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. 2022 Aug 2;57(10):6336–6349. doi: 10.1111/ijfs.15979

Linkages between soil, crop, livestock, and human selenium status in Sub‐Saharan Africa: a scoping review

Beaula Mutonhodza 1,✉, Edward J M Joy 2,3, Elizabeth H Bailey 4, Murray R Lark 4, Muneta G M Kangara 3, Martin R Broadley 3,4, Tonderayi M Matsungo 1, Prosper Chopera 1
PMCID: PMC9804181  PMID: 36605250

Summary

Selenium (Se) is essential for human health, however, data on population Se status and agriculture‐nutrition‐health linkages are limited in sub‐Saharan Africa (SSA). The scoping review aims to identify linkages between Se in soils/crops, dietary Se intakes, and livestock and human Se status in SSA. Online databases, organisational websites and grey literature were used to identify articles. Articles were screened at title, abstract and full text levels using eligibility criteria. The search yielded 166 articles from which 112 were excluded during abstract screening and 54 full text articles were assessed for eligibility. The scoping review included 34 primary studies published between 1984 and 2021. The studies covered Se concentrations in soils (n = 7), crops (n = 9), animal tissues (n = 2), livestock (n = 3), and human Se status (n = 15). The evidence showed that soil/crop Se concentrations affected Se concentration in dietary sources, dietary Se intake and biomarkers of Se status. Soil types are a primary driver of human Se status and crop Se concentration correlates positively with biomarkers of Se dietary status. Although data sets of Se concentrations exist across the food system in SSA, there is limited evidence on linkages across the agriculture‐nutrition nexus. Extensive research on Se linkages across the food chain is warranted.

Keywords: Animal tissue, cereals and grains, dietary, nutrition, se deficiency

Selenium linkages in Sub‐Saharan Africa.

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Introduction

The global burden of malnutrition is unacceptably high and affects every country in the world (UNICEF, 2018). Malnutrition encompasses undernutrition, including micronutrient deficiencies (MNDs), which is often linked to stunting and wasting in children (UNICEF, 2020). Children aged 6–59 months and women of reproductive age (WRA; 15–49 years) are typically the most vulnerable to mineral deficiencies (Kumssa et al., 2015; Willett et al., 2019). In general, iron (Fe), zinc (Zn), and iodine (I) have been the minerals of greatest public health concern based on global disease burdens in sub‐Saharan Africa (SSA; Wang et al., 2016). Although not usually included in national surveys, Se is an essential mineral and widespread deficiencies are likely in SSA (Joy et al., 2015a; Ligowe et al., 2020b).

Se deficiencies can lead to gestational complications, miscarriages, and damage to the nervous and immune systems of a fetus (Pieczyńska & Grajeta, 2015). Low concentrations of Se in blood serum in the early stage of pregnancy is a predictor of low‐birth weight babies (Pieczyńska & Grajeta, 2015). These factors can predispose populations to inter‐generational malnutrition if not addressed (Imdad et al., 2017). Se is a potent antioxidant that plays an important role in inflammatory or immune‐related diseases including anti‐viral immunity, autoimmunity, sepsis, allergic asthma (Huang et al., 2012), and metabolic signalling in strokes (Amani et al., 2019). Poor quality diets can lead to micronutrient deficiencies, especially in populations with low total food intake (Biesalski & Black, 2016). In SSA, the diets of forty countries consist primarily of carbohydrates (>55% of the overall dietary energy supply (DES) with protein supplying less than 15% of overall DES; Abrahams et al., 2011). Carbohydrate rich foods typically have low concentrations of Se compared with protein‐based foods (Combs, 1988; Donovan et al., 1991; Joy et al., 2015a). Low dietary diversification is an important contributory factor to inadequate human dietary Se intake which consequently leads to human Se deficiency (Gashu et al., 2018; Ngigi et al., 2020). The World Health Organisation (WHO) recommends a daily intake of 55 μg of Se for adults (WHO, 2016). The Se requirements for pregnant and lactating women are considered to be higher, and range from 60 to 70 μg day−1. Requirements for children range between 15 and 40 μg day−1 depending on age (NRC and IOM, 2000; Institute of Medicine, 2010).

Dietary Se intake and the Se status of people varies markedly around the world (Fordyce, 2012). This is due to differences in geological, soil geochemical, and climatic factors which result in variation in soil and crop Se status (Fordyce et al., 2000; Rayman, 2000; Fairweather‐Tait et al., 2011; Ligowe et al., 2020a). Among national food systems in Africa, the prevalence of inadequate dietary Se intake from Se supplies was estimated to be 28% using food composition data from a national and/or regional level (Joy et al., 2014). Greater risks of Se deficiency were estimated to occur in the Eastern (52%) and Middle (49%) regions of Africa, followed by Southern (26%), Northern (12%), and Western (6%) regions (Joy et al., 2014).

Ligowe et al. (2020b) reported circumstantial evidence to support the hypotheses that many soils in SSA provide inadequate Se to food crops for optimal human health, especially where access to animal source foods is limited (Ligowe et al., 2020b). However, they concluded that data on Se concentration in soil, crops, livestock, and human biomarkers are generally sparse across SSA. Evidence of the linkages across the agriculture‐nutrition‐health nexus will enable the translation of information across countries and may help inform micronutrient surveillance programs and policy interventions. The aim of this review was therefore to identify linkages between Se in soils/crops, dietary Se concentration, and livestock and human Se status in SSA. The review puts a focus on the Se status of women and children as this group is deemed most vulnerable to micronutrient deficiencies (Kumssa et al., 2015; Willett et al., 2019) and any nutrition programming targeting this group will have a greater impact on public health outcomes (von Grebmer et al., 2014).

Methods

Study design

This study is a scoping literature review of mainly published articles and grey literature. The grey literature consisted of project reports and unpublished research theses and data. The inclusion criteria were as follows; Quantitative studies written in, or translated to English were considered with no restrictions on the year of publication. The review featured studies that focused on soil, crop, livestock, women, and children in SSA. The design of the scoping review was carried out according to the 2018 PRISMA Extension for Scoping Reviews (PRISMA‐ScR) checklist which clarifies the purpose and comprehensiveness of the scoping process (Tricco et al., 2018).

Search strategy

Electronic searches for all experimental studies, reviews, meta‐analyses, and reports on Se concentration in soil, crop, livestock, and people in SSA were conducted. Searches were conducted using the following electronic databases: Web of Science, Semantic Scholar, PubMed, Mendeley, and official websites for the United Nations, WHO and the Food and Agricultural Organisation (FAO). Search terms were developed under two headings: Se status of livestock, women and children and Se concentration of soils, crops and animal tissue. Individual livestock names, key words and combinations were used to perform a comprehensive search of the databases, e.g., Se AND soil AND Sub‐Saharan Africa; Se AND crop AND Sub‐Saharan Africa; Maternal OR mother OR women Se status and Sub‐Saharan Africa; Child OR baby OR children Se status AND Sub‐Saharan Africa; staple diets Sub Saharan Africa; Se and plasma or serum or blood and Sub‐Saharan Africa. The electronic database was supplemented with scanning of reference lists of relevant reviews. Overlapping data sets were removed using Mendeley remove duplicate function. Articles and review articles with dates of coverage spanning from 1972 to 2021 were identified.

Eligibility

Studies conducted in SSA were included if they reported on Se concentration of soils (n = 7 studies), crops (n = 9), animal tissues (n = 2), livestock (n = 3), or human Se status (n = 15). Human studies that used whole blood (1), plasma (8), or serum (6) Se as biomarkers of Se status were included but those that used hair and toenails were excluded. Measurement of Se in hair or toenails may provide a useful indication of longer‐term exposure to Se, however due to the lack of widely used thresholds to define Se adequacy or deficiency and the high risk of contamination with exogenous Se from hair products or soil particles these studies were excluded (Phiri et al., 2019). Human studies with age and sex dis‐aggregated data were used and data were considered only for apparently healthy children and WRA. For clinical studies, data were only considered for the control groups that did not exhibit clinical symptoms. Soil Se extraction methods were considered for comparability of results, for this study soil data are based on strong acid/peroxide digests. Studies included typically determined Se concentration by inductively coupled plasma mass spectrometry (ICP‐MS; n = 26). Other analysis methods used were, atomic absorption spectrophotometry (AAS; n = 3) including hydride generation (HG‐AAS; n = 4) and electrothermal (EAAS; n = 1), inductively coupled plasma atomic emission spectrometry (ICP‐AES; n = 1), inductively coupled plasma optical emission spectrometry (ICP‐OES; n = 1), spectrofluorimetry (n = 1), neutron activation analysis (NAA; n = 2), and particle induced X‐ray emission (PIXE; n = 1).

Data extraction

Data were extracted manually into tables in Microsoft Excel to record the required information, including from supplementary material where these were available. Data extracted included: total soil Se concentration, Se concentration of commonly consumed crops and animal source foods in SSA, Se status of livestock, children and WRA, and publication characteristics. Total soil Se concentration data used in this study are all based on acid digests and not being compared with other data based on extractions designed to measure ‘available’ Se or similar. Crop and animal edible tissue Se concentrations were calculated, with dry weight (DW) data converted into wet weight (WW) the form in which the food is consumed using the formula (US‐EPA, 2011):

WW=DW×100−%moisture/100 (1)

Moisture content was based on data from a fitted United States Department of Agriculture (USDA) food item (USDA, 2013). Livestock and human Se status as indicated by plasma/whole blood Se concentrations was used. Units of measurement were standardised for comparability purposes and converted to micrograms per litre (μg L−1) or micrograms per kilogram (μg kg−1). Measures of central tendency, mean and standard of variation (SD) or medians and ranges rounded to the nearest whole number were used to report Se concentrations.

Outcomes

The primary outcomes were Se concentration in soil, crop, and edible animal tissue, and Se status of livestock, children and WRA. Total soil Se concentration less than 400 μg kg−1 was considered low (Fordyce, 2012). Commonly consumed foods with Se concentrations less than 45 μg kg−1 WW were regarded as having low Se concentration (Courtman et al., 2012). Livestock was considered Se deficient if the plasma Se concentration was less than 30 μg L−1 (Mpofu et al., 1999) and marginally deficient if less than 60 μg L−1 (Dermauw et al., 2014). In the human body, deficiency of Se was noted when its amount in plasma was less than 70 μg L−1 (Thomson, 2004; Phiri et al., 2019; Belay et al., 2020).

Ethical approval

Ethical approval was not required as the study did not have a direct involvement with human participants and the human studies included were based on published data.

Protocol registration

This protocol was not registered with any health research entity as it is does not report on a health‐related outcome (CRD, 2016).

Study selection

A total of 166 articles were identified upon removal of duplicates; 112 were excluded at title and abstract screening. The remaining 54 full text articles were assessed for eligibility and 34 articles were retained for the scoping review (Fig. 1).

Figure 1.

Figure 1

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PRISMA study selection. A total of 166 papers were identified upon removal of duplicates, 112 were excluded at title and abstract screening. The remaining 54 full text articles were assessed for eligibility and only 34 papers were included for the scoping review.

Results

Total soil se concentration in SSA

Total soil Se data from six SSA countries were identified (Table 1). Among the six countries Zimbabwe, Zambia and Malawi had sub‐optimal (˂400 μg kg−1) soil Se concentrations, with the lowest being observed in North east Zimbabwe (Fordyce et al., 1994). Kenya had intermediate values of total Se concentration that ranged from 215 to 703 μg kg−1 (Ngigi et al., 2020). Subjective to the small sample size used compared with the other countries Nigeria and Tanzania had optimal (>600 μg kg−1) total soil Se concentrations with Tanzania having the highest maximum total soil Se concentrations.

Table 1.

Total Soil Se concentration ranges in SSA

Location Sample description Soil Se concentration (μg kg−1), median (range) Digestion method Analytical method References
Kenya n = 160 8 sites 465 (215–703) Nitric and hydrofluoric acids ICP‐MS Ngigi et al. (2020)
Malawi n = 88 10 soil types 162 (50–620) Nitric, hydrofluoric, and perchloric acids ICP‐MS Joy et al. (2015b)
Nigeria n = 3 3 zones 1130 (1070–1190) Hydrochloric, nitric, hydrofluoric, and perchloric acids AAS Aremu et al. (2010)
Tanzania n = 31 3 soil types 600 (210–4700) Nitric and hydrofluoric acids ICP‐MS Mills & Milewski (2007)
Zambia n = 190 3 zones 70 (trace–790) Hydrogen peroxide, perchloric, and nitric acids AAS Melse‐Boonstra et al. (2007)
Zimbabwe n = 70 3 zones 56 (10–163) Hydrochloric acid, nitric, and perchloric acids HG‐AAS Fordyce et al. (1994)
Zimbabwe n = 350 2 zones 430 (30–1100) Hydrochloric acid, nitric, hydrofluoric, and perchloric acids ICP‐MS Manzeke‐Kangara MG, unpublished results
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ICP‐MS, inductively coupled plasma mass spectrometry; AAS, atomic absorption spectrophotometry; HG‐AAS, hydride generation‐atomic absorption spectrophotometry.

Crop se concentration in SSA

Maize (Zea mays) is the principal staple in most countries within Eastern and Southern Africa (FAO, 2016). Maize grain in Kenya, Malawi, South Africa and Zambia typically had suboptimal Se concentrations (˂45 μg kg−1 WW) with median values ranging from 13 to 26 μg kg−1 WW (Table 2). However, maize grain from Tanzania (Watts et al., 2019) and Uganda (Bevis & Hestrin, 2021) had adequate Se concentration with median values >45 μg kg−1 WW. Among the grains; rice (Oryza sativa), finger millet (Eleusine coracana) and wheat (Triticum aestivum) sorghum (Sorghum bicolor) had relatively higher median Se concentrations across all countries followed by maize. Roots and tubers are an important constituent of SSA diets, particularly in Central and Western Africa (FAO, 2016). The lowest median Se concentration in cassava (Manihot esculenta) was observed in Kenya and the highest in Tanzania. Sweet potatoes (Ipomoea batatas) from Malawi had the lowest median Se concentration and Tanzania had the highest. Median Se concentration in beans (Phaseolus vulgaris) were optimal (>45 μg kg−1 WW) in Tanzania and Uganda and sub‐optimal (˂45 μg kg−1 WW) in Kenya and Malawi. Cowpea (Vigna unguiculata) Se concentration was highest in Malawi and lowest in Kenya. Generally, Tanzania had higher Se concentration across all crops compared with the other countries.

Table 2.

Se concentrations in common plant foods in SSA

Crop Country Sample description Se concentration (μg kg−1 WW), mean (SD) or median (range) Analytical method References
Beans Kenya Field (n = 130) 33 (4–1280) ICP‐MS Watts et al. (2019)
Malawi Field (n = 6) 37 (4–69) ICP‐MS Joy et al. (2015b)
Tanzania Field (n = 27) 135 (19–1800) ICP‐MS Watts et al. (2019)
Uganda Field (n = 92) 90 (13–3098) ICP‐MS Bevis & Hestrin (2021)
Cassava Kenya Field (n = 18) 4 (2–17) ICP‐MS Watts et al. (2019)
Nigeria Field (n = 13) 5 (6) HG‐AAS Zarmai et al. (2013)
Tanzania Field (n = 2) 29 (6–51) ICP‐MS Watts et al. (2019)
Uganda Field (n = 108) 9 (2–174) ICP‐MS Bevis & Hestrin (2021)
Cowpea Kenya Field (n = 44) 32 (4–308) ICP‐MS Watts et al. (2019)
Malawi Field (n = 17) 100 (13–400) ICP‐MS Joy et al. (2015b)
Zimbabwe Field (n = 23) 35 (10–88) ICP‐MS Manzeke‐Kangara (unpublished results)
Finger millet Kenya Field (n = 13) 21 (10–557) ICP‐MS Watts et al. (2019)
Malawi Field (n = 7) 29 (11–77) ICP‐MS Joy et al. (2015b)
Zimbabwe Field (n = 8) 27 (10–79) ICP‐MS Manzeke‐Kangara (unpublished results)
Maize Kenya Field (n = 217) 26 (5–1201) ICP‐MS Watts et al. (2019)
Malawi Field (n = 88) 17 (4–478) ICP‐MS Chilimba et al. (2011)
Malawi Field (n = 1608) 45 (90) ICP‐MS Gashu et al. (2021)
South Africa Silos (n = 896) 13 (0–53) HG‐AAS Courtman et al. (2012)
Tanzania Field (n = 42) 143 (5–1187) ICP‐MS Watts et al. (2019)
Uganda Field (n = 112) 65 (8–970) ICP‐MS Bevis & Hestrin (2021)
Zambia Field (n = 36) Market (n = 31) 14 (8–43) ICP‐MS Gondwe (2018)
Rice Malawi Field and stores White (n = 21) Brown (n = 33) 22 (<4–261) ICP‐MS Joy et al. (2017)
Tanzania Field (n = 2) 11 (9–13) ICP‐MS Watts et al. (2019)
Sorghum Kenya Field (n = 44) 39 (5–726) ICP‐MS Watts et al. (2019)
Malawi Field (n = 45) 205 (5–697) ICP‐MS Joy et al. (2015b)
Uganda Field (n = 155) 109 (1–2127) ICP‐MS Bevis & Hestrin (2021)
Zimbabwe Field (n = 11) 64 (5–170) ICP‐MS Manzeke‐Kangara (unpublished results)
Sweet potato Kenya Field (n = 19) 3 (1–24) ICP‐MS Watts et al. (2019)
Malawi Field (n = 3) 1 (trace–2) ICP‐MS Joy et al. (2015b)
Nigeria Field (n = 13) 8 (5) HG‐AAS Zarmai et al. (2013)
Tanzania Field (n = 3) 20 (6–35) ICP‐MS Watts et al. (2019)
Uganda Field (n = 87) 11 (1–232) ICP‐MS Bevis & Hestrin (2021)
Wheat Ethiopia Field (n = 328) 80 (107) ICP‐MS Gashu et al. (2021)
Zambia Field (n = 6) 7 (6–13) ICP‐MS Gondwe (2018)
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SD, standard deviation; ICP‐MS, inductively coupled plasma mass spectrometry; HG‐AAS, hydride generation‐ atomic absorption spectrophotometry.

Se concentrations in animal edible tissues

Se is associated with protein in animal tissues (Combs, 1988; Kieliszek, 2019). Generally, cattle in Burundi had higher Se concentrations in both muscle and organ tissue compared with Ethiopia (Table 3). Organ meat had higher Se concentrations compared with muscle meat across all countries. Among the organs kidneys had the greatest Se concentration in both cows and goats. Se concentration of commonly consumed fish in Malawi including catfish (Clarias gariepinus), Chambo (Oreochromis shiranus), Usipa (Engraulicypris breianalis), and Matemba (Barbus paludinosus) had the lowest Se concentrations compared with cow and goat meat. Milk had sub‐optimal (<45 μg kg−1 WW) Se concentrations. Goat milk had lower Se concentration compared with cow milk. Largely, meat (cow, goat, and fish) had higher Se concentration compared with milk.

Table 3.

Se concentrations in animal edible tissues in SSA

Country Meat/milk type Se concentration (μg kg−1 WW), mean (SD) or median (range) Analytical method References
Burundi Cattle (beef) Abattoir (n = 11)
Muscle 169 (110) HG‐AAS Benemariya et al. (1993)
Liver 316 (99) Benemariya et al. (1993)
Heart 335 (40) Benemariya et al. (1993)
Pancreas 488 (19) Benemariya et al. (1993)
Spleen 292 (65) Benemariya et al. (1993)
Kidney 1428 (21) Benemariya et al. (1993)
Ethiopia Cattle (beef) Abattoir (n = 60)
Liver 180 (154–209) ICP‐MS and ICP‐OES. Dermauw et al. (2014)
Kidney 1138 (1019–1256) Dermauw et al. (2014)
Semitendinosus muscle 88 (55–116) Dermauw et al. (2014)
Cardiac muscle 175 (140–204) Dermauw et al. (2014)
Burundi Goat farm (n = 11)
Muscle 103 (70) HG‐AAS Benemariya et al. (1993)
Heart 189 (93) Benemariya et al. (1993)
Liver 427 (349) Benemariya et al. (1993)
Pancreas 372 (149) Benemariya et al. (1993)
Spleen 288 (107) Benemariya et al. (1993)
Kidney 1194 (235) Benemariya et al. (1993)
Milk (mature unprocessed)
Cow (n = 19) 26 (5) Benemariya et al. (1993)
Goat (n = 5) 23 (5) Benemariya et al. (1993)
Fish
Malawi Catfish (n = 4) 250 (5) NAA Donovan et al. (1991)
Chambo (n = 7) 260 (5) Donovan et al. (1991)
Usipa, dried (n = 10) 730 (2) Donovan et al. (1991)
Matemba dried (n = 10) 480 (6) Donovan et al. (1991)
Matemba fresh (n = 10) 110 (4) Donovan et al. (1991)
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μg kg−1 WW, micrograms per kilogram Wet Weight; SD, standard deviation; HG‐AAS, hydride generation‐atomic absorption spectrophotometry; ICP‐MS, inductively coupled plasma mass spectrometry; ICP‐OES, inductively coupled plasma optical emission spectrometry; NAA, neutron activation analysis.

Se status of livestock in SSA

Livestock Se deficiency (<30 μg L−1) and marginal deficiency (<60 μg L−1) was observed in Ethiopia, South Africa, and Zimbabwe (Table 4). In Zimbabwe, livestock Se deficiency was reported in calves, steers and cows while in Ethiopia marginal deficiency was observed in bulls. In South Africa, 10% of the sheep were marginally deficient (Erasmus, 1984).

Table 4.

Se status in livestock in SSA

Country Livestock Source and sample size Analytical method Se concentration (μg L−1), mean (SD) or median (range) Se status References
Ethiopia Bull Abattoir (n = 28) Plasma ICP‐MS and 2ICP‐OES 45 (36–54) Marginally deficient Dermauw et al. (2014)
South Africa Sheep Farm (n = 115) Whole blood NAA 320 (50–750) Marginally deficient‐adequate Erasmus (1984)
Zimbabwe Calves Farm (n = 40) Plasma ICP‐AES 17 (5) Deficient Mpofu et al. (1999)
Cows Farm (n = 40) 17 (5) Deficient Mpofu et al. (1999)
Steers Farm (n = 40) 25 (5) Deficient Mpofu et al. (1999)
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SD, standard deviation; ICP‐MS, inductively coupled plasma mass spectrometry; ICP‐OES, inductively coupled plasma optical emission spectrometry; NAA, neutron activation analysis; ICP‐AES, inductively coupled plasma atomic emission spectrometry.

Prevalence of Se deficiency in children and women in SSA

Inadequate serum/plasma Se concentrations were found in various SSA countries (Ngigi et al., 2020; Ligowe et al., 2020b). Se deficient status defined as mean serum/plasma Se concentrations less than 70 μg L−1 (Phiri et al., 2019; Belay et al., 2020) was observed in Ethiopia, Democratic Republic of Congo (DRC), Ivory Coast, Malawi, Nigeria, South Africa, Zambia, and Zimbabwe, with mean serum/plasma concentrations among women and children ranging from 29 to 69 μg L−1 (Table 5). In northwest Ethiopia, 60% of children were reported to be Se deficient (Amare et al., 2012) and in Malawi, 77% (n = 348) of children aged between 6 and 23 months were Se deficient (Gebremedhin, 2020). The prevalence of Se deficiency in Zimbabwe was found to be high with almost half of the children (n = 269) being Se deficient (Kuona et al., 2014).

Table 5.

Se deficiency in women and children in SSA

Country Sampling Age group Se concentration (μg L−1), mean (SD) or median (range) Analytical method References
Ethiopia Simple random (n = 100) 54–78 months 63 (26) Serum ICP‐MS Amare et al. (2012)
Ethiopia Cluster (n = 628) 6–60 months 61 (11–291) Serum ICP‐MS Gashu et al. (2016)
Ethiopia Stratified (n = 521) 6–59 months 67 (42–95) Plasma ICP‐MS Belay et al. (2020)
Democratic Republic of Congo (DRC) Convenience (n = 87) 4–17 years 35 (23) Serum ICP‐MS Bumoko et al. (2015)
Democratic Republic of Congo (DRC) Convenience (n = 109) 15–49 years 40 (22) Serum spectrofluorimetry Ngo et al. (1997)
Ivory Coast Cluster (n = 47) 18–69 years 29 (17) Plasma EAAS Tiahou et al. (2004)
Malawi Convenience (n = 54) Infants (24 weeks) 60 (13) Plasma ICP‐MS Flax et al. (2014)
Malawi Cluster (n = 494) Preschool children 61 (25) Plasma ICP‐MS Phiri et al. (2019)
Malawi Survey (n = 348) 6–23 months 57 (23) Plasma ICP‐MS Gebremedhin (2020)
Malawi Convenience (n = 148) 14–45 years 62 (61–65) Plasma ICP‐MS Gibson et al. (2011a)
Malawi Convenience (n = 49) 17–38 years 67 (53–76) Plasma and erythrocyte ICP‐MS Stefanowicz et al. (2013)
Niger Convenience (n = 71) 15–49 years 77 (16) Plasma NAA and PIXE Cénac et al. (1992)
Nigeria Convenience (n = 30) 13–14 years 63 (27) Serum ICP‐MS Olopade et al. (2009)
South Africa Convenience (n = 111) 47–58 years 68 (58–71) Plasma AAS Jaskiewicz et al. (1988)
Zambia Convenience (n = 476) 6 months 50 (47–52) Serum ICP‐MS Gibson et al. (2011b
Zimbabwe Convenience (n = 269) 7–10 years 85 (16) Serum ICP‐MS Kuona et al. (2014)
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SD, standard deviation; ICP‐MS, inductively coupled plasma mass spectrometry; NAA, neutron activation analysis; PIXE, particle induced X‐ray emission; AAS, atomic absorption spectrophotometry.

Discussion

Total soil Se concentration and implications on dietary Se supply in SSA

Low total soil Se concentration is common in SSA (Ligowe et al., 2020b) and, in some contexts, this may constrain concentrations of Se in crops and livestock feed (Courtman et al., 2012). Total Se concentrations might not be a good indicator of how much Se is available to be taken up by a crop or into forage however, it provides an upper bound for the available soil Se. In soils with total Se concentrations <2000 μg kg−1, the relationship between total and bioavailable Se was found to be linear (Statwick & Sher, 2017).

The total Se concentration in the soil, however, may not correlate with Se concentration in plants (Fordyce et al., 1994; Courtman et al., 2012). Se availability to plants is strongly influenced by physio‐chemical factors. Low soil pH, clay soil texture, high organic matter and iron oxyhydroxides reduce Se availability and subsequently Se concentration in food crops (Stroud et al., 2010; Gashu et al., 2018). The low total soil Se concentrations observed in Zimbabwe could be attributed to the low soil pH observed in Zimbabwean soils (Fordyce et al., 1994) which potentially influences dietary Se availability in the human population (Kuona et al., 2014). In Malawi, women from villages with acidic soils in the Zombwe Extension Planning Area (EPA) had lower median dietary Se intakes of 6.5 μg day−1 and a lower median plasma Se concentration of 53.7 μg L−1 (n = 60) compared with women in Mikalango EPA where crops were grown on vertisol soils (characteristically with high pH) and their Se intake was eight‐fold higher (median plasma Se concentration 117 μg L−1, n = 60; Hurst et al., 2013).

Soil types are a primary driver of human Se status (Ligowe et al., 2020b). Vertisol soil types have been linked with much higher grain Se concentrations (Chilimba et al., 2011; Ligowe et al., 2020a, 2020b). The higher Se concentrations in crops observed in Tanzania could be attributable to dominance of vertisol soils (Bationo et al., 2012) while much lower Se concentrations are observed in the same crops grown in Malawi where vertisol soil type comprise only 0.5% of the land area (Hurst et al., 2013).Vertisols are mostly found in semi‐arid and sub‐humid regions of Ethiopia and Tanzania, though they are also scattered throughout much of southern Africa (Hudson, 1987).

Although vertisol soil types are not widely found in SSA they may have a greater capacity to supply Se to crops than do other soils characteristic of southern Africa. The lack of predominant vertisol soil types in SSA points to the need for implementation of strategies that enhance soil‐to‐crop transfer of Se such as, low organic matter and iron oxyhydroxides and high soil pH could improve supply of the micronutrient in SSA. Application of lime is a low‐cost strategy that could be considered, although there is no robust evidence that this strategy would work. However, the enzymes of the antioxidant system that protect cells by eliminating reactive oxygen species (ROS) including catalase, superoxide dismutase, and numerous peroxidases are more efficient in selenite than selenate (Cartes et al., 2005; Mora et al., 2015). Selenate predominates in alkaline soils and has higher mobility than selenite, which commonly occurs in neutral or acid soils and is easily adsorbed on oxy hydroxides (Kabata‐Pendias, 2010; El‐Sayed et al., 2020). On the other hand, Chen et al. (2014) observed that Se in the form of selenite caused a generally increased generation of ROS in plants (Chen et al., 2014), while the generation of ROS (hydrogen peroxide) was stronger during the interaction of Se in the form of selenate than selenite (Ríos et al., 2009). It would be important to know if there might be Se antioxidant losses from increased soil alkalinity that may offset the gains from improved bioavailability of Se. Future studies that explore these linkages and dynamics are essential.

It would also be beneficial to know if soil pH only affects Se status of soil and crop, or if it expands to other micronutrients. If this finding expands to other elements there is a strong case to be made for investment in enhancing soil pH.

Crop Se concentration and implications on dietary Se intake

Positive relationships exist between the Se concentration of grain and biomarkers of Se dietary status (Gashu et al., 2021). Cereals, roots and tubers are primary energy sources across SSA (Joy et al., 2014; Food and Agriculture Organisation, 2016; Gondwe, 2018). For example, in Malawi, >50% of dietary energy is derived from maize (Joy et al., 2015a) making it a major contributor to dietary Se intake even where grain Se concentrations are low (Chilimba et al., 2011). Pulses (beans) also make a large contribution to total protein intake in SSA (Ligowe et al., 2020b) and Eastern Africa has a high total intake of 22 kg capita−1 (Food and Agriculture Organisation, 2016). Grains and grasses are non‐Se accumulating thus often contain low concentrations of Se on many soil types (Mayland et al., 1989; Saha, 2017).

It is therefore apparent that crop‐based foods are not good sources of Se in many soil types, and this could result in low dietary Se intakes in SSA communities typically dependent on plant‐based diets (Donovan et al., 1991; Courtman et al., 2012; Gashu et al., 2020). Hence, a need to increase crop and human Se intake through employment of agronomic biofortification strategies that ensure loading of Se into staple grains through fertilisation. Se plays a key role in the antioxidant systems in plants, studies have shown that application of Se at low doses protect the plants from variety of abiotic stresses such as temperature (Chu et al., 2010; Djanaguiraman et al., 2010, 2018), drought (Hasanuzzaman et al., 2011), desiccation (Pukacka et al., 2011), aging (Djanaguiraman et al., 2005; Hartikainen, 2005), and metal stress (Kumar et al., 2012; Pandey & Gupta, 2015; Ghorai et al., 2022). Se, applied at low concentrations, enhances growth and antioxidative capacity of both mono and dicotyledonous plants (Kavalcová et al., 2015; Shalaby et al., 2017; El‐Sayed et al., 2020; Cipriano et al., 2022). However, at high Se doses, it acts as pro‐oxidant and causes oxidative stress in plants (Mroczek‐Zdyrska & Wójcik, 2012; Mora et al., 2015; Mroczek‐Zdyrska et al., 2017). Thus, it is important to have a balance in the amount of Se concentrations added in Se biofortification of crops, as the occurrence of an antioxidant or pro‐oxidative effect depends on the concentration of Se to be used. Food processing methods that improve the bioavailability of nutrients for example fermentation and advocacy for targeted Se fortification of the commonly consumed crop products at population level can be also be employed to improve human Se dietary intake.

Significance of animal source foods to dietary se supply

A substantial proportion of dietary Se intake in several countries in SSA has been attributed to fish consumption (Donovan et al., 1991; Eick et al., 2009; Joy et al., 2015a; Food and Agriculture Organisation, 2016). In the staple foods of most SSA countries' diets, fish has the highest Se concentration (Donovan et al., 1991). Meat and organ consumption form an important contribution to dietary Se supply, as these tissues have a high concentration of Se (Combs, 1988; Donovan et al., 1991; Fairweather‐Tait et al., 2010; Kieliszek, 2019).Animal consumption is low in SSA where per capita meat consumption is approximately 11 kg annum−1, a third less than the global average per capita meat consumption (Food and Agriculture Organisation, 2016). Milk (cattle and goat) had low Se concentration and yet dairy represents a primary protein source to SSA consumers with fresh dairy products accounting for more than 90% of total dairy consumption (Food and Agriculture Organisation, 2016). In addition, animal milk (from goats and cows) a poor source of Se, is a common complementary food given to infants and young children (Benemariya et al., 1993). Se from animal source foods is unlikely to contribute substantially to the diets of most people in SSA, especially among women and children.

However, consumption of organ meat can be advocated for to improve Se supply, as organ meat is deemed affordable to poor populations in most developing countries (van Heerden & Morey, 2014; Bester et al., 2018).Organ meat can serve as an alternative Se dense animal source which could potentially improve the Se status of populations. Data on Se concentrations in animal source foods and consequently the direct linkage between animal protein intake, Se intake and Se status are inadequate in SSA. More research is warranted for a wider range of animal products including poultry which accounts for the largest proportion of total meat consumption in SSA (Food and Agriculture Organisation, 2016).

Livestock Se deficiency and its implication on dietary Se supply in SSA

There is a paucity of studies on livestock Se status in SSA and relationships between trace element concentrations in plasma and edible tissues have not been widely studied. Dietary Se intake by livestock is essential to secure animal health and prevent Se deficiency, and also to increase Se levels in meat, eggs and milk (Combs, 1988; Haug et al., 2007). Current findings suggest that Se deficient livestock potentially yield meat and meat products with low Se concentrations (Dermauw et al., 2014), which will in turn influence human dietary Se intakes.

The linkages between livestock Se concentration in plasma and edible tissue could be important for human nutrition, as plasma concentrations might form a more practical tool for early evaluation of Se concentrations in meat, essential for optimal human health (Dermauw et al., 2014). Se supplementation of livestock through addition of Se to animal feed or Se enrichment of pastures is a promising approach to improve livestock health and productivity, and increase dietary Se intakes (Alfthan et al., 2015). However, a direct link between the sensitivity of human Se status with respect to the variation in livestock Se status is yet to be fully established.

Se deficiency in women and children in SSA

Se deficiency is widespread among women and children in SSA. The vicious cycle of malnutrition can be addressed by interventions that target this group. However, dietary intake and consumption studies in this group have not been widely assessed and there are a few nationally representative surveys documenting human Se status (Gashu et al., 2018; Phiri et al., 2019; Belay et al., 2020). Se speciation significantly affects the potential benefits of this element to mammal health, being the organic Se forms (selenocysteine and/or selenomethionine) the most effective bioavailable Se species for the animal and human nutrition (Thomson, 2004; Fairweather‐Tait et al., 2010; Surai et al., 2019). There are several factors that can contribute to Se deficiency other than reduced intake. Antagonistic effects exist between Se, vitamins (A, K, and C; Watts, 1994) and other minerals; copper (Cu; Ranches et al., 2021), heavy metals, zinc (Zn), sodium (Na), magnesium (Mg), manganese (Mn; Watts, 1994; Thomson, 2004; Stress et al., 2014), and iron (Fe; Petkova‐Marinova et al., 2017; Larvie et al., 2019). Although Se might be flowing adequately in the environment its absorption in the human body might be inhibited. There is a need to review dietary recommendations in light of dietary forms of Se and the Se interactions that influence Se uptake.

Summary of evidence

In this scoping review, 34 primary studies published between 1984 and 2021, conducted in SSA addressing Se concentrations in soils (n = 7 studies), crops (n = 9), animal tissues (n = 2), livestock (n = 3), and human Se status (n = 15) were identified. Although independent data sets of Se concentrations exist across the food system in SSA, our findings indicate a paucity of research focusing specifically on the linkages across the agriculture nutrition nexus. However, the evidence gathered showed that Se deficiency is widespread in SSA food systems. Soil characteristics such as soil type were reported to influence soil and crop Se concentrations. For example, vertisol soils in Malawi were positively correlated with crop Se concentration which consequently affected human dietary Se intake and biomarkers of Se status. Crop Se concentration influences animal and human Se status. Although animal source foods are a better source of dietary Se than most crops, they are unlikely to be consumed widely among lower‐income groups. Cereals and legumes are the primary source of dietary Se for most of the population in SSA.

Limitations

The scoping review had some limitations. To meet the objective of reviewing data for apparently healthy children and WRA, only age and gender dis‐aggregated studies were utilised. As such, the results are not fully representative of all the human Se studies conducted in SSA. Furthermore, due to limited published Se data the scoping review used unpublished results that are not publicly available and articles published beyond the last 30 years.

Conclusions

There is evidence that the majority of agricultural soils in SSA have low concentrations of plant‐available Se which limits entry of Se into food systems. Crop based foods are a major component of diets across SSA, with low consumption of animal‐source foods that serve as good sources of Se compared with plant‐based foods. Crop and livestock Se concentrations are proxy indicators of dietary Se intake and may reflect human Se status. It is apparent that dietary Se supply is typically low in rural populations of SSA compared with their urban counterparts and that Se deficiency in women and children is widespread in SSA. It is therefore, recommended that strategies be identified and implemented that span the whole food system from soil to crop to humans.

However, in resource limited settings as is the case in most SSA countries appropriate strategies can only be implemented effectively after surveys at national and regional levels have identified hot spots and population groups at risk of Se deficiency. More empirical evidence of linkages across the agriculture–nutrition–health domains is required in SSA, as data remain scarce on livestock Se status, bioavailability of Se in foods and spatial variability of human Se status in SSA.

Funding information

The authors acknowledge funding from the UK Research and Innovation (UKRI) Global Challenges Research Fund (GCRF) [grant number EP/T015667/1], ‘Translating GeoNutrition: Reducing mineral micronutrient deficiencies (MMNDs) in Zimbabwe’, the ‘GeoNutrition’ project funded by the Bill & Melinda Gates Foundation [BMGF INV‐009129], the University of Nottingham, and the University of Zimbabwe.

Conflict of interest

The authors declare no conflicts of interest.

Author contributions

Beaula Mutonhodza: Conceptualization (lead); visualization (lead); writing – original draft (lead); writing – review and editing (lead). Prosper Chopera: Conceptualization (equal); supervision (lead); validation (lead); visualization (equal); writing – original draft (equal); writing – review and editing (equal). Tonderayi M. Matsungo: Conceptualization (equal); supervision (lead); validation (equal); visualization (equal); writing – original draft (equal); writing – review and editing (equal). Martin R. Broadley: Conceptualization (supporting); funding acquisition (lead); validation (supporting); visualization (supporting); writing – review and editing (supporting). Muneta G. M. Kangara: Data curation (supporting); writing – review and editing (supporting). Murray R. Lark: Validation (supporting); visualization (supporting); writing – review and editing (supporting). Elizabeth H. Bailey: Validation (supporting); visualization (supporting); writing – review and editing (supporting). Edward J. M. Joy: Validation (supporting); visualization (supporting); writing – review and editing (supporting).

Ethics approval and consent to participate

Not applicable.

Peer review

The peer review history for this article is available at https://publons.com/publon/10.1111/ijfs.15979.

Acknowledgments

The authors would like to acknowledge Fiona Fordyce for providing project reports and guidance on Se work conducted in Zimbabwe (British Geological Survey Technical Report WC/94/3) that were not available online.

Co‐corresponding author: Email: martin.broadley@notting hham.ac.uk

Data availability statement

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.

References

Courtman et al. (2012) analysed the concentration of maize grain and deduced factors that could influence the Se concentration of the grain. The information obtained from the analysis was used to compile a regional distribution map of the Se content of the maize. The findings of the study indicated that maize was a poor source of Se and this was attributed to soil type. Contradictory to other studies Courtman et al. (2012) established that it was not always the case that soil with high levels in Se would yield plants or crops with adequate Se content. This study was significant to the review as it was conducted in South Africa and gave descriptions of the Se concentration (50 μg Se/kg DM) of foods that was considered adequate from an animal and human nutritional point of view which became a reference point in the current paper. Courtman et al. (2012) concluded that the consumption of low Se grain could contribute to inadequate intake of Se in livestock and humans and subsequently cause Se deficiency. However, in light of his earlier finding that Se rich soil does not always yield Se rich plants, it can also be said that consumption of Se deficient maize may not translate to animal or human Se deficiency. Empirical evidence at population level would be required to establish this linkage.

Hurst et al. (2013) show that Se deficiency is likely endemic in Malawi based on the Se status of adults consuming food from contrasting soil types. They established that soil pH markedly affected dietary Se intake and biomarkers of Se status and that the Malawian population was at risk of dietary Se inadequacy. These results were consistent with seven other countries in Southern Africa, indicating the magnitude of the problem in that setting. The linkages shown in the research prompted and fed into the scope of the current review. The limitation of the study was in the use of indirect methods in establishing the Se content of foods which may not yield accurate estimates of actual Se intake because of the geographic variation in food Se contents not captured in food tables. Therefore, determination of food Se by actual analysis of representative samples is necessary to produce Se intake valid estimates. The authorship contributed to the literature of the current study.

Kuona et al. (2014) conducted a cross‐sectional study that assessed the prevalence of, and factors associated with stunting, thinness, overweight, underweight and Se deficiency in school aged children from Zimbabwe, a low‐income country. It attempted to make associations between nutritional status as measured by physical measurements and micronutrient status. Stunting and Se deficiency were prevalent in this cohort of children indicating the chronic nature of the emergence of Se deficiency in the human population. By considering three different socioeconomic settings, the study highlighted the dominant prevalence of Se deficiency in low‐income settings which is the case with most SSA countries hence the inclusion of the paper in the current review. Also, it was the only paper that could be found that reported on human Se deficiency in Zimbabwe which allowed for comparison with soil, crop and livestock Se studies conducted in the same country.

Ngigi et al. (2020) took a holistic approach in establishing the risk of Se deficiency in Kenya. They assessed Se status across the food chain of the local population and investigated the soil–food Se concentration and Se intake–individual Se status relationships from which they established that the vast majority of women and children had inadequate Se intake and were Se deficient. However, the limitation of the study was in the use of hair as a Se biomarker. Measurement of Se in hair may provide a useful indication of longer‐term exposure to Se, however it lacks widely used thresholds to define Se adequacy or deficiency and has a high risk of contamination with exogenous Se from hair products. Due to this the study was excluded from results and only cited in literature.

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