Heavy Metals in Seafood and Farm Produce from Uyo, Nigeria

Abstract

Objectives:

This study aimed to obtain representative data on the levels of heavy metals in seafood and farm produce consumed by the general population in Uyo, Akwa Ibom State, Nigeria, a region known for the exploration and exploitation of crude oil.

Methods:

In May 2012, 25 food items, including common types of seafood, cereals, root crops and vegetables, were purchased in Uyo or collected from farmland in the region. Dried samples were ground, digested and centrifuged. Levels of heavy metals (lead, cadmium, nickel, cobalt and chromium) were analysed using an atomic absorption spectrophotometer. Average daily intake and target hazard quotients (THQ) were estimated.

Results:

Eight food items (millet, maize, periwinkle, crayfish, stock fish, sabina fish, bonga fish and pumpkin leaf) had THQ values over 1.0 for cadmium, indicating a potential health risk in their consumption. All other heavy metals had THQ values below 1.0, indicating insignificant health risks. The total THQ for the heavy metals ranged from 0.389 to 2.986. There were 14 items with total THQ values greater than 1.0, indicating potential health risks in their consumption.

Conclusion:

The regular consumption of certain types of farm produce and seafood available in Uyo, Akwa Ibom State, Nigeria, is likely adding to the body burden of heavy metals among those living in this region.

Advances in Knowledge

• - Heavy metals, particularly lead and cadmium, have been implicated in metabolic syndrome and other chronic diseases which have been increasing in sub-Saharan Africa. Although food, water, air and the soil are known sources of these heavy metals, the exact individual contribution of these is unknown. The present study therefore sought to determine the levels of heavy metals in seafood and farm produce in Uyo, Nigeria.

Application to Patient Care

• - Determining the health risks of consuming seafood and farm produce from Uyo, Nigeria, is of the utmost importance to public health since the results of this study indicate that these foods likely add to the body burden of heavy metals among individuals residing in this region.

The southeastern coastal region of Akwa Ibom State is currently the highest oil and gas producing state in Nigeria. It is characterised by rich marine biodiversity as well as agricultural and crude oil exploration activities. Heavy metals are ubiquitous in the environment, arising from both natural and anthropogenic activities.1 Humans are exposed to these metals through various pathways.1

Seafood is widely consumed as part of the local diet in Nigeria, due in part to its high protein content, low saturated fats and omega fatty acids which are known to contribute to good health.2 Fish can be very nutritious as they are rich in many vitamins, with a good selection of minerals and all of the essential amino acids in the right proportions.3 However, fish absorb heavy metals from the surrounding environment depending on a variety of factors, such as the characteristics of a species, the exposure period and concentration of the element, as well as abiotic factors such as temperature, salinity, pH and seasonal changes.4 Hence, harmful substances like heavy metals released by anthropogenic activities may accumulate in marine organisms; as a result, the consumption of fish contaminated by toxic chemicals may present a risk to human health.

Vegetables constitute an essential dietary component by contributing protein, vitamins, iron, calcium and other nutrients which are usually in short supply;5,6 furthermore, this component also acts as a buffer for acidic substances produced during digestion. Vegetables, fruit and cereals can accumulate heavy metals, potentially posing a direct threat to human health.6–8 Plants can absorb heavy metals from contaminated soil or via exposure to polluted air.9 It has been reported that nearly half of the mean ingestion of lead (Pb), cadmium (Cd) and mercury through food is due to the consumption of plant products.9 Moreover, some population groups may be more exposed to these heavy metals, e.g. vegetarians, since they consume higher quantities of these types of food within their daily diets.9

Risk assessments of the bioaccumulation of heavy metals in various foods are important. This study attempted to obtain representative data on the levels of heavy metals in seafood and farm produce consumed by the general population in Uyo, Akwa Ibom State, Nigeria. Nevertheless, while sometimes the levels of contaminants in food items exceed the legal limits set by various regulatory bodies, they may not always represent a significant risk to human health. For that reason, target hazard quotients (THQs) were estimated in order to evaluate potential hazards to human health. This study also aimed to estimate the dietary intake and THQ of heavy metals via the consumption of selected seafood and farm produce items from Uyo.

Methods

Samples of 25 commonly consumed food crops, vegetables and seafood were either purchased from the metropolis in Uyo or collected from farmlands in the region in May 2012. Only edible parts of the selected food items were used for the analysis. Where purchased, the sources of the food items were authenticated by suppliers in order to ensure they were locally caught or grown in Uyo.

All samples were first washed with deionised water and then oven-dried at 70–80 °C for 24 hours. Dried samples were ground using a pestle and mortar and sieved through a muslin cloth. For each sample, 0.5 g was placed in crucibles. Samples were then placed for ashing in a furnace for four hours at 550 °C. The ash was digested in a solution of perchloric acid and nitric acid (at a volume ratio of 1:4). Following this, samples were left to cool. A final volume of 25 mL was made by adding deionised water. The hydrolysed samples were then well shaken and transferred to a centrifuge tube for centrifugation at a rate of 3,000 x g to remove solid particles. The resulting samples were thoroughly mixed before sub-samples were taken to ensure homogeneity.

The presence of Pb, Cd, nickel (Ni), cobalt (Co) and chromium (Cr) were analysed using an atomic absorption spectrophotometer (model 929 UNICAM, Spectronic Unicam, Cambridge, UK) at wavelengths of 217.0, 228.8, 232.0, 242.5 and 357.9 nm, respectively. The limit of detection for the heavy metals were 0.005 µg/g, with blank values reading as 0.00 µg/g for the metals in deionised water with an electrical conductivity value of <5 μS cm⁻¹. Appropriate quality procedures and precautions were carried out to reduce the risk of contamination and assure the reliability of the results. Double-distilled deionised water was used throughout the study. Glassware was properly cleaned and all reagents were of analytical grade. Blank determinations of reagents were used to correct instrument readings. For validation of the analytical procedure, several samples that had already been analysed were spiked and homogenised with various amounts of standard solutions of the metals.

Daily intake rate (DIR) in g person⁻¹ day⁻¹ was calculated based on the following formula:10

$$\mathit{DIR}=C_{\mathit{metal}}\times D_{\mathit{food}\hspace{0.17em}\mathit{intake}}/B_{\mathit{average}\hspace{0.17em}\mathit{weight}}$$

Where C_metal is the metal concentration in food in μg g⁻¹, D_{food intake} is the daily intake of food in kg person⁻¹ and B_{average weight} is average body weight in kg person⁻¹.

Using an adapted method, an average daily consumption of 345 g of rice was assumed.11 This method was originally calculated by Wang et al. in a Chinese population where rice is eaten in different forms, as it is in the Nigerian population.11 Since the local population of Uyo is predominantly comprised of subsistence farmers who do not have the luxury of variety in their menu, daily consumption of other farm produce and seafood was assumed to be 345 g/person/day and 34.5 g/person/day, respectively.11,12 Average adult body weight was considered to be 60 kg.11,12

Noncarcinogenic risk estimation of heavy metal consumption was determined using THQ values. THQ is a ratio of the determined dose of a pollutant to a reference level considered harmful. THQ values were determined based on the following formula:10

$$\mathit{THQ}=\mathit{EFr}\times \mathit{ED}\times \mathit{FIR}\times C/{\mathit{RfD}}_o\times B_{\mathit{average}\hspace{0.17em}\mathit{weight}}\times A{T}_n\times {10}^{-3}$$

Where EFr is exposure frequency in 365 days year⁻¹, ED is exposure duration in 70 years (equivalent to an average lifetime),13 FIR is average daily consumption in g person⁻¹ day⁻¹, C is concentration of metal in food sample in μg g⁻¹, RfD_o is reference dose in μg g⁻¹ day⁻¹ and AT_n is average exposure time for noncarcinogens in days. The following reference doses were used: Cr = 1.5 μg g⁻¹ day⁻¹, Ni = 2.0 × 10⁻² μg g⁻¹ day⁻¹, Pb = 4.0 × 10⁻³ μg g⁻¹ day⁻¹ and Cd = 1.0 × 10⁻³ μg g⁻¹ day⁻¹.12

AT_n was based on the following formula:

$$A{t}_n=\mathit{EFr}\times {\mathit{ED}}_{\mathit{total}}$$

Where ED_total is the total number of exposure years, which was assumed to be 70 years.13

A THQ value of >1.0 was considered to indicate that the level of exposure was smaller than the reference dose, implying that daily exposure at this level was unlikely to cause any harmful effects in a human subject during their lifetime. Thus, the exposed population was considered unlikely to experience obvious adverse effects at this level.11,14 THQs were calculated according to the methodology described by the Environmental Protection Agency (EPA) in the USA.15 Doses were calculated using the standard assumption for an integrated risk analysis and an average adult body weight of 60 kg.11,15 In addition, based on EPA guidelines, it was assumed that ingested doses were equal to absorbed contaminant doses.16,17

The total THQ of heavy metals for individual food samples was calculated as the sum of the individual THQ of the toxic metals.12 A reference value for tolerable weekly intakes of heavy metals has been established.18,19 Therefore, the daily intake of these heavy metals was compared with the provisional tolerable weekly intake (PTWI) and the proposed maximum permissible level.18,19

Results

The concentrations of heavy metals (Pb, Cd, Ni, Co and Cr) in selected farm produce and seafood from Uyo are shown in Table 1. Periwinkle, crayfish and stockfish had the highest Pb content (0.09 μg g⁻¹), followed by sabina and bonga fish (0.08 μg g⁻¹ each) and pumpkin leaf (0.07 μg g⁻¹). All other food items had Pb levels <0.05 μg g⁻¹. Pb ranged from 0.08–0.09 μg g⁻¹ in seafood samples and <0.05–0.07 μg g⁻¹ in vegetable samples.

Table 1:

Levels of selected heavy metals in farm produce and seafood from Uyo, Nigeria

Food item	Heavy metal levels in μg g−1
	Pb	Cd	Ni	Co	Cr
Cereals
Rice	<0.05	0.13	0.84	<0.05	0.41
Wheat	<0.05	0.16	0.41	<0.05	0.42
Soya bean	<0.05	0.07	0.24	<0.05	0.58
Millet	<0.05	0.21	0.39	<0.05	0.43
Maize	<0.05	0.22	0.81	<0.05	0.47
Fruit
Cucumber	<0.05	<0.05	0.11	<0.05	0.09
Orange	<0.05	<0.05	0.10	<0.05	0.09
Guava	<0.05	<0.05	0.15	<0.05	0.11
Watermelon	<0.05	<0.05	0.14	<0.05	0.12
Pawpaw	<0.05	<0.05	0.13	<0.05	0.09
Root crops
Potato	<0.05	0.11	0.56	<0.05	0.64
Yam	<0.05	0.14	0.74	<0.05	0.83
Cocoyam	<0.05	0.14	0.83	<0.05	0.84
Sweet potato	<0.05	0.15	0.91	<0.05	0.86
Water yam	<0.05	0.16	0.74	<0.05	0.81
Vegetables
Pumpkin leaf	0.07	0.35	1.45	0.12	1.08
Water leaf	<0.05	<0.05	0.74	<0.05	0.53
Scent leaf	<0.05	<0.05	0.68	<0.05	0.41
Bitter leaf	<0.05	<0.05	0.75	<0.05	0.38
Editan leaf	<0.05	<0.05	0.69	<0.05	0.39
Seafood
Periwinkle	0.09	0.34	0.05	0.13	1.07
Crayfish	0.09	0.21	0.11	0.15	1.08
Stockfish	0.09	0.43	1.32	0.07	1.06
Sabina fish	0.08	0.33	1.41	0.11	1.05
Bonga fish	0.08	0.33	1.43	0.14	1.07

Pb = lead; Cd = cadmium; Ni = nickel; Co = cobalt; Cr = chromium.

Concentrations of Cd ranged from <0.05–0.35 μg g⁻¹ in vegetable samples, with the highest levels in pumpkin leaf (0.35 μg g⁻¹). In seafood samples, concentrations ranged from 0.21–0.43 μg g⁻¹. Higher concentrations of Cd were noted in the fish samples, with the highest amount found in stockfish (0.43 μg g⁻¹). Cd concentrations in cereals, fruit and root crops ranged from <0.05–0.22 μg g⁻¹.

Ni levels were highest in pumpkin leaf, with a concentration of 1.45 μg g⁻¹. Concentrations ranged from 0.68–1.45 μg g⁻¹ in vegetable samples and 0.05–1.43 μg g⁻¹ in seafood samples. Concentrations of Ni in cereals, fruit and root crops ranged from 0.10–0.91 μg g⁻¹.

The highest concentration of Co was recorded in crayfish (0.15 μg g⁻¹). Concentrations ranged from <0.05–0.12 μg g⁻¹ in vegetable samples and 0.07–0.15 μg g⁻¹ in seafood samples. In root crop, fruit and cereal samples, concentrations of Co were <0.05 μg g⁻¹.

For Cr, crayfish and pumpkin leaf had the highest concentrations (1.08 μg g⁻¹ each). Cr content ranged from 0.38–1.08 μg g⁻¹ in vegetable samples and 1.05–1.08 μg g⁻¹ in seafood samples. Among root crop, fruit and cereal samples, sweet potato had the highest level of Cr (0.86 μg g⁻¹) while cucumber, oranges and pawpaw had the lowest concentration (0.09 μg g⁻¹) each.

As can be seen in Table 2, the daily intake of heavy metals from the consumption of analysed food items showed large variations. Cr levels were found to be above permissible levels in all of the food items. While still at permissible values, crayfish and pumpkin leaf contained the highest levels of Cr (0.00062 and 0.00621 g person⁻¹ day⁻¹, respectively). Table 3 shows the permissible intake levels of selected heavy metals as per recommendations from the Food and Agriculture Organization (FAO) and the World Health Organization (WHO).13,18

Table 2:

Estimated daily intake rates of selected heavy metals through the consumption of farm produce and seafood from Uyo, Nigeria

Food item	Daily intake rate of heavy metals in g person−1 day−1
	Pb	Cd	Ni	Co	Cr
Cereals
Rice	0.00029	0.00075	0.00483	0.00029	0.00236
Wheat	0.00029	0.00092	0.00236	0.00029	0.00241
Soya bean	0.00029	0.00004	0.00138	0.00029	0.00334
Millet	0.00029	0.00121	0.00225	0.00029	0.00247
Maize	0.00029	0.00127	0.00466	0.00029	0.00270
Fruit
Cucumber	0.00029	0.00029	0.00063	0.00029	0.00052
Orange	0.00029	0.00029	0.00029	0.00029	0.00052
Guava	0.00029	0.00029	0.00087	0.00029	0.00063
Watermelon	0.00029	0.00029	0.00080	0.00029	0.00069
Pawpaw	0.00029	0.00029	0.00075	0.00029	0.00052
Root crops
Potato	0.00029	0.00063	0.00322	0.00029	0.00368
Yam	0.00029	0.00080	0.00426	0.00029	0.00477
Cocoyam	0.00029	0.00080	0.00477	0.00029	0.00483
Sweet potato	0.00029	0.00087	0.00524	0.00029	0.00495
Water yam	0.00040	0.00092	0.00426	0.00029	0.00466
Vegetables
Pumpkin leaf	0.00040	0.00201	0.00895	0.00069	0.00621
Water leaf	0.00029	0.00029	0.00425	0.00029	0.00305
Scent leaf	0.00029	0.00029	0.00391	0.00029	0.00236
Bitter leaf	0.00029	0.00029	0.00431	0.00029	0.00219
Editan leaf	0.00029	0.00029	0.00397	0.00029	0.00225
Seafood
Periwinkle	0.00005	0.00020	0.00003	0.00007	0.00061
Crayfish	0.00005	0.00012	0.00006	0.00009	0.00062
Stockfish	0.00005	0.00025	0.00008	0.00004	0.00061
Sabina fish	0.00005	0.00019	0.09398	0.00006	0.00060
Bonga fish	0.00005	0.00019	0.00082	0.00008	0.00061

Pb = lead; Cd = cadmium; Ni = nickel; Co = cobalt; Cr = chromium.

Table 3:

Permissible intake levels of heavy metals as per Food and Agriculture Organization and World Health Organization recommendations13,18

Heavy metal	PTWI in μg kg−1 week−1	Daily intake in μg kg−1 day−1	Intake for a 60 kg individual in μg day−1
Pb	25.0	5.0	300.0
Ni	1.0	0.2	12.0
Cd	7.0	0.4–2.0	60.0
Cr	0.5	0.1	6.0

PTWI = provisional tolerable weekly intake; Pb = lead; Cd = cadmium; Ni = nickel; Cr = chromium.

Figure 1 shows the estimated total THQs caused by consumption of the food items. Estimated THQ values for the selected heavy metals ranged from 0.000–2.473. Total THQ values for the metals ranged from 0.389–2.986. A total of 14 of the food items had total THQ values greater than 1.0, indicating some health risks. For Cd, eight food items, comprising of cereals (millet and maize), vegetables (pumpkin leaf) and seafood (periwinkle, crayfish, stockfish, sabina and bonga fish), had THQ values greater than 1.0, indicating potential health risks. All other metals had total THQ values below 1.0, indicating an insignificant health risk.

Figure 1:

Target hazard quotient values of selected heavy metals via consumption of farm produce and seafood from Uyo, Nigeria.

THQ = target hazard quotient; Pb = lead; Cd = cadmium; Ni = nickel; Cr = chromium.

Discussion

In order to assess the health risk of any pollutant, it is essential to estimate the level of exposure by quantifying the exposure routes of the pollutant to the target organisms. While there are various possible exposure pathways of pollutants to humans, the food chain is one of the most important. As for many dietary components, the intake of metals can be both beneficial and harmful. For example, many Nigerian families exhibit low levels of dietary calcium due to poverty, which has potentially harmful effects.20 However, preventative measures should be in place to avoid the surplus ingestion of potentially toxic metals. Many developed countries spend significant resources to avoid excessive metal intake by the general population, from monitoring endogenous metal levels in foods to detecting contamination during food preparation processes.21,22 Cases of excessive intake of trace metals have been implicated in pathological events and inflammation.23

In the current study, Pb levels were observed to be lower than those noted in a similar study by Orisakwe et al., which reported a Pb range of 0.00–61.17 μg g⁻¹ in different foodstuffs from southeastern Nigeria.24 However, like the current study, Pb concentrations were found to be highest in seafood. According to the FAO and WHO, the safe limit for Pb in fruit, root crops and cereals is 0.1 μg g⁻¹, 0.1 μg g⁻¹ and 1.00 μg g⁻¹, respectively.25 Therefore, the food items analysed could be considered to have safe levels of Pb.

A PTWI of 25 μg kg⁻¹, equalling 1,500 μg Pb/week for a 60 kg person, has been established.18 In Taiwan, Huang et al. demonstrated that low-level prenatal exposure to Pb among fetuses can lead to decreased intelligence and delayed cognitive function.26 Furthermore, a recent Nigeria-based study found that prenatal Pb exposure was significantly associated with crown rump length at birth.27 Among individuals of reproductive age, exposure to Pb is a public health issue of great concern, since evidence indicates that even low levels of exposure to this metal can affect fetal growth and development.28 Conditions associated with Pb poisoning (impaired mental and physical development, poor school performance, anaemia, under-nutrition, cardiovascular diseases, metabolic syndrome, infertility, etc.) represent a significant social, financial and health burden on affected individuals and their families and communities.29 In Nigeria, there are multiple sources of exposure to Pb, including automobiles which burn leaded petrol.30,31 Despite the introduction of a planned Clean Air Initiative to reduce levels of Pb in Nigerian petrol from 0.74 g/L to 0.15 g/L by 2002, there is as yet no evidence to suggest that the programme has been implemented.30

Concentrations of Cd observed in the current study agreed with a similar study in southeastern Nigeria which reported a range of 0.00–0.24 μg g⁻¹ of Cd in various items of food.32 Safe limits of Cd in cereals, fruit and root crops are 0.2 μg g⁻¹, 0.05 μg g⁻¹ and 0.1 μg g⁻¹, respectively, according to the European Commisson.25 In the current study, Cd was below the permissible level in all the food items. The recommended value of Cd is within the range of 7.0 μg kg⁻¹ body weight week⁻¹ for adults.19 Considering the accumulative properties and long biological half-life of Cd, a level of 0.4–2.0 μg kg⁻¹ body weight day⁻¹ has been set as permissible.33 This equals 60 μg day⁻¹ for an individual of 60 kg. Absorption following the oral exposure of Cd likely depends on physiological status, such as age and levels of iron, calcium and zinc stored in the body. There is epidemiological evidence that in utero exposure to Cd may have adverse effects on a newborn’s health.34

Mostly present in the pancreas, Ni plays an important role in the production of insulin and is required in small quantities by the body; a deficiency of Ni can result in liver disorders.35 However, increased concentrations of Ni can have many adverse effects, namely kidney damage, cancers of the lung and nasal sinus, pneumonitis, chronic bronchitis, allergic reactions and mild skin rashes.35 Concentrations of Ni in cereals, fruit and root crops in the current study were in line with another study which reported a range of 0.00–3.13 μg g⁻¹ among various food items.24 The recommended daily intake of Ni is approximately 0.2 μg kg⁻¹ body weight day⁻¹.18 Unfortunately, in the current study, Ni levels in the cereals, root crops, vegetables, stockfish, Sabina fish and bonga fish were found to be higher than recommended for all food items.

While the body requires trace amounts of Co, this metal is toxic in elevated concentrations. For diabetic patients, a total of 2.0 μg day⁻¹ is required in the form of vitamin B₁₂. High intake of Co can lead to gastrointestinal, vision and heart problems, as well as thyroid damage.25,36 The Co concentrations of the studied samples conformed with those reported by Dabeka et al. among Canadians (<0.003–0.0759 μg g⁻¹).37 According to another Nigerian study, normal daily intake of Co is between 2.5–3.0 mg day⁻¹, which is equivalent to 180 mg kg⁻¹ body weight day⁻¹ for an adult of 60 kg.38 Poisoning therefore occurs when daily Co intake is greater than this range.

Exposure to Cr can occur through air, water, food or skin contact. In human beings and animals, it is considered an essential metal for carbohydrate and lipid metabolism within a certain concentration (up to 200 μg day⁻¹).39 However, exceeding this concentration leads to an accumulation and toxicity that can result in hepatitis, gastritis, ulcers and lung cancer.39 In the current study, cucumber, oranges and pawpaw had the lowest concentration of Cr and were within acceptable limits of 250 μg day⁻¹ for adults equivalent to 1,500 μg g⁻¹ for an average weight of 60 kg.40 However the Cr concentration was above the permissible level in all the other tested food items. The recommended value of Cr is 0.5 μg kg⁻¹ body weight week⁻¹ and 0.1 μg kg⁻¹ body weight day⁻¹ for adults.18

Levels of Pb, Cd, and Ni have been previously investigated in three popular types of seafood in Ondo State, Nigeria (fish, crab and periwinkle), in order to evaluate the ecosystem of this oil-polluted coastal region.30,41 Increasing levels of Pb, Cd, and Ni could pose a potential threat to the ecology of the area and the health of the local population.30,41 Using reference doses, estimates of THQs for heavy metals in eight food items analysed in the current study found Cd values of over 1.0, indicating a potential health risk.14,15 Multiple exposure to heavy metals or pesticides may lead to additive and/or interactive effects according to the risk addition hypothesis.12,42

There are several limitations of the current study that should be considered. These include the unavailability of national food diaries and nutrition data in Nigeria and the absence of national standard nutritional limits and guidelines. In humans, the degree of toxicity of heavy metals is dependent on the daily intake.32 In view of the paucity of food consumption data and the absence of food diaries in Nigeria, the current study assumed a daily consumption of 345 g of rice based on an adaptation of Wang et al.’s study of a Chinese population.11 Rice is a staple food throughout West Africa. Like China, rice is one of the most important cereals in Nigeria and is a staple food for both urban and rural dwellers. In Nigeria, urban consumers have developed a preference for imported rice, mainly due to a perception of superior quality. Most of the rice consumed in Nigeria is imported from China.12 Uyo is a riverine community with a local population of subsistence farmers; it was therefore assumed that intake of rice would be complemented by farm produce and seafood. Daily ingestion rates of farm produce and seafood were therefore estimated to be 345 g/person/day and 34.5 g/person/day, respectively.11,12 These assumptions require a cautionary interpretation of the results as they may not give an accurate view of health concerns associated with heavy metal consumption in Nigeria. Further studies are recommended to investigate consumption patterns in this region.

Conclusion

Consumption of certain types of seafood and farm produce is likely to add to the body burden of heavy metals among individuals living in Uyo, Nigeria. Heavy metal analysis of millet, maize, periwinkle, crayfish, stockfish, sabina fish, bonga fish and pumpkin leaf indicated a potential health risk with regards to Cd content. As the excessive ingestion of heavy metals can have severe public health implications, the monitoring of these metals in seafood and farm produce in Nigeria is of the utmost importance.