Description
ABSTRACT
This study assessed the levels of seven heavy metals: lead, cadmium, zinc, copper, chromium,
nickel and iron in the muscle, liver, kidney, intestine and tripe of cow slaughtered and consumed
in Enugu State, Nigeria. The objective of the study was to compare the distribution of these
metals in the different parts of the body, compare their concentration with international
guidelines and estimate the dietary intake and exposure of the consumers to these metals from
cow meat.
One hundred and fifty samples made up of thirty each of muscle, liver, kidney, intestine and tripe
were procured from different markets in Enugu State. Two grams (2g) of each dried sample was
digested with a 3:2 HNO 3 /HClO 4 mixture and made to 20 mL with de-ionized water, and
analysed for heavy metals with a GBC Avanta ver 2.0 AAS, model A6600 AVANTA PM. The
dietary exposure analysis was carried out using a food frequency questionnaire, filled by
different categories of the populace. The data obtained were subjected to Monte-Carlo
probabilistic modeling. Results showed that range of detectable values in µgg -1 of the studied
metals were: Pb(0.01-0.72), Cd(0.01-0.80), Zn(456.10-601.11), Cu(0.92-5.28), Cr(0.56-10.68),
Ni(0.24-2.36), Fe(41.99-151.64) in muscle; Pb(0.08-501.79), Cd(0.02-0.90), Zn(104.49-653.04),
Cu(3.90-192.29), Cr(0.26-22.05), Ni(0.03-2.26), Fe(112.87-1697.37) in liver; Pb(0.040-44.89),
Cd(0.10-1.12), Zn(400.02-560.19), Cu(8.93-17.50), Cr(7.56-17.96), Ni(0.40-3.30),
Fe(146.86916.47) in kidney; Pb(0.01-108.02), Cd(0.01-0.90), Zn(328.54 -477.06), Cu(1.78-
9.62), Cr(12.13-21.47), Ni(0.64-2.75), Fe(44.52-526.61) in intestine and Pb(0.01-127.90),
Cd(0.011.10), Zn(410.26-530.47), Cu(2.13-5.94), Cr(15.80-30.27), Ni(0.27-2.98), Fe(76.23-
2408.07) in tripe. Pb, Fe, Cu, Zn were accumulated mostly in liver; Cd and Ni in kidney; and Cr
in tripe. Cd,
Zn, Cr and Ni concentrations were above international maximum permissible levels in most
samples. Pearson’s correlation showed correlation between some of the metals in the various
meat parts and showed a consistent negative correlation between cadmium and chromium in
most of the tissues, thus showing that cadmium interferes with the availability of chromium. One
way analysis of variance showed no significant difference between Pb and Cd in the various
meat parts (p>0.05) while levels of Zn, Cu, Cr, Ni and Fe were all significantly different
(p<0.05) in the various tissues.
The dietary exposure analysis revealed low weekly consumption: 0.162 kg for muscle, 0.149 kg
for liver, 0.076 kg for kidney, 0.198 kg for intestine and 0.182 kg for tripe. The estimated mean
dietary intake in µg per kg body weight per week for the various categories of the studied
population are in the following ranges: Pb (0.12-1.16); Cd (0.40-1.93); Zn (420-2362); Cu
(6345); Cr (9.25-93); Ni (1.4-6.9) and Fe (153-1692). With the exception of Cr, the intakes of
other metals were lower than the provisional tolerable weekly intake (PTWI).The dietary intake
of the metals was therefore leading to low exposure from cow meat to most of the metals.
However, exposure to chromium ranged from 62-340% of the recommended daily intake (RDI).
On the whole, with the exception of chromium, the dietary intake of these metals from cow meat
poses no danger to the consumers.
LIST OF FIGURES
Figure 1: Absorption and distribution of inorganic
chemical elements in animals………………………………………6
Figure 2. Dietary intake assessment methods………………………………..66
Figure 3: Bar chart representation of the mean concentration
of lead in the different part of cow meat……………………………..92
Figure 4: Bar chart representation of the mean concentration of
cadmium in the different part of cow meat…………………………92 Figure 5: Bar
chart representation of the mean concentration
of zinc in the different part of cow meat………………………………93
Figure 6: Bar chart representation of the mean concentration of copper
in the different part of cow meat……………………………93 Figure 7: Bar chart
representation of the mean concentration of chromium in the different
part of cow meat……………………….94 Figure 8: Bar chart representation of the
mean concentration of nickel in the different part of cow
meat…………………………..94
Figure 9: Bar chart representation of the mean concentration of iron in
the different part of cow meat…………………………95 Figure 10: Bar chart
representation of mean dietary intake of lead by the different
categories of the population……………………….100 Figure 11: Bar chart
representation of mean dietary intake of cadmium by the different
categories of the population……………………….103
Figure 12: Bar chart representation of mean dietary intake of zinc by
the different categories of the population……………………….106
Figure 13: Bar chart representation of mean dietary intake of copper
by the different categories of the population……………………….109
Figure 14: Bar chart representation of mean dietary intake of chromium
by the different categories of the population……………………….112
Figure 15: Bar chart representation of mean dietary intake of nickel by
the different categories of the population……………………….115
Figure 16: Bar chart representation of mean dietary intake of iron by
the different categories of the population……………………….118
LIST OF TABLES
Table 1: Livestock population in Nigeria 2001-2007…………………………………….8
Table 2: Major areas of poultry and livestock production in Nigeria……………………8
Table 3: Livestock management systems in Nigeria…………………………………….9
Table 4: Biological Significance of the Classification of Metals
Based on the periodic Table……………………………………………………14
Table 5: Summary of reports on losses during dry ashing……………………………….42
Table 6: Comparison of wet oxidation and dry ashing techniques…………………..…..46
Table 7: Contribution (% of daily intake) of various food groups to
dietary intakes of Pb, Cd, Hg…………………………………………………..62
Table 8: Estimated intake of dietary toxic metals (µg per person -1 day -1 )
from total diet…………………………………………………………………..64
Table 9: Standard Analytical Condition for a GBC Avanta ver 2.02 AAS………………77
Table 10: % Recovery for Pb, Cd, Zn, Cu, Cr, Ni and Fe in liver samples………………83
Table 11: Co-efficient of variation for Pb, Cd, Zn, Cu, Cr, Ni and Fe…………………..84
Table 12: Mean moisture content of the various meat parts in %……………………………85
Table 13: Concentrations of Pb, Cd, Zn, Cu, Cr, Ni and Fe (µg/g) in muscle………….86
Table 14: Concentrations of Pb, Cd, Zn, Cu, Cr, Ni and Fe (µg/g) in liver…………….87
Table 15: Concentrations of Pb, Cd, Zn, Cu, Cr, Ni and Fe (µg/g) in kidney………….88
Table 16: Concentrations of Pb, Cd, Zn, Cu, Cr, Ni and Fe (µg/g) in intestine………..89
Table 17: Concentrations of Pb, Cd, Zn, Cu, Cr, Ni and Fe (µg/g) in tripe…………….90
Table 18: Mean concentrations of the metals in the
different parts of cow meat in µg/g……………………………………………91
Table 19: Mean concentration (µg/g) of some heavy metals reported elsewhere……….96
Table 20: Mean daily consumption (g/day/person) of the different
parts of cow meat by different categories of the populace……………………97 Table
21: Mean dietary intake of lead by the different
categories of the population………………………………………………………99
Table 22: Mean dietary intake of cadmium by the different
categories of the population……………………………………………………..102
Table 23: Mean dietary intake of zinc by the different
categories of the population……………………………………………………..105
Table 24: Mean dietary intake of copper by the different
categories of the population……………………………………………………..108
Table 25: Mean dietary intake of chromium by the different
categories of the population……………………………………………………..111
Table 26: Mean dietary intake of nickel by the different
categories of the population……………………………………………………..114
Table 27: Mean dietary intake of iron by the different
categories of the population……………………………………………………..117
TABLE OF CONTENTS
Title page………………………………………………………………….i
Approval page……………………………………………………………ii
Certification…………………………………………………………….iii
Dedication………………………………………………………………iv
Acknowledgement……………………………………………………….v
Abstract…………………………………………………………………..vii
List of figures…………………………………………………………….ix
List of tables………………………………………………………………xi
Table of contents…………………………………………………………xiii
CHAPTER ONE
INTRODUCTION
1.0 General introduction………………………………………………………….1-2
1.1 Bioavailability and distribution of chemical pollutants……………………..2-4
1.2 Chemical toxicity……………………………………………………………..4-7
1.3 Need for study of heavy metals concentrations in animals…………………7-9
1.3.1. White Fulani (breed of study)…………………………………………………9-10
1.3.2. Heavy metals in cattle…………………………………………………………10-11
1.4 Objectives………………………………………………………………………..11
1.4.1. General Objective………………………………………………………………11
1.4.2. Specific Objectives…………………………………………………………….12
1.5 Scope of study………………………………………………………………12
CHAPTER TWO
LITERATURE REVIEW
2.0 Metals………………………………………………………………………..13
2.1 Classification of metals……………………………………………………….13
2.1.1. Classification based on Periodic Table…………………………………….13-14
2.1.2 Classification based on Lewis acid behavior………………………………14-16
2.1.3. Classification based on the standpoint of environmental
and biological studies…………………………………………………16-17
2.2 Pharmacologic and therapeutic uses of metals…………………………….17-20
2.3 Heavy metals………………………………………………………………..20-21
2.4 Sources and toxicological effects of some heavy metals…………………..21
2.4.1 Lead……………………………………………………………………….21
2.4.1.1. Toxicity of Lead…………………………………………………………22-24 2.4.2
Cadmium…………………………………………………………………..24
2.4.2.1. Toxicity of Cadmium…………………………………………………….24-26
2.4.3. Mercury…………………………………………………………………….26-27
2.4.3.1. Toxicity of Mercury………………………………………………………..27-28
2.4.4. Arsenic……………………………………………………………………..28-29
2.4.4.1 Toxicity of Arsenic………………………………………………………….29
2.5 Essential Elements……………………………………………………………29-30
2.5.1 Copper…………………………………………………………………………30
2.5.2 Chromium…………………………………………………………………….30-31
2.5.3 Zinc……………………………………………………………………………31
2.5.4 Nickel……………………………………………………………………………31
2.5.4.1. Toxicity of Nickel……………………………………………………………..32
2.5.5. Selenium………………………………………………………………………..32-33
2.5.6 Iron……………………………………………………………………………….33-34
2.6 Bioaccumulation of heavy metals……………………………………………….34-35
2.6.1. Factors affecting Bio-accumulation of heavy metals……………………………35
2.6.1.1 Persistence………………………………………………………………………..36
2.6.1.2 Size of organism……………………………………………………………….36
2.6.1.3 Biological factors………………………………………………………………36
2.6.2. Mechanism for bio-accumulation……………………………………………36-37
2.6.2.1 Direct absorption………………………………………………………………37
2.6.2.2 Food chain……………………………………………………………………..37
2.7 Analytical techniques for heavy metal detection………………………………..37
2.7.1. Sample preparation………………………………………………………………..37-40
2.7.1.1. Dry ashing……………………………………………………………………….40-42
2.7.1.2. Modified Dry Ashing……………………………………………………………43
2.7.1.3. Wet Ashing…………………………………………………………………….44-46
2.7.1.4. Microwave Digestion……………………………………………………………46
2.7.2. Detection of heavy metals…………………………………………………………46-47
2.7.2.1. Inductively Coupled Plasma Atomic Emission
Spectroscopy (ICP-AES)…………………………………………………………47-48
2.7.2.2. X-ray Fluorescence………………………………………………………………48-50
2.7.2.3. Neutron activation analysis……………………………………………………..50-52
2.7.2.4. Spark-source mass spectrometry………………………………………………..52
2.7.2.5. Anodic stripping voltametry……………………………………………………52-53
2.7.2.6. Gas –liquid chromatography…………………………………………………..53-54
2.7.2.7. Electron microprobe……………………………………………………………54-55
2.7.2.8. Atomic absorption spectrometry……………………………………………….55-57
2.7.2.8.1. The cold vapor mercury technique………………………………………….57-59
2.7.2.8.2. Hydride generation technique……………………………………………….59-60
2.7.2.8.3. Graphite furnace atomic absorption………………………………………….61-62
2.8. Dietary intake of heavy metals………………………………………………….62-65
2.9. Methods of estimating dietary intake……………………………………………65-66
2.9.1. Household surveys……………………………………………………………….66
2.9.1.1. Food account method………………………………………………………….67
2.9.1.2. Inventory method………………………………………………………………67
2.9.1.3. Household record……………………………………………………………….67
2.9.2. Individual surveys………………………………………………………………67-68
2.9.2.1. Food Records or Diaries………………………………………………………..68
2.9.2.2. 24-Hour Recall…………………………………………………………………68-69
2.9.2.3. Food Frequency Questionnaire (FFQ)……………………………………..69-71
2.10. Portion size in dietary assessment………………………………………………71
2.10.1. Direct Weighing of Food Portions…………………………………………….72
2.10.2. Visual Estimation of Weights…………………………………………………72-73
2.10.3. Visual Estimation of Sizes……………………………………………………73
2.10.3.1. Household measures……………………………………………………….73
2.10.3.2. Food Models………………………………………………………………….73-74
2.10.3.3. Photographs………………………………………………………………..74
CHAPTER THREE
EXPERIMENTAL
3.0. Analysis……………………………………………………………………………75
3.1. Heavy metal analysis……………………………………………………………..75
3.1.1. Sample collection ………………………………………………………………75
3.1.2. Washing of glass wares…………………………………………………………75-76
3.1.3. Sample preparation………………………………………………………………76
3.1.3.1. Digestion of samples………………………………………………………….76
3.1.4. Instrumentation…………………………………………………………………77
3.1.5. Preparation of standard solutions for heavy metal recovery experiment………78
3.1.5.1. Lead standard solutions………………………………………………………..78-79
3.1.5.2. Cadmium standard solutions………………………………………………….79
3.1.5.3. Zinc standard solutions…………………………………………………………79
3.1.5.4. Copper standard solutions……………………………………………………..79
3.1.5.5. Chromium standard solutions…………………………………………………79-80
3.1.5.6. Nickel standard solutions………………………………………………………80
3.1.5.7. Iron standard solutions…………………………………………………………80
3.1.5.8. Preparation of Mixed Standard Solution………………………………………80
3.1.5.9. Recovery Experiments………………………………………………………….81
3.1.5.10. Precision/ repeatability………………………………………………………..81 3.1.6.
Statistical Analysis……………………………………………………………….81
3.2. Dietary Exposure Analysis…………………………………………………………82
3.2.1 Statistical Analysis…………………………………………………………………82
CHAPTER FOUR
RESULTS
4.0. Precision and Accuracy…………………………………………………………….83-85
4.1. Metal analysis………………………………………………………………………86-96
4.2. Results on dietary intake…………………………………………………………..97-119
CHAPTER FIVE
DISCUSSION AND CONCLUSION
5.0. Heavy metals in cow meat………………………………………………………120-128
5.1. Dietary intake……………………………………………………………………128-129
5.1.1 Dietary intake of lead from cow meat………………………………………….130-131
5.1.2. Dietary intake of cadmium from cow meat…………………………………….131-132
5.1.3. Dietary intake of zinc from cow meat………………………………………….132-133
5.1.4. Dietary intake of copper from cow meat…………………………………………133-134
5.1.5. Dietary intake of chromium from cow meat……………………………………134-135
5.1.6. Dietary intake of nickel from cow meat…………………………………………135
5.1.7. Dietary intake of iron from cow meat……………………………………………136-139
5.2. Conclusion…………………………………………………………………………139-140
5.3. Contribution to knowledge………………………………………………………..140-141
5.4. Recommendation………………………………………………………………….141
REFERENCES………………………………………………………………………….142-160
APPENDIX
1. Statistical Data…………………………………………………………………………161-169
2. Mean daily consumption of meat and dietary exposure to heavy metals
questionnaire………………………………………………………………………………170-178
CHAPTER ONE
INTRODUCTION
1.0. General Introduction
For decades, there has been increasing global concern over public health with emphasis on
impacts of environmental pollution, in particular, the global burden of disease. The World
Health Organization (WHO) estimates that about a quarter of the diseases facing mankind
today occur due to prolonged exposure to environmental pollution (UNEP,
http://www.unep.org/urban_environment/PDF/DandoraWasteDump_ReportSummary.Pdf).
Most of the environment-related diseases are however not easily detected and may be acquired
during childhood which will later manifest in adulthood.
Metals, a major category of globally – distributed pollutants, are elements that have been
extracted from the earth and harnessed for human industry and products for millennia. The
industrialization of the world has dramatically increased the overall environmental load of
heavy metals toxins. Industry and commercial processes have actively mined, refined,
manufactured, burnt and manipulated heavy metal compounds for a number of reasons. Today
heavy metals concentrations have increased in our drinking water, air, soil and food due to our
increased use of metallic products. They are present in virtually every area of modern
consumerism, from construction materials to cosmetics, medicine to processed food, fuel
sources to agents of destruction, appliances to personal care products (Extreme Health, 2005).
The toxicity of heavy metals depends on a number of factors. Specific symptomatology
varies according to the metal in question, the total dose absorbed, and whether the exposure
was acute or chronic. The age of the person can also influence toxicity. For example, young
children are more susceptible to the effects of lead exposure because they absorb several times
the percent ingested compared with adults and because their brains are more plastic such that
even brief exposures may influence developmental processes (Soghonian and Sinert, 2008).
The route of exposure is also important. Elemental mercury is relatively inert in the
gastrointestinal tract and also poorly absorbed through intact skin, yet inhaled or injected
elemental mercury may have disastrous effects (Soghonian and Sinert, 2008). Exposure to
metals may occur through the diet, from medications, from the environment, or in the course
of work or play. Where heavy metal toxicity is suspected, time is taken to study dietary,
occupational, and recreational history since identification and removal of the source of
exposure is frequently the only therapy required.
A full dietary and lifestyle history may reveal hidden sources of metal exposure. Metals
may be contaminants in dietary supplements, or they may leach into food and drink stored in
metal containers like lead decanters. Persons intentionally taking colloidal metals for their
purported health benefits may ultimately develop toxicity. Metal toxicity may complicate
some forms of drug abuse, for example, beer drinker's cardiomyopathy was diagnosed in
alcoholics in Quebec, and later Minnesota, during a brief period in the 1970s when cobalt was
added to beer on tap to stabilize the head (Soghonian and Sinert, 2008). More recently, a
parkinsonian syndrome among Latvian injection drug users of methcathinone has been linked
to manganese toxicity.
The uptake of chemical elements by living organisms is a dynamic and complex process.
Plants and animals absorb these elements from soil, sediments and water by contact with their
external surfaces, through ingestion and also from inhalation of air borne particles and
vaporized metals. The assimilation of an element (i.e the bioavailability fraction) depends on a
number of chemical and physicochemical factors such as chemical speciation, solubility in
organic medium, pH e.t.c.
In soils, metals and metalloids can occur in both solid and aqueous (i.e. soil solution) phases.
In solution, these elements can exist, either as free ions or as various complexes associated
with organic or inorganic ligands or as suspended colloidal particles. In the solid phase, they
can be adsorbed or absorbed on organic and inorganic soil components, exist as minerals or
precipitated with other minerals. In general, ions in solution are more available for plant and
animal uptake, immediately entering the food chain. However, metal ions present in the solid
phase may be available under certain biological and physico-chemical conditions such as
exudation of special chelators, desorption, redox and pH changes e.t.c. (Silva et al., 2005).
Significant contamination of seeds, plants and plant products with toxic elements due to
contaminated soil and water has been observed as a result of release of these toxicants into the
sea, rivers, lakes or even irrigation channels (Millis et al., 2004). The consumption of
contaminated vegetation constitutes an important route of animal exposure to heavy metals.
Animals are exposed to these toxicants through a number of other routes. The most important
among these are respiratory mostly for gases and particulate matters; dermal contact with
chemicals able to cross the skin barrier, and from various food sources.
Absorption of metals and metal compounds inhaled as particles are influenced by several
processes that include deposition, mucociliary and alveolar clearance, solubilization and
chemical binding (Silva et al., 2005). After entering the body, the metals deposited in
nasopharygeal, tracheobronchial, or pulmonary compartments may be transported by
mucociliary action to the gastrointestinal tract. Metals can also be phagocyted by macrophages
(Silva et al., 2005).
Food is the most important route for accumulating most chemical elements (essential and
toxic). Certain elements, such as mercury, present in organisms of lower trophic levels can be
efficiently transferred to higher levels organisms, becoming more concentrated at the top of
the food chain, a phenomenon known as biomagnification (Silva et al., 2005).
Transition metals readily form stable covalent complexes and usually interact as part of
macromolecules (protein, enzymes, hormones, e.t.c) according to their chemical
characteristics including oxidation state (IEH, 1998; Schoof, 2003a, b). This tendency ensures
that in vivo, these metals are complexed with particular biological groups, such as sulphydryl
(-SH), amino (-NH), hydroxyl (-OH), disulphide (-SS), and carboxylic (-COOH) groups of
amino acids, peptides, proteins, phospholipids, citrate, ascorbate, and other tissue constituents.
These groups are also found in important biomolecules with catalytic, structural or transport
functions (Silva et al., 2005). Each transition metal possesses its affinity for organic binding.
In general, elevated values of equilibrium constants are observed for biomolecules rich in –SH
groups, towards which metals such as Pb, As, and Hg show particular reactivity (Silva et al.,
2005). Proteins such as metallothioneins, ferritin, transferrin, lactoferrin, melanotransferrin,
hemosiderin, ceruloplasmin, and amino acids (glutathione (GSH), cysteine, histidine and
others) are examples of biomolecules able to bind toxic metals in biological matrixes. The
reactivity for a wide range of biological ligands is the basis of the damaging actions of many
metal ions at molecular level, and determines the characteristic toxicity of the absorbed metal.
The knowledge of mechanisms of action is relevant for identifying possible targets and
possible related biomarkers of effects. Health effects induced by toxic metals vary greatly;
from irritant and acute or chronic systemic toxic effects to teratogenic, mutagenic and
carcinogenic effects (Silva et al., 2005).
The reactive elements occurring in food, mostly as organic complexes or associated with
fibers often have a low solubility within the intestinal lumen and are frequently poorly
absorbed. Absorption of these minerals would benefit from lower concentrations or absence of
agents such as phytates, oxalates and also fiber in diet (Hazell, 1985). Additionally, the effect
of other micronutrients on metal absorption/toxicity is also important. Micronutrients can
interact with toxic metals in several ways in the body. These include, absorption and excretion
of toxic metals; transport of metals in the body; binding to target proteins; metabolism and
sequestration of toxic metals; and finally, in secondary mechanisms such as oxidative stress.
Therefore, a diet poor in micronutrient can have an important influence on the toxicity of
nonessential metals such as cadmium, lead, mercury, arsenic (Peraza et al., 1998; Hurrell,
2001).
The gastrointestinal absorption of inorganic salts is dependent on a number of factors such as
presence of transport systems, particle size, solubility, dose, simultaneous administration of
other substances, pH, rate of transit and host factors such as species, age, nutritional status and
sex. It can vary from less than 10% for Pb, Cd, In, and U to almost complete absorption (90-
100%) for water soluble inorganic salts of trivalent ions of arsenic, germanium, and thallium
(Silva et al., 2005). Some complex interference in intestinal absorption of a given metal such
as that observed for Pb in the presence of Ca, Zn, and Fe have been attributed to a competition
for carrier proteins (Powell et al., 1999; Bressler et al., 2004)
Faeces
In biological fluids and tissues, most metals and metalloids are not present as free cations. The
level of elements in blood is influenced by the level of current exposure-absorption, body
burden and excretion rate of the element. In blood they are usually bound to red cells or
plasma proteins. Lead and cadmium are almost completely bound to red blood cells (Silva et
al., 2005). The chemical elements bound to plasma proteins constitute the fraction available
for transport into and out of tissues. The passage of these elements from blood into the
interstitial fluid and intracellular compartments is fully dependent on the diffusible fraction.
The distribution of those metals present in the ionized and unbound form is directly related to
the concentration gradient in the different compartments. Albumin, a plasma protein, has a
great capacity to bind several metals, yet, for some metals, binding occurs with proteins
having a specific transport function such as transferrin or ceruloplasmin. These bindings
constitute a major protective mechanism against metal toxicity. Thus, the availability of toxic
elements is regulated to a large extent by the availability and relative concentrations of
biological ligands as well as by the ability of the resulting metal complexes to serve as
substrates for the various organic solute transporters (Powell et al., 1999; Bressler et al.,
2004).
The essential elements virtually do not produce toxic effects on human and animals as a
result of homeostatic mechanisms for example, the intestinal control of absorption. In fact,
living organisms have evolved transport mechanisms for active uptake and /or extrusion,
enabling cells to regulate their intracellular concentrations.
Meanwhile, toxic elements can compete with the essential ones for protein binding sites and
this is the underlying cause of toxicity effects of many of them (IEH, 1998).The toxic effects
of metals and metalloids are partly due to the direct inhibition of enzymatic systems, and also
to the indirect alteration of the essential metal-ion equilibrium. As a consequence their
biological availability is inhibited and damage to the cell membrane can occur by the
disruption of ion transport across it.
Livestock play a very important role n Nigerian agriculture contributing about 12.7% of the
total agricultural GDP (CBN, 1999). Nigeria is one of the four leading livestock producers in
sub-Sahara. In 1990, the livestock population comprised about 14 million cattle, 23 million
goats and 13 million sheep (RIM, 1990), however, these figures have since increased as
shown in Table 1. Cattle are found throughout the country but are commonest in the Northern
region. Table 2 shows the distribution of livestock in the country.
In Nigeria, cattle, goat and sheep production systems are predominantly traditional; nomadic
or pastoral system; mixed farming and the peri-urban and modern ruminant husbandry.
Traditionally managed stock is over 85% for all species (Tewe and Bokanga, 2001) and
ruminants’ livestock under the extensive system rely on natural grass and forage legumes for
subsistence. The traditional production systems include scavenging, cut-and-carry production
system; seasonal tethering, fattening and compound dairying.
In the nomadic or pastoral system, the traditional grazing pattern is that at the early dry
season, the animals are either near permanent villages feeding on the dry forage and browse as
far enough South to find range and water. The migration continues as long as the grass ahead
is as green as the pastures at hand. When the northernmost grass and water are consumed
(usually in November or December), there is slow movement Southwards where there are
crop stubble and a full growth of grass to carry the animals through the dry season (Clyburn,
1974). Table 3 presents the livestock management system in Nigeria.
1.3.1. White Fulani (breed of study): White Fulani are a breed of humped cattle herded by
the nomadic and semi-sedentary Fulani people of the Sudo-Sahelian region of Africa. There
are a number of different Fulani breeds but white Fulani or Bunaji cattle refer to the white
long horned zebu cattle of Northern Nigeria. The Fulani people are thought to be a branch of
the Northern Hamite people who spread their influence over the Western Sudanese and
Sahelian region during the days of the ancient Ghana empire about 600-700 AD (Sinclair et
al., 1993). These nomadic tribes’ people found their way to Northern Nigeria during the 13th
century and found it to be an ideal region to practice their transhumant lifestyle. There has
been a suggestion that humped white Fulani cattle did not appear until Arab traders brought
zebu stock to Kano and other Northern Nigerian settlements during the 15th century (Payne,
1970). In the 1990’s, White Fulani cattle were estimated to be the most numerous breed in
Nigeria with total population then at approximately five million head or 37% of the total cattle
population (Blench, 1993). A typical White Fulani animal has a white coat with black ears,
muzzle and feet. Black or red spots on the limb and sides of the body are quite common also.
It has long, slender, lyre-shaped horns and a well-developed hump which is neither clearly
thoracic nor cervico-thoracic in aspect. This has prompted Mason in 1951 to consider the
breed not to be true zebu, but belonging to the “zeboid” group with the other Fulani breed and
the Sanga cattle of Southern Africa. The average live weights for male and female animals are
500 kg and 340kg respectively and the breed is used primarily for milk and beef production
but also for draught. Baker and Manwell (1980) classified the breed as admixed African
Humped Fulani.
1.3.2. Heavy metals in cattle:
Farm animals, such as cattle serve as a major source of meat in Nigeria. Heavy metal
contamination can be transferred to these animals through direct exposure, drinking polluted
water, eating crops grown on sewerage, contact with industrial effluents, and inhalation of
vehicle and other emissions as they move from one place to the other. Thus, toxic metals can
bioaccumulate in the tissues and organs of these animals.
The prevalence of heavy metals in the Nigerian environment has been extensively studied.
The presence of these heavy metals at enhanced concentrations has been reported in fish, water,
soil, and plants (Kakulu et al., 1987; Okoye et al, 1991; Okoye, 1991; Okoye, 1994; Obodo, 1994;
Amusa et al., 2003; Ano et al., 2007).
Some reports have been made on levels of some heavy metals in different animals across the
world. Miranda et al (2005) reported high levels of some toxic and trace metals in calves from
a polluted area of Northern Spain. Jukna et al. (2006) reported moderate levels of heavy
metals in the viscera and muscles of Lithuanian cattle while Korenekova et al (2002) reported
high concentrations of some heavy metals in cattle reared in the vicinity of a metallurgic
industry. Others include report of low copper and mercury levels in local Jordanian and
imported sheep meat and organs (Sharif et al., 2005) and Skalicka et al (2002) who reported
low cadmium levels in poultry meat. Lawal et al (2006) has reported high levels of cadmium,
and lead in milk from cows grazed in open fields in Nigeria. There are however very few
literature of the levels of these heavy metals in muscles and internal organs in animals reared
in the Nigerian environment (Okoye and Ugwu, 2010).
Meat is a food material, which is composed mainly of proteins, fats and essential elements. It
is an essential source of protein needed for growth and maintenance of good health. It
becomes necessary to study the concentrations of toxic heavy metals in meat in order to assess
the levels of exposure of consumer to toxic metals, and henceforth, maintain an ongoing
knowledge on the levels of these metals both in the environment and in meat.
1.4 OBJECTIVES
1.4.1. General Objective:
This research aims at determining the concentrations and patterns of distribution of
some essential and some toxic heavy metals in meat and edible offal’s of cow meat consumed in
Enugu State, Nigeria.
1.4.2. Specific Objectives: The study would involve:
1. Determination of the concentrations of seven heavy metals: lead, cadmium, copper, zinc,
nickel, chromium, and iron in the muscles, kidneys, livers, intestines and tripe of cows reared
in Nigeria.
2. Comparison of the distribution of these metals in the various meat parts.
3. Determination of any correlation between the metals in the different meat parts.
4. Comparison of the concentrations of these metals with some regulatory guideline values.
5. Estimation of the mean daily consumption of the different parts of the meat under study by
different categories of the populace.
6. Estimation of the dietary intake of the studied metals from meat by the consumers.
7. Assessment of the dietary exposure of the consumers and any health risk due to these heavy
metals as a result of consumption of cow meat.
1.5 SCOPE OF STUDY
Heavy metal analysis was carried out using meat samples from cows slaughtered and sold in
Nsukka and Enugu markets. Atomic absorption spectrophotometer was used in determining
the concentration of the heavy metals in the samples. It was expected that the analysis would
give an overview of the toxicological data for cows reared in Nigeria; since these animals are
reared mostly by the nomadic Fulani’s of Northern Nigeria, who take their cattle from one
place to the other southwards for grazing and selling.
Dietary exposure assessment was carried out on the urban population of Enugu and Nsukka using a
validated food frequency questionnaire.