification is the process where by heavy metals and other substances increase in concentration as they successively move up the food chain. Bio-magnification occurs normally because these substances are not easily broken down or because the rate of internal degradation or excretion is low or non-existent. Most heavy metals are lipophilic and since fat soluble substances cannot be excreted via water-based medium, they accumulate in fat tissues of living organisms (Gray, 2002). When eaten by another organism, the fats carrying heavy metals are absorbed in the gut of the predator which then accumulates in the predator organism. Heavy metals may be present in lakes or sea water in very low concentration. These metals in lakes or sea water are absorbed but very slowly excreted by photoautotrophs. Consequently bioaccumulation results in a buildup of heavy metals in adipose tissues of successive trophic levels: zooplanktons, invertebrates, small fish and large fish following that order. Anything that consumes these large fish including man consumes high levels of heavy metals (Teague, 1999; Gray, 2002; Tirkey et al., 2012; Babatunde et al., 2012).

Still fish are important sources of proteins and essential nutrients particularly Omega-3 fatty acids. They have low fat content as well and because of these reasons fish is considered an important component of a healthy meal (Damodhar & Reddy, 2012). But with the current trends of heavy metal pollution and given that heavy metals are not easily biodegradable (Rejomon et al., 2008) it implies that increased fish consumption may be exposing consumers to high concentrations of heavy metals. Whereas trace elements like copper, manganese and zinc are essential for enzymatic activity in the human body, exposure to high concentration of these chemical substances is toxic (Mwashote, 2003). Other metals including lead, cadmium, and mercury are not required by the human body and thus exposure to these chemical substances may induce acute or chronic toxic outcomes (Zukowska & Biziuk, 2008).

In Kenya studies on pollution originating from heavy metals have been done in the marine, inland and man-made aquatic ecosystems. These studies show elevated levels of heavy metals in the water, sediments, fish and other fauna and flora of these systems (Muohi et al., 2003; Ochieng et al., 2007; Oyogi et al., 2011). Muiruri et al., (2013) revealed that heavy metals may accumulate in fish in extremely high levels even when the concentration of the same metals are relatively low in the water. In addition, studies done in other parts of the world indicate that fish may have additional sources of heavy metal contamination other than aquatic pollution. Practices like fishing, handling, processing, transporting and storing may also elevate the levels of various heavy metals in fish. (Khansari et al., 2005; Hasyimah et al., 2011; Baboli & Velayatzadeh, 2013)

Because of the toxic nature of heavy metals, international agencies including FAO, WHO and USEPA have developed guidelines and scientific methodology to be used to determine if heavy metals in food items and beverages are within the recommended safe limits (Zukowska & Biziuk, 2008). When dietary exposure to heavy metals exceed the recommended limits adverse health effects associated with these heavy metals may occur. Each heavy metal exhibit unique signs of its toxicity. Nonetheless, the general long-term health effects associated with heavy metals include; neurotoxicity, developmental delays among children and young adults, high blood pressure, impaired hearing acuity, impaired hemoglobin synthesis, bone, muscle and joint disorders, cardiovascular disorders, male reproductive disorders, and brain damage. In addition, long term exposure to some heavy metals including lead (Pb) and copper (Cu) is considered a high risk factor for various forms of cancer. The short-term health effects associated with exposure to high concentration of heavy metals include; gastrointestinal disorders, nausea, diarrhoea, stomach ache, stomatitis, tremor, ataxia, paralysis, vomiting, convulsion, depression, headache and fever (Järup et al., 1998; Duruibe et al., 2007).

Many environmental and health disasters associated with heavy metal pollution have been document around the world. Well-known among them is the Minamata disaster that happened from the 1950s in Minamata city, Japan. Minamata disease was caused by the release of the highly toxic methylmercury into the aquatic ecosystems from the Chisso Corporation's chemical factory. This toxic substance bio-accumulated in fish and other sea food in Minamata bay and Shiranui sea which when consumed by the locals resulted into acute and chronic mercury poisoning. According to the Japanese government, 2,955 people contracted Minamata disease and 1,784 people have since died (Tsubaki & Katsuro, 1977; Harada, 1995).

Another major incident associated with heavy metal pollution is the Itai Itai disease caused by cadmium poisoning due to mining activities in Toyoma prefecture in Japan. Cadmium was being released in significant quantities into Jinzu River and its tributaries by mining operations. And because the same river was being used for irrigation of rice fields, drinking, washing, fishing by users downstream, high dietary and dermal exposure resulted into cadmium poisoning among the population downstream. Common symptoms associated with Itai itai disease include weak and brittle bones, spinal and leg pain and waddling gait that develops due to bone deformities caused by cadmium. The pain eventually becomes unbearable, with fractures becoming more common as the bone weakens. Other complications include coughing, anemia, and kidney failure, leading to death (Kobayashi, 1978; Nogawa, 1981).

Statement of the Problem

There is sufficient evidence from literature to indicate that fish from the aquatic ecosystems in Kenya are contaminated with heavy metals. Literature also indicate that fish consumption is a dietary source of exposure to heavy metals. Nonetheless, little research in Kenya has been conducted on the extent of human exposure to heavy metals through fish consumption. Thus, it is difficult to establish if fish consumers are exposed to doses of heavy metals in fish that could result into adverse health effects. This is despite the fact that more and more Kenyans are shifting to white meat due to the known health benefits and that the Kenyan government is promoting aquaculture and fish consumption now more than ever before. Limited information on the extent of human exposure to heavy metals in fish makes it difficult to establish if fish consumption predisposes consumers to adverse health effects associated with these toxic substances. Therefore this study sought to determine the level of human exposure to heavy metals by determining the concentration of heavy metals in fish in relation to fish consumption patterns of fish consumers.

Objectives

Broad objective

This study contributed towards generating information on the extent of human dietary exposure to heavy metals through consumption of fish sold in Nakuru town.

Specific objectives

1 To determine the consumption patterns of the widely consumed fish species in Nakuru town

2 To determine the concentration of lead, cadmium and copper in the edible portions of the widely consumed fish species

3 To compute exposure to lead, cadmium and copper through consumption of the widely consumed fish species and compare with recommended standards

Research Questions

1. What are the consumption patterns of the widely consumed fish species in Nakuru town?

2. What is the concentration of lead, cadmium and copper in the edible portions of the widely consumed fish species in Nakuru town?

3. What is the level of exposure to lead, copper and cadmium through consumption of the widely consumed fish species and how does it compare to the recommended standards?

Justification of the Study

Increased air, water and soil pollution has made contamination of food items with heavy metal an unavoidable problem in the recent past. The greatest concern with these environmental toxicants is they are not easily degradable and thus once they accumulate in living organisms, they are bio-magnified up the food chain (Murtala et al., 2012). When one is exposed to concentration of heavy metals above recommended doses, adverse health effects may occur. The general health effects associated with ingestion of lead beyond the recommended limit include increased risk of both systolic and diastolic blood pressure, increased susceptibility to dental caries (tooth decay), increased bone deformities, decreased fertility among men, increased pre- term birth among expectant women, decreased body stature and delayed sexual maturation among adolescents who have reached puberty. Among children, lead affects brain and cognitive development, visual-motor integration and intelligence quotient among others (ATSDR, 2007; Chen et al., 2014).

The general health effects associated with ingestion of cadmium beyond the stipulated limit include renal tubular damage which is the critical health effect of cadmium exposure. It may also cause cardiovascular problems that include disorders of the cardiac conduction system, lower blood pressure, ventricular fibrillation and coronary heart diseases. Cadmium has also been classified by the United States Environmental Protection Agency as a group B1 probable human carcinogen (USEPA, 2014). In addition, oral exposure to cadmium may cause severe irritation of gastro-intestinal system, it may also reduce gastro-intestinal intake of iron and this could cause anemia. Other chronic health effects of cadmium include osteoporosis, osteomalacia and other bone, joint and muscle defects that have been observed in many different populations where dietary cadmium intake is high (Järup et al., 1998; ATSDR, 2012; Bernhoft, 2013).

The general health effects as a result of dietary copper intake beyond the recommended limit include gastro-intestinal disorders including vomiting, nausea, malaise, diarrhoea, stomach ache and abdominal pain. Long-term exposure to copper could cause irritation of the nose, mouth and eyes, headaches, dizziness, liver and kidney damage and this could possibly lead to death. Chronic exposure to copper results into Wilson’s disease characterized by hepatic cirrhosis, brain damage, renal disease, muscle rigidity, psychiatric disturbances and involuntary muscle movements (Mbuthia et al., 2014). In addition, a lot of scientific studies have postulated a possible link between long term dietary copper exposures to declining intelligence among young adults. It was thus necessary for this study to be conducted in order to establish if fish consumption predisposes fish consumers in Nakuru town to the abovementioned adverse health effects associated with lead, copper and cadmium.

This study supports the social pillar of the Kenya’s Vision 2030 which seeks to provide clean, safe and healthy environment for all Kenyans by the year 2030. The findings of this study provides baseline information for food safety and quality control organizations like Kenya Bureau of Standards (KEBs) and Department of Public Health (DoPH) whose mandates include enhancing consumer safety in Kenya. These entities can use the findings of this study as a premise upon which to prescribe safe quantity of fish to consume with minimal risk of exposing consumers to concentrations beyond internationally recommended levels. In addition the study supports goal number 3 of the sustainable development goals that seek to secure healthy lives and promote wellbeing for all at all ages.

Scope of the Study

This study was conducted in a period of nine months. The study focused on assessing human exposure to heavy metals through consumption of the widely consumed fish species from 5 major fish markets located in Nakuru town, Kenya. Survey based cross-sectional study was used alongside laboratory heavy metal analysis to obtain information on the patterns of consumption of the widely consumed fish species in Nakuru town and the concentration of lead, cadmium and copper in the samples of these fish species. This study had two target populations. The first target population consisted of fish consumers who buy and consume fish from the main fish markets in Nakuru town. The second target population consisted of fish species that are widely consumed by fish consumers and bought from the main fish markets in Nakuru town.

A multistage purposive sampling procedure was used in the selection of fish consumers with the main sampling units being five main fish markets in Nakuru town. Three hundred and eighty five (385) fish consumers who buy and consume fish from the five identified markets were randomly sampled to participate in the survey; 77 from each market. Ohio Environmental

Protection Agency sampling procedure for Whole Body Composites (WBC) was adopted to sample the composite fish samples of the widely consumed fish species. National Health and Nutrition Examination Survey (NHANES) food frequency questionnaire was adopted, modified and used to collect data on the consumption patterns of the widely consumed fish species. The concentration of copper, cadmium and lead in the composite samples of the widely consumed fish species was obtained by Atomic Absorption Spectrophotometer (Thermo Jarrell Ash S11) fitted with appropriate hollow cathode tubes for element-specific lamps. The level of human exposure to lead, copper and cadmium through consumption of the widely consumed fish species was computed using estimated weekly intake (EWI); target hazard quotient (THQ) and hazard index (HI).

Limitations of the Study

The main limitation for the study was the probability that respondents would not provide accurate information on the amount of fish they consume and this could lead to underestimation or over estimation of heavy metal intake through fish consumption. To address this limitation, various sizes of fish were used during questionnaire administration in order to help the respondents estimate to a feasible extent the fish they normally purchase and consume. The sources of fish (the lakes, rivers, ponds where the fish were captured or harvested) could not be established because there are so many cartels involved along the market chain of fish in Kenya. Thus it was not possible to relate heavy metal concentration in the samples of some widely consumed fish species fish with specific habitats where these fish were captured/harvested.

Assumptions

The assumption of this study was that the respondents would be willing and able to participate in the survey and truthfully provide information on their fish consumption patterns.

Definition of Terms

Consumption patterns: refers to the preference and purchasing behaviour as well as the consumption frequency and amount of the widely consumed fish species

Estimated Weekly Intake: is a calculation of the likely weekly consumption of a hazardous substance based on the estimated levels of the hazardous substance in foods, the amount of those foods which are eaten by the different groups and their body weight.

Hazard index: is the summation of the hazard quotients (target hazard quotient) for all chemicals to which an individual is exposed. It is used to determine the risk of developing adverse health effects caused by exposure to multiple hazardous substances.

Hazardous substance(s): Substances which, upon release into the atmosphere, water, or soil, or which, in direct contact with the skin, eyes, or mucous membranes, or as additives to food, cause health risks to humans or animals through absorption, inhalation, or ingestion.

Heavy metals: Refers to any metallic chemical element that is toxic and poisonous at low concentrations.

Human exposure: the subjection of human beings through inhalation, oral or dermal contact to contaminants or hazardous substances taking into consideration the concentration of contaminant and duration of exposure.

Human exposure assessment: is the scientific process of measuring or estimating the magnitude, frequency, and duration of human exposure to an agent in the environment.

Pathway: Path, course, route or way through which chemicals reach their receptors.

Target Hazard Quotient: is the ratio between the potential exposure to a substance and the reference dose – the level at which no adverse effects are expected. It is used to calculate a risk based concentration for non-carcinogenic contaminants.

Toxicity: the quality or condition of being toxic. The degree to which a substance is toxic

Widely consumed fish species: fish species eaten by majority of fish consumers in Nakuru town.

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