Studies of selenium geochemistry and distribution in relation to iodine deficiency disorders in Sri Lanka. (WC/98/028).
This report is intended for scientific and medical specialists, especially in Sri Lanka (and elsewhere in the world where iodine deficiency disorders and selenium deficiency are major problems).
This report outlines the results of a collaborative study undertaken by the British Geological Survey (BGS), the Dept. of Geology, University of Peradeniya, Sri Lanka and the Institute of Fundamental Studies, Sri Lanka (IFS), to investigate the relationship between human and environmental Se status and IDD in Sri Lanka. The current study was designed to address the knowledge gaps in terms of Se and iodine levels in the Sri Lankan environment and their relationship to goitre in the human population and to recommend possible remediation strategies.
During the present study, fifteen villages were selected for sampling on the basis of the incidence of goitre in each village as follows: five no/low IDD incidence (NIDD) (25% goitre).
This sampling strategy was designed to determine the chemical differences in the environment (demonstrated by soil, staple foodstuffs (rice) and drinking water samples) and the Se status of the human population (shown by hair samples) between villages of different IDD incidence. The villages were located in three regions, namely Anuradhapura in the Dry Zone, Kandy in the uplands of the Wet Zone and Kalutara in the coastal plain of the Wet Zone.
This study presents for the first time, information on levels of Se in soil, rice, water and hair samples and systematic data on the levels of iodine in soils and rice in Sri Lanka. Results show that soil iodine concentrations in the Wet and Dry Zones are similar, therefore the previous theory that iodine is washed out of soils in the Wet Zone by heavy rainfall is not borne out by the present study. Highest levels of iodine in soil occur in the Kalutara coastal region, an area where IDD is prevalent. Soils in the Kalutara region contain highest levels of organic matter and soil iodine concentrations in this region appear to correlate with organic matter content.
This study has shown for the first time that significant proportions of the Sri Lankan population may be Se deficient (34%, 22% and 28% in the HIDD, MIDD and NIDD villages respectively). Although it is not the case that Se deficiency only occurs in areas where IDD is prevalent, these results show that a combination of iodine and Se deficiency could be involved in the etiopathogenesis of goitre in Sri Lanka. Although no difference in Se status was detected between women suffering from goitre and those who were not, the highest hair Se values were determined in women not suffering from goitre. It is unlikely that Se deficiency is the main controlling factor in IDD but it could contribute to the onset of goitre along with iodine deficiency and other factors such as poor nutritional status in general and the presence of goitrogenic substances in the diet.
The results of the present study have shown that the distribution of red rice cultivation in Sri Lanka is coincident with high IDD incidence regions. No studies have been carried out into the goitrogenic properties of red rice therefore it is not possible to assess the impact of this potential source of goitrogens on IDD in Sri Lanka.
Levels of total Se and iodine in Sri Lankan soils are not low compared to other areas of the world where IDD do not occur. However, the results of this study have demonstrated the important role that geochemical factors, such as soil pH and organic matter and clay content, play in controlling the amount of Se and iodine available for plant uptake.
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