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Maize grain and soil surveys reveal suboptimal dietary selenium intake is widespread in Malawi

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This national survey measured selenium in maize grain and matching soils from 88 Malawian fields and used soil maps and population data to estimate how much selenium people obtain from maize. Grain selenium was very low at most sites, so the authors conclude that suboptimal selenium intake is widespread. Grain from high-pH Eutric Vertisol soils had up to 10-fold more selenium.

Key numbers

  • Median maize grain selenium across all 88 sites: 0.019 mg/kg dry weight (range 0.005 to 0.533) (Abstract).
  • 2009 survey of 73 sites: median 0.016 mg/kg (range 0.0045 to 0.533) (Results, p. 3).
  • 2010 Shire Valley Eutric Vertisol sites (soil pH 6.97 to 8.02): 0.173 to 0.413 mg/kg at 13 of 15 sites; two sites at 0.0054 mg/kg (Results, p. 4).
  • Total soil selenium: median 0.1623 mg/kg, mean 0.1941 mg/kg, range 0.0521 to 0.6195 mg/kg; phosphate-extractable selenium median 0.0046 mg/kg (Results, pp. 4 to 5).
  • Estimated median selenium intake from maize: 6.7 µg per person per day (range 1.6 to 189), assuming 0.354 kg maize per person per day (Abstract; Results p. 4).
  • Modeled distribution: 50% of the population below 6 µg/day, 90% below 7.5 µg/day from maize; non-maize sources estimated at 15 to 22 µg/day (Results, p. 5).

Methods (brief)

Composite samples of eight cobs and matching topsoil (0 to 0.15 m) were taken per field. Grain was digested in nitric acid and hydrogen peroxide and analyzed by ICP-MS with a wheat flour reference material; soils were analyzed for total and phosphate-extractable selenium. Intakes were extrapolated from median grain values per soil type and district population.

Evidence fitness

Supports staple-grain selenium concentrations and the role of soil pH in grain selenium in Malawi. Intakes are modeled from maize alone and are not measured total diets or biomarkers.

Limitations

Funding came primarily from Yara GmbH, a fertilizer company, and one author worked for Yara International; the authors disclose this as a competing interest. The intake model assumes soil properties drive grain selenium and that maize consumption is uniform within districts. Detection limits are not stated.

Update history

No substantive edit history is available in this build. The full commit record is available in git.

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