Soil-type influences human selenium status and underlies widespread selenium deficiency risks in Malawi

Abstract

Selenium (Se) is an essential human micronutrient with critical roles in immune functioning and antioxidant defence. Estimates of dietary Se intakes and status are scarce for Africa although crop surveys indicate deficiency is probably widespread in Malawi. Here we show that Se deficiency is likely endemic in Malawi based on the Se status of adults consuming food from contrasting soil types. These data are consistent with food balance sheets and composition tables revealing that >80% of the Malawi population is at risk of dietary Se inadequacy. Risk of dietary Se inadequacy is >60% in seven other countries in Southern Africa, and 22% across Africa as a whole. Given that most Malawi soils cannot supply sufficient Se to crops for adequate human nutrition, the cost and benefits of interventions to alleviate Se deficiency should be determined; for example, Se-enriched nitrogen fertilisers could be adopted as in Finland.

Figure 1. Soil-to-maize grain Se transfer.

(a, b) Most Malawi soils (grey symbols) have low soil-to-maize Se transfer linked to soil type including a low soil pH, in contrast to Eutric Vertisols (open symbols). In (a) boxes represent 25 and 75%-iles with median lines; whiskers are 5 and 95%-iles; circles are outliers. Inset in (a), pie chart shows that Eutric Vertisols comprise <0.5% of all soil types in Malawi. Data redrawn from Chilimba et al. (2011; ref. 10).

Figure 2. Dietary Se intake and biomarkers of Se status in adult females in Malawi.

(a) Dietary Se intake, (b) blood plasma Se concentration, (c) urine Se concentration and (d) blood plasma glutathione peroxidase 3 (GPx3) activity. Grey bars represent six villages from Zombwe Extension Planning Area (EPA) with low pH soils; open bars represent six villages from Mikalango EPA with high pH soils. Insets show these villages grouped by EPA. Boxes are 25 and 75%-iles with median lines; whiskers are 5 and 95%-iles; circles are outliers.

Figure 3. Selenium biomarkers as functions of dietary Se intake among adult females in Malawi.

(a) Blood plasma Se concentration and (b) urine Se concentration. Grey circles are adults from villages with low pH soils; open circles are adults from villages with high pH soils. Regression lines are (a) y = 39.9 + (92.6*(1 − (3.2*10⁻¹⁵)^x)), r² = 0.648, P < 0.0001; (b) y = 7.8 + (318*x), r² = 0.5707, P < 0.0001.

Figure 4. Mapping dietary Se availability in Africa.

(a) Mean dietary Se availability and (b) estimated risk of inadequate Se intake based on US Estimated Average Requirement (EAR) of 45 and 23 μg Se person⁻¹ d⁻¹ for those aged >10 and <10 yr, respectively, and 49 and 59 μg Se person⁻¹ d⁻¹ for pregnant and lactating women, respectively. Data are calculated from national-level food balance sheets for 46 African countries in 2007 and regional food composition tables using methods described previously.