SELENIUM IN NIGERIAN FOODS

Francis Olawale Abulude, Mary Omofolarin Ogunkoya, Toluwalope Alaba Orojo
Department of General Studies, Federal College of Agriculture, Akure, Nigeria

ABSTRACT

The amount of selenium in some Nigerian foods was determined using an atomic absorption spectrophotometer. The analytical results showed that foods which were particularly rich in selenium included: cereals, milk, kolanut, pawpaw, beans and yam (5 mg · kg^-1) while fish and sugar, nuts, legumes, vegetables and other foods produced no values for selenium. In comparing our results with the literature values, our results were higher. The differences could be due to the geographical origin, the soil content of selenium and analytical methods used. The values obtained were far below those intakes referred to as undesirable dose.

Key words: Selenium, Nigerian foods, glutathione peroxidose.

INTRODUCTION

Selenium is a group VI element and has both metallic and non-metallic properties. It can exist in 4 oxidation states (-2, +1, + 2, +6) and forms chemical components analogous to those of sulphur. Selenium is abundant in the earth’s crust at concentrations of 50 to 90 mg · kg^-1. High concentrations may be found in volcanic, sedimentary and some carbonate rocks. The concentration of selenium in soil varies from 5 to 1200000 mg · kg^-1.

Human exposure to selenium is mainly through food. The intake in the U.K is 29-39 mg · day^-1, in Nordic countries it is estimated to be 71-152 mg · day^-1 [13].

Selenium is an essential trace element for human and animals. It is necessary for the functioning of the enzyme glutathione peroxidose (GPX) which protects against oxidative damage to intracellular structures. Selenium is an essential component of the enzyme tetrathiodothyronone 5- deiodinase I and is thus involved in iodine metabolism. Selenium compounds are readity absorbed from the human gastro-intestinal tract. The absorption and bioavailability can be affected by the physical and chemical form of the selenium compound or the dosing regimen [2]. Yeast containing organic selenium has been shown to be more bioavailable in nursing rats [16], than inorganic sodium selenite [21]. Absorbed selenium is rapidly distributed and does not accumulate in any specific organs, although the concentration is higher in the liver and kidney [7].

Selenium occurs in various chemical forms (selenite or selenate) in plants and animals it is in an inorganic form such as selenomethionine or other selenium- containing amino acids [22].

The estimated safe and daily dietary allowances of selenium have been documented [8, 9, 14, 18, 19]. In Nigeria, there may be little or no information available for selenium content of varieties of foods. In this work, we therefore obtained different food samples and determined their selenium content it is hoped that these results would add to nutrition data.

MATERIAL AND METHODS

The food samples (Table 1) were obtained in Akure, Ondo State, Nigeria in March, 2005. They were cleaned to remove dirt, washed in deionised water, oven dried at 60°C for 2-4 h depending on sample, ground in Kenwood blender, sieved with a mesh of aperture 425 ľm and stored prior to analysis.

Each sample (0.5 g) was dry ashed in a muffle furnace with a temperature of 550°C for 2-4 h (depending on the sample). The ashed sample was dissolved in a little quantity of 2M HCL, filtered and made up to 50 cm³ using 2M HCL. The selenium content was determined using Pye Unicam atomic absorption spectrophotometer. Determinations were in duplicate. Statistical analysis was performed using SPSS 10 for windows.

RESULTS AND DISCUSSION

The mean selenium contents of each food group are shown in Table 1. The meat and fish group which include quantities of dry fish (Tilapia nicotilus), cray fish (Procambaris clarkii), snail, (Achatina fulica) and albino rat was richest in selenium. Although snail and rat contained little or none. In milk and dairy products analysed, milk had 5 ± 3.5 mg · kg^-1, egg had no selenium. In all the vegetables analyzed, selenium was also not detectable. In sugar, fruits and nuts group, Elaeis guinensis was shown to be the highest source with 10ą4.55 mg · kg^-1, while Cola nitida and Carica papaya produced 5 mg · kg^-1each. In this report, seeds and legumes group which included Vigna unguiculata (white and brown) and Gossypium hirsutum and Parkia biglobosa produced 5mg · kg^-1, and 10mg · kg^-1, respectively. In tuber and roots group, only Manihot utilizina had selenium content (5mg · kg^-1). Maggi cube also contained a significant amount (5mg · kg^-1) which showed that it could provide a significant quantity of selenium for some people.

Comparing, the value we obtained with literatures cited, it was observed that our results were higher than values reported for foods in the UK (cereal 0.03-0.23 mg · kg^-1, meat and fish 0.09-0.16 mg · kg^-1, milk <0.01-0.01 mg · kg^-1, vegetable (0.01-0.02 mg · kg^-1, [8] and fruits 0.29 mg · kg^-1, offal 0.42 mg · kg^-1, fish 0.39 mg · kg^-1, eggs 0.19 mg · kg^-1, poultry 0.15 mg · kg^-1, [1]: FSANZ [9] recorded low mean value of 0.01 mg · kg^-1 for seafoods found in Australia. ATSDR [2] quoted 0.44 mg · kg^-1 (wheat bread) and 0.32 mg · kg^-1 (white bread) obtained in the US. Wardlaw [23] reported between 0.8-681 mg · kg^-1 for some food items with canned tuna and sirloin steak having the highest value and Anhwange et al. [1] recorded 0.023-0.035 mg · kg^-1 for seed of Moringa oleifera, Detarium microcarpum and Bauhinia monandra found in Nigeria.

The differences in the selenium contents of the foods we selected and those in literatures may be due to the geographical origin, the soil content of selenium and analytical methods employed by different authors. However, since we consume a varied diet obtained from many geographical areas in Nigeria, it is unlikely that selenium deficiency in the soil in a few areas will cause selenium deficient in Nigeria diets. Individual intakes will vary considerably about the calculated mean, depending on the amount of protein in the diet and in particular on the amount of selenium rich foods, which are eaten. Vegetarian and other food advocates might obtain substantially more than average, but still for below those intake postulated as toxic [8].

In human and animals, deficiency and symptom include acute chronic heart disorder, growth retardation and reproductive failure and dysfunction, development of cardiovascular disease, reduction in the level of GPX protein, muscle pain and muscle wasting selenium deficiency is thought to be involved in Keshan disease, an endemic juvenile cardiac myopathy occurring in the Keshan region in China where selenium intake is extremely low [4, 23].

Nutritional benefits of selenium have been reported, its increase intake may reduce cancer incidence and heart deterioration, but it can not correct the heart disorder once they have occurred [5]. It has also been reviewed that selenium is a contributory factor in male sub-fertility and in conjunction with other antioxidant vitamins, improve symptoms in AIDs sufferers and may show the progression of the disease. It has been suggested to reduce the adverse effects of chemotherapy or radiotherapy [15] enhances the chemotherapeutic effects of Taxol and Doxorubicin [20] and presents the drug-resistance in cancer patients [3].

The anti-cancer effects and toxicity depend upon the concentration and the chemical form of selenium. The toxicity is cumulative. Selenium, selenate and selenomethionine are among the most toxic selenium compounds whereas selenium sulphide is much less toxic due to its insolubility. Administration of high level of selenium compounds can lead to its toxicity in human [11]. Koppel et al. [10] reported that a 10 g oral dose of selenium dioxide can cause death in human. Selenite has been shown to be toxic at just 5 mg · kg^-1, of dietary supplementation [17]. The acute effects of selenium toxicity have been summarized thus: Patients suffer from diarrhea, hyper salivation, garlic odor to the breadth, burns and erosions may occur in the mouth and peripheral circulatory failure [12].

There have been several recommended maximum intakes of selenium. The COMA [24] panel agreed on 450 µg Se/day, the Nordic project [14] considered an intake of 280-350 µg Se/day white the USEPA [19] established a maximum safe level of 5 µg Se/kg between day, FSANZ [9] reported Australian RDI to be 85 µg/day and WHO (1996) recommended 400 µg Se/day.

Comparison of our analytical results with the literature, it was observed that good food sources of selenium were cereals, meat, fish, milk, fruits, nuts, seeds and legumes and it is unlikely that selenium deficiency occurs in Nigeria. The values obtained were far below those intakes referred to as the undesirable dose.

ACKNOWLEDGEMENT

The authors are grateful to Mr. Ajao (2004/2005 NYSC member) of Department of General Studies, Federal College of Agriculture, Akure, Nigeria for the provision of Wistar strain albino rats.

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Accepted for print: 08.08.2006

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