A Review of the Health Implications of Heavy Metals in Food Chain in Nigeria

Heavy metals such as Zn, Pb, Fe, and Cu are abundant in the environment and contribute largely to the sustainability and equilibrium of ecosystem processes. However, because of their bioaccumulation, nondegradability, and the excessive amounts in which they exist, these metals contaminate the food chain and subsequently become a source of toxicity to human beings and the entire ecological function. This is a major issue of concern within the study of environmental science and geochemistry. Although there is a global significance to the issue, it seems more immediate for the developing countries (DCs) such as Nigeria, where the pressure of the teeming population escalates the exigency for human sustainability, food security, and total eradication of hunger. Within the Nigerian context, many studies have examined this all-important issue, but most of these studies are fragmented and limited within the purview of mostly individual states and localities within the country. Taken on a wider geographical scale, the discussions and perspectives of these studies on heavy metal contamination of the food chain offer insufficient insight and expose merely a snapshot of the actual situation. As a result of this, a country-wide knowledge base of the implications of heavy metals on the food chain is lacking. Thus, the present study synthesises existing literature and their findings to create a knowledge base on the vulnerability of the food chain in Nigeria. Aquatic foods, fruits, vegetables, and major staple food such as tubers are the major host of carcinogenic and mutagenic components of heavy metals in Nigeria. This study motivates the Standard Organisation of Nigeria (SON), along with other food and agricultural agencies, to intensify their efforts in monitoring and analysing food components, and we advise consumers to eat with certain degrees of caveat.


Introduction
Since his famous publication titled, "What is a heavy metal?," Hawkes [1] has spurred far greater interests for more and more debates within environmental science, geochemistry, and toxicology research. Although Hawkes' study, from the stand point of natural and pure sciences research, primarily constructed a theoretical paradigm that lays the foundation for a critical understanding of heavy metals, it has definitely instigated a rising academic discussion, which now focuses on the abundance and implications of heavy metals within the human-ecological systems. Hawkes' study is also a seminal tool to drive a deeper and more cognate exploration of heavy metals. e context by which he introduced and expounded heavy metals as the block of metals and metalloids found in Groups 3 to 16 and in Periods 4 and greater of the periodic table makes sense to conclude that a metal is heavy not necessarily because of its density, rather its chemistry. is has remained a building block which various studies, in environmental science literature, have used to define heavy metals comprising metals and metalloids which are associated with adverse environmental effects, mostly pollution, toxicity, and contamination. Still, the diversity of definitions accorded to heavy metals in the literature creates an ideological conflict because of the dominance of density, relative atomic mass, and atomic number in the conceptualisation of heavy metals.
Recently, Ali and Khan [2] argued on a more comprehensive idea which suggests that heavy metals are naturally occurring metals having an atomic number greater than 20 and an elemental density greater than 5 gcm −3 . e study provided a list of fifty one elements that it referred to as heavy metals-with the exclusion of tin and arsenic. Obviously, there is an active debate on the conceptualisations of heavy metals which, needless to say, is not the focus of this study. However, many ideas drawn from extant research are logical pointers to what might be the likely implications of heavy metals on the food chain.
Basically, heavy metals are unarguably the transition and posttransition metals, and the examples which are common in various literature are lead (Pb), cadmium (Cd), vanadium (V), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), arsenic (As), nickel (Ni), manganese (Mn), tin (Sn), zinc (Zn), and mercury (Hg). e availability and accessibility of these metals and metalloids through natural and anthropogenic pathways remain a major global concern in the ecosystem [3][4][5]. e sources of heavy metals in the surface environment are natural and anthropogenic. Natural sources include parent rocks and metallic minerals. Anthropogenic sources include agriculture (fertilizers, pesticides, etc.), metallurgy (mining, smelting etc.), energy production (power plant, leaded gasoline, etc.), and sewage disposal [6,7]. A study at Ishiagu in Nigeria showed that while the Pb-Zn deposit is the natural source of heavy metals in the area, anthropogenic sources (from the use of power plants) could also be significant as power plants have been up to the time of the study the only source of electricity in the area [7]. In the North China Plain, the application of farmyard and chemical fertilizer was the pathway through which some heavy metals migrated into the soil [8]. In assessing the sources of heavy metals in the surface environment in a study area, anthropogenic activities of the habitants remain a major factor. In Nigeria, mining of ore deposits, oil and gas exploration and exploitation, and agriculture, besides manufacturing, construction, and production activities are among the several commercial and artisanal extractions of natural resources, capable of influencing the disruption, dispersion, and distribution of naturally placed heavy metals and metalloid to the environment [9]. ese metals and metalloid are further influenced by the conditions of the environment of deposition such as acidity, alkalinity, pH, adsorption, speciation, and temperature which inform their solubility, mobility, availability, and accessibility. ey also make their way into the different environmental media such as soil, water, rock, and sediment from which we grow most of our food. e chemistry of heavy metals is a major contribution to their implications within the human-ecological context. In terms of usefulness, the universe had been largely blessed by the abundance of heavy metals and the roles they play both in keeping the equilibrium and sustainability of ecosystem functions [10][11][12]. For example, Fe which is the most abundant metal on Earth and very electropositive can combine easily with O2 (oxygen gas) and thus forms a rich compound for industrial and biological purposes [13]. In their experimental research, Bae et al. [14] assiduously showed the crucial roles that Zn plays in dermatology. Cu is effectively used in the treatment for fish diseases [15] and to treat radiation sickness [16]. Nagajyoti et al. [17] argued that in trace amounts, most of the heavy metals are of significance to plant metabolism. However, these metals and metalloids adversely affect lives in the ecosystem, including humans, plants, and animals through their bioaccumulation and bioaugmentation in the food chain [18,19]. Alkorta et al. [20] and Volesky [21] were of the opinion that most heavy metals are toxic even at low concentration of about 0.1 to 0.3 mg/l. However, pH level plays a vital role in heavy metal ion adsorption from an aqueous solution, and speciation and adsorption level influence the mobility of these metals [22]. e soil type can also influence adsorption of heavy metals; for instance, soil clay minerals have a greater influence on adsorption and inactivation compared with soil organic matter [23].
In view of these impacts, Ali et al. [24] observed that some heavy metals possess the ability to disrupt metabolic activities and genetic makeup, while others affect embryonic or foetal development. is often results in cancer, developmental disorders, and neurological and behavioural changes often found in children. So, the health impacts of heavy metals on the food chain, which include the soil, plants, and aquatic life, cannot be more obvious. As a major source of human exposure to heavy metal toxicity, the food chain is a critical component that now draws the attention of major discussions within the academy of nutritional and environmental sciences (see, for example, [25][26][27]). With the increasing human populations and rising issues of food insecurity which have heightened the demand for genetically engineered food, it is now ever more crucial that current research should examine the typology of heavy metals found in the food chain and the pathway in which these metals contaminate a range of staple foods consumed by many people within the diversity of geographical contexts.
is study focuses on the negative impacts of heavy metals and metalloids in the food chain regarding their total concentration levels in the environment in Nigeria. With the early industrialisation stage in Nigeria and poor implementation of environmental protection policies and regulations, its environmental pollution and contamination have been a great concern. According to Onakpa et al. [28], both natural and anthropogenic sources are pathways by which heavy metals and metalloids contaminate many Nigeria food crops and vegetables. Previous studies on the health implications of heavy metals are fragmented and mostly focused on individual states and localities within Nigeria [29][30][31]. e findings of these studies are consistent with the general view that heavy metals are nonbiodegradable, and metallic elements with relatively high density are toxic or poisonous even at low concentrations. ey accumulate in the environment and over time contaminate the food chain, which is a major source of environmental and human health risk [1,24,32,33]. Despite the obvious significance of these studies, a country-wide knowledge base of the implication of heavy metals on the food chain within different regional and socioeconomic settings of Nigeria is lacking. is is a major gap in science, which this review makes efforts to address. It provides important insight into the toxicity of heavy metals 2 e Scientific World Journal and the particular foods that are increasingly vulnerable to heavy metals and hazardous for consumption with respect to different geographical regions in Nigeria. e authors synthesise the findings of the current studies to create a knowledge base which somewhat simplifies an understanding of the impacts of heavy metals on the food chain within the context of Nigeria. Given the high population of Nigeria, its extreme poverty level, and environmental and public health challenges [34], food availability and consumption are key factors to its continuous existence. erefore, research must inform decision towards ensuring that health and sustainability are core in the choice of foods that Nigerians consume.
is will mitigate several health issues that constrain economic and human capital development within the country.

Methods and Data
To achieve the objectives of this review, the authors conducted a structured search process to identify the body of the current literature relevant to the study. In collecting relevant literature, the authors ensured that they maintained quality and academic standard, which are often the basis of most literature review studies, for example, Manikas and Hansen [35]; Nkwunonwo et al. [36]; onemann and Schumann [37]. erefore, the present literature search considered mainly research articles published in highly reputed journals (indexed by Scopus and Scimago and ranked in ompson Reuter). e authors consulted EBSCO's environmental database, which hosts top open access environmental research journals: Greenfile, which archives well-researched debates covering all aspects of human impact on the environment. e study also considered several other libraries including DOAJ (Directory of Open Access Journals) digital Library, Google Scholar, PubMed, E-resources, IEEE Explore, Spring-erLink, ScienceDirect, and omson Reuters' Web of Science. Key terms such as "heavy metals," "food chain," and "food poisoning," along with the combination of terms such as "heavy metal in Nigeria," "heavy metal and food chain in Nigeria," "heavy metals and health in Nigeria," and "heavy metals and food chain contamination" were applied to the search. Authors adopted the idea by Manikas and Hansen [35], with which they designed and applied a set of five inclusion and exclusion criteria: (1) the literature should address heavy metals as an area of core research, (2) the literature must be a research paper, i.e., being published after a rigorous peer review, (3) the language expressed in the selected article must be English, (4) the literature must not be more than ten years of age from the time of the present research, except when it is a seminal study, and finally (5) the literature must be none of these: an abstract with extended perspective, short communications, letters, and presentations. Although authors relished bringing in the global focus of environmental geochemistry and heavy metal toxicity, they maintained that a good number of the sourced studies must focus on heavy metals in Nigeria. With this aim, the study had to consider some locally based journals in situations where local ideas seemed unassailable and inevitable, but authors made sure that they maintained the same criteria for exclusion and inclusion.
On the whole, studies derived from the literature search have acknowledged the richness and usefulness of heavy metals within the human-ecological system. It also acknowledged significantly the negative implications of these metals, especially in the food chain. Discussions on the toxicity of heavy metals also include the means to assess and mitigate these impacts. ough Kumar et al. [38] discussed the process of phytoextraction, Ali et al. [24] presented phytoremediation of heavy metals. It also shows that other methodologies including photocoagulation [39], biosorption [40], and rhizofiltration [41] have been found in the extant research literature. ese are proven scientific techniques that have now formed the basis of best practices in the study of the impacts of heavy metals on the food chain system. Finally, the literature search shows that despite the wide-ranging discussions on heavy metals and food chain, research has developed little regarding the application of more scientific techniques to mitigate the heavy metal impacts on the food chain in Nigeria.

Results and Discussion
Nigeria is one of the DCs facing massive environmental contamination through natural constituents such as heavy metals and metalloids. e concentration levels of heavy metals alongside their bioavailability and bioaccessibility can cause contamination of trophic levels, as shown in Table 1. is table also shows different regions where foods could serve as good dietary sources due to essential trace metals, besides the levels for human consumption safety margin. Animals, leafy vegetables, and fruits are consumed widely because of their nutritional value and culinary purposes. erefore, they are part of the daily diet in many Nigerian households. e consumer's perception determines the quality of such food and vegetables. Although, this perception is subjective, it is mostly based on the measure of the colouration and leaf size of vegetables. Hence, there is a general assumption that vegetations with dark green and big leaves are of better quality. However, Mapanda et al. [63] have shown that external morphology is inadequate for leafy vegetables qualitative assessment as heavy metals rank high amongst the major contaminants. In the soil, heavy metal accumulation depends on plant species. However, the efficiency of plants in absorbing metals is determined by either plant uptake or soil-to-plant transfer factors of the metals. Plant tissues absorb and accumulate heavy metals based on the available pH, moisture content, temperature, organic matter, and nutrient [64]. Table 2 shows some risks associated with food chain contamination for humans and some trophic levels in Nigeria.  4 e Scientific World Journal involved in the biosynthesis of heme) and developmental problems such as impaired cognitive function, behavioral disorder, stunted growth, and impaired hearing at blood lead level as low as 5 μg/l [9,78]. Lead also interferes with calcium metabolism, both directly and by interfering with vitamin D metabolism; it concentrated largely in bones in animals and humans and interferes with the normal maturation of erythroid elements in the bone marrow [79]. ese effects have been observed in children at blood lead levels ranging from 12 to 120 μg/l. Lead is toxic to both the central and peripheral nervous systems, inducing subencephalopathic neurological and behavioral effects. Other effects include epidemiological effects (blood lead level of 30 μg/l), intelligence quotient deficits of about four points in children due to prenatal/postnatal exposure to lead (blood lead level ranging from 11 to 33 μg/l), headache, irritability, constipation, weight loss, fatigue, hypertension, miscarriages, stillbirths, and renal tumors [80]. However, there is evidence from studies in humans that adverse neurotoxic effects other than cancer may occur at very low concentrations of lead, and that a guideline value derived on this basis would also be protective for carcinogenic effects [81]. A report of prevalence dental caries in children in the Tamar Valley, England, and in Ceredigion, Wales, was associated with a high level of Pb in soils which were available to plants [79].

Manganese (Mn).
Adverse effects of Mn can result from both deficiency and overexposure, causing neurological effects due to high level in drinking water, which can cause tremor, gait disorders (seen in primate), psychological symptoms such as irritability, and emotional liability [80]. For instance, amyotrophic lateral sclerosis, according to Hubbs-Tait et al. [80], is a progressive neurological disorder, which appears to be a disease that may reflect a deficiency of Mg or Mn. e decreased intake of Mg or Mn leads to a decreased ability to store and use thiamin (vitamin B 1 ). ese authors further noted that there is some evidence suggesting that Parkinson's disease may be casually related to Mn, but in this case, to an excess of it.

Nickel (Ni).
Nickel is an essential trace element in animals. Some of its health risk includes fibrosis, chronic bronchitis, impaired pulmonary function, and emphysema [82]. Allergic contact dermatitis is the most prevalent effect of toxicity of nickel in the general population [78]. However, it is suspected to be an essential element for some plants and animals [83]. According to Plant and ornton [84] and Carla [85], Ni deficiency results in decreased plasma cholesterol, increased liver cholesterol, ultrastructural changes in the liver calls, rough hair, impaired reproduction, and poor growth of the offspring.

Zinc (Zn).
Zinc is an essential trace element whose threshold value in surface and groundwater normally do not exceed 0.01 and 0.05 μg/l, respectively [86]. However, concentrations in tap water can be much higher as a result of dissolution of zinc from pipes. It should be noted that drinking water containing zinc levels above 3 μg/l may not be acceptable for consumers [87]. Izah et al. [62] e Scientific World Journal 5

Cobalt (Co).
Cobalt is an essential microelement for humans in the form of vitamin B 12 with a complex pathway through the food to man [79,86]. Deficiency of Co results in pernicious anemia syndrome, whereby the intrinsic factor from the stomach that facilitates B12 absorption is absent, characterized by larger than normal (macrocytic) red blood cells plus neurologic abnormalities. Excessive Co added to the foam stabilizer in beer however produced severe cardiomyopathy, haematologic, neurologic, and thyroid abnormalities in humans [79,88]. A relationship between Co/ iodine (I) ratios in geochemical environment showed an inverse correlation between Co in water and soil and thyroid enlargement in animal and man.

Copper (Cu) and Chromium (Cr).
ese are important essential elements but when consumed in excess, they cause toxicity [89,90]. It should be noted that the threshold for the effects of copper on the gastrointestinal tract still leaves some uncertainty regarding the long-term effects of Cu on sensitive populations, such as carriers of gene for Wilson disease and other metabolic disorders of copper homeostasis [81]. North: Pb, Zn, Cr, Fe, Mn, Cu, Co, Ni, Some heavy metals are introduced into the environment from farmyard and chemical fertilizer application (agrochemicals). Others include domestic by-products, worn automobile tires, roofs, brake linings, and waste food. ere are also iron mining sites (Itakpe, Kogi State). Sample of tested sediment from the river Ngada, Maiduguri in Borno State revealed provocative amounts of heavy metals. Evidence of mobility of heavy metals from dumpsites to farmlands through leaching and runoff in Lafia, Nasarawa State Increase concentration of zinc in pasture fields. Anthropogenic sources usually produce heavy metals that are high in instability and solubility and in turn, result in high bioavailability which increases potential health risk. Decrease in soil productivity by high Pb concentration, while extreme low concentration can inhibit some essential plant processes such as mitosis, water adsorption, and photosynthesis. is can lead to brown short roots and stunted growth/foliage. Toxicity of heavy metals on the other hand can lead to reduced ability in leguminous plants to fix molecular nitrogen Copper deficiency results in kinky and steely hair syndrome in humans and abnormal wool in sheep, while excessive Cu intake results to hepatolenticular degeneration with progressive impairment of Cu-laden tissues until death results [81]. It also helps in interconversion of the major neurotransmitters, dopamine, noradrenaline, and adrenaline, and in pigment production. Zinc-Cu interaction has shown hypothesis of ischemic heart disease, which proposes that decreased Cu intake with excessive Zn may play an aetiologic role in cardiac deaths in both animals and man [79,88]. Contamination and pollution of vegetable and soils near smelters does occur, and excessive Cu in drinking water has been reported to have caused a toxic syndrome in an infant called pink disease [84]. Chromium helps to maintain blood glucose levels, but its toxicity can result in allergic dermatitis such as eczema [91].

Cadmium (Cd).
Cadmium may also be adsorbed onto organic substances, such as humic and fulvic acids, and therefore organic-rich waters may have higher Cd concentrations given a local Cd source [79,85]. Chronic exposure to the metal can lead to kidney disorders, anemia, emphysema, anosmia (loss of sense and smell), cardiovascular diseases, renal problems, and hypertension [84]. Itaiitai disease appears to be a Cd-related disease, which is very painful and causes the wastage and embrittlement of bones [78].

Arsenic (As).
Arsenic is a metalloid whose chronic exposure effects include tingling, numbness, and peripheral neuropathy according to Plant and ornton [84]. ese authors also argued that arsenic toxicity in cattle has been found to cause dysentery and respiratory distress. An ecological correlation between the arsenic level of well water and mortality from various malignant neoplasm in China (Province of Taiwan) demonstrated a significant association with the arsenic level in well water ranging from 0.35 to 1.14 mg/l with a median of 0.78 mg/l for cancer of the liver, nasal cavity, lung, skin, bladder, and kidney and hyperpigmentation, hyperkeratosis, Blackfoot disease (a type of gangrene) in both males and females, and prostate cancer in males [92].
Heavy metal contamination is a global issue [93][94][95][96][97] and of high significance in Nigeria. Evidence from research within the context of Nigeria suggests a growing concern for protecting different varieties of food from the toxic effects of heavy metals [30,69]. e present outcome from various laboratory investigations, such as those of Ogunkunle et al. [56] and Obiora et al. [29], show that some of these heavy metal contaminations, particularly those affecting dairies, most of the aquatic habitat, and the beverage industry, are within safety level, according to international standards. However, chances are that they would heighten their toxicity because of the ever-increasing industrial and anthropogenic activities. erefore, future toxicology assessment and food chain safety measures are being recommended [62].
Of all the heavy metals found in the literature, Pb, Cd, Hg, and Mn have the highest health risk index (HRI) in Nigeria because of their bioavailability. ese heavy metals are present in most of the staple foods, such as cassava found in the southern and southeastern parts of Nigeria. is is a risky situation for people who live in these areas where the consumption of such foods is part of the endeared human culture. Other findings and concerns from the review include the following:

Conclusion
Heavy metal poisoning of the food chain has stimulated increasing research interests within the field of environmental chemistry and toxicology. Earth materials comprise an abundance of heavy metals that support the ecosystem processes. Several cases of human health impacts have resulted because of bioavailability, bioaccessibility, and excessive concentrations and distribution of contaminants in the food chain. ese heavy metals contaminate most of the soils, farmlands, and aquatic habitats. e chemistry of these metals, rather than their density, is a major contributor to their usefulness, more so their negative impacts on the human-ecological systems. e Scientific World Journal 7 Although existing research on food chain contamination in Nigeria is fragmented and localised, this review has shown that heavy metal contamination affects Nigeria's food chain although at present, the amount of these toxic substances found in beverages, food condiments, and some aquatic animals is at a safety level. Sources of contaminants are both natural and anthropogenic.
ese toxic substances make their way to the farmland, water, and food stuff exposed in the market by mostly the process of leaching and runoff. Anthropogenic activities contaminate different environmental media such as the soil, water, and air which support the growth, production, and supply of foods. Ingestion, inhalation, and dermal are the major exposure pathways through which heavy metals in locally produced foods in most of the regions get to the consumers. Toxicity of these heavy metals can cause potential health hazards. us, longterm consumption of these foods is of public health significance. ere is the need for better quality control for food crops to protect consumers from contamination exposure. In addition, close monitoring of the significant possibility of significant harm of these heavy metals on the health of the population in these regions is essential. It is also important to use remediation measures in contaminated areas where local foods are grown or produced.
In view of the implications of heavy metals in the food chain in Nigeria, researchers showed insufficient fundamental discussions on the understanding of the true nature of the health risk to inform practical and implementable decision and policy. It therefore shows that a country-wide knowledge base is lacking. is is a major limitation, which possess potential constraints on insight for future research focus. erefore, this prevention study aimed to address the gaps in knowledge by synthesising existing knowledge and drawing from the examples presented in extant research. In addition, this review shows that more scientific techniques for mitigating the impacts of these heavy metals' contamination are lacking in Nigeria although research documents a few primordial and conventional toxicity reduction techniques. e spatial distribution of these toxicities in Nigeria's food chain is the major issue that suggest the need for further research, which should map and delineate spatially the variations in heavy metal toxicity within the whole of Nigeria. is will provide insight into those states and localities that mostly need food intervention. It will further enhance quality decision and policy making towards country-based food safety adaptation and capacity to cope with food toxicity and diseases resulting from heavy metal contamination of the food chain.

Conflicts of Interest
e authors declare that they have no conflicts of interest.