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

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This study measured selenium in duplicate diets, blood plasma and urine of women living in two Malawian areas with contrasting soils. Women in villages on acid soils ate about one eighth as much selenium as women near calcareous high-pH soils, and their plasma selenium was less than half as high. The authors also used food balance sheets to estimate that more than 80% of the Malawi population, and 22% of Africans overall, are at risk of inadequate selenium intake.

Key numbers

  • Duplicate-diet selenium intake, median: 6.5 µg/day (SD 9.4, range 1.1 to 62.3, n = 56) on acid soils versus 55.3 µg/day (SD 44.9, range 5.8 to 192, n = 58) on calcareous soils (Results, p. 2; Fig. 2).
  • Plasma selenium, median: 53.7 µg/L (range 32.3 to 78.4, n = 60) versus 117 µg/L (range 82.6 to 204, n = 60) (Results, p. 2).
  • Urine selenium, median: 7.3 µg/L (n = 59) versus 25.3 µg/L (n = 56) (Results, pp. 2 to 3).
  • Detection limits: plasma 0.055 µg/L (LOQ 0.182); urine 0.095 µg/L (LOQ 0.316) (Methods).
  • Estimated average requirement used for risk estimates: 45 µg/day for people over 10 years, 23 µg/day under 10, 49 µg/day in pregnancy, 59 µg/day in lactation (Fig. 4 legend).
  • Modeled mean selenium supply in Africa in 2007: 71 µg per person per day, from 27 (Djibouti) to 264 (Ghana) (Results, p. 3).

Methods (brief)

Women were recruited from six villages in each area. Duplicate diet composites were freeze-dried, digested in nitric acid and hydrogen peroxide and analyzed by ICP-MS, as were plasma and urine, with three serum reference materials recovering 102 to 109%. Plasma glutathione peroxidase activity was also measured. Continental risk estimates combine 2007 food balance sheets with regional food composition tables.

Evidence fitness

Supports measured dietary intake and biomarker contrasts tied to soil type in rural Malawi. The national and continental inadequacy estimates are modeled from food supply data and carry the uncertainty the authors flag for food composition tables.

Limitations

Women only; two areas; casual urine samples; food balance sheets describe supply, not consumption.

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