Geochemical dynamics and bioavailability of iodine and selenium

Ahmad, Saeed (2020) Geochemical dynamics and bioavailability of iodine and selenium. PhD thesis, University of Nottingham.

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Iodine (I) and selenium (Se) are vital dietary micronutrients for human and animal health. Approximately 35% of the world population is living at risk of I deficiency and more than half a billion people worldwide are thought to be facing Se deficiency. Historically, I deficiency has been reported in Gilgit-Baltistan (GB), Pakistan. The deficiencies of both I and Se are dietary and people of GB are largely consuming locally grown agricultural produce. Soil characteristics control the availability of I and Se to plants. Therefore, better understanding of I and Se geochemistry is required to enhance the dietary intake of these essential micronutrients via the food chain.

This study aimed to assess the factors controlling I and Se status in the population of GB. The objectives of this study were to: (i) to investigate the status of I and Se in soil, water and plants of GB; (ii) gain greater understanding of I and Se speciation in water and soil, and factors controlling their availability for plant uptake; (iii) assess the feasibility of Se biofortification in a wheat crop with the use isotopically enriched 77Se as a tracer and thereby determine the fate of residual 77Se; (iv) assess the nutrition status of I and Se of the local population with a survey of a human biomarker and use of iodised salt.

A survey was undertaken to collect soil (n = 26), water (n = 107) and plant (n = 281) samples from different districts of GB. Iodine and Se concentration in water ranged from 0 – 10.2 µg L-1 and 0.016-3.04 µg L-1 respectively with iodate (IO3-) and selenate (SeVI) being the dominant inorganic species. Average concentrations of I and Se in soil were 685 ± 247 µg kg-1 and 209 ± 96 µg kg-1 respectively, which are much lower than global mean values. Soil samples were alkaline but with a coarse texture and had low organic carbon; these properties have probably contributed to low retention of I and Se in soil. The potential availability of I and Se for plant uptake was assessed by measuring their reactive fractions (‘soluble’ and ‘adsorbed’) and speciation in these fractions. The reactive fractions of I and Se accounted for < 7% and < 5% of their total concentrations in soil respectively. Speciation analysis indicated that I and Se were present both in organic and inorganic forms in soil. More than 90% of the reactive I fractions (soluble and adsorbed) were present as organic species; the inorganic I (<10%) had a higher proportion of iodide (I-) compared to IO3-. Inorganic Se was largely present as selenite (SeIV), which is strongly adsorbed on soil particle surfaces and less available for plant uptake than SeVI. The low concentrations of I and Se in water and soil were also reflected in all plant samples. Typically, all plant samples had low concentrations of I and Se compared to similar species from other parts of the world. Based on a typical Pakistani food basket it was estimated that consumption of locally grown plants and drinking water would only provide 32% and 61% of the recommended daily allowances (RDA) of I (150 µg day-1) and Se (55 µg day-1) respectively.

Selenium biofortification of wheat crop was undertaken in a rotational crop system (wheat, maize, wheat) with a single application of isotopically enriched 77Se species (SeIV and SeVI) at three different levels (0, 10 and 20 g ha-1) at the crop’s early stem extension stage. With application of 10 and 20 g ha-1 SeIV, grain Se concentration in the first wheat crop was increased by 14 and 32 times while application of SeVI at the same rate as that of SeIV resulted in a 35 and 95 times increase compared to a Se concentration of only 1.42 µg kg-1 in grain from control plots. There was no added Se (77Se) detectable in the second crop (maize) or the third crop (wheat) indicating that fresh applications of Se would be necessary for each crop to adequately biofortify grain. Considering the average daily consumption of wheat, the grains obtained from control plots would provide <1% of Se RDA to an adult.

Iodine concentration in iodised salt was 8.85 (±10.4) µg kg-1, which is less than the minimum threshold of WHO recommended I concentration in iodised salt. The population average salt intake (13.0 ±5.64 µg kg-1 day-1) largely fell within the usual range for daily worldwide salt consumption however it was almost three times higher than the WHO recommended daily salt intake.

Average daily I and Se intake estimated from urinary I and Se concentration was found to indicate that the I and Se RDAs were not being met and that the GB population is living at risk of I and Se deficiency.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Young, SD
Bailey, EH
Subjects: Q Science > QP Physiology > QP501 Animal biochemistry
Faculties/Schools: UK Campuses > Faculty of Science > School of Biosciences
Item ID: 60480
Depositing User: Ahmad, Saeed
Date Deposited: 24 Jul 2020 04:40
Last Modified: 24 Jul 2020 04:40

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