Based on both epidemiological data and clinical trials, selenium deficiency is increasingly linked to the development of certain cancers. Variation in the soil concentrations of selenium can determine the selenium intakes of the local population, and certain areas of China and New Zealand are known to have low selenium intakes because of the low selenium soils used for crop production. Therefore an accurate marker of selenium status is important in order to assess the selenium status of a given population. In addition, administration of selenium in clinical trials requires assessment of the selenium intake of the subjects in order to related dietary changes to the selenium status. Short term assessment of selenium status is possible using plasma samples that can measure the selenoprotein P and glutathione peroxidase levels in blood cells. However, toenail clipping can be used as longer term marker because weeks or months are required to add minerals to the toe nail ends from the beds.
Researchers have assessed the use of toenail clippings from subjects to measure long term selenium status. In one study1, 12 male subjects were fed either a high dose (4.91 μmol) a medium dose (2.61 μmol) or a low dose (0.41 μmol) of selenium per day in whole wheat bread for 1 year, while selenium blood status was assessed every 12 weeks for 2 years. Blood selenium started to rise after 2 weeks on the high and medium dose bread and continued to rise until week 24 where it remained constant throughout the study before falling 12 weeks after the end of supplementation. For the first 3 months selenium supplementation had no effects on toenail selenium status but after this time selenium concentrations in toenail clippings started to rise and continued to do so until 1 year. By 1 year after the end of the study, toenail selenium had not returned to baseline. The low dose bread did not cause changes in blood levels of selenium or toenail selenium suggesting it was too low to cause accumulation of body selenium stores.
These data show that toe nail clippings reflect the long term selenium intakes and show that levels are unaffected by recent dietary changes in selenium intake. The growth rate of the toenails is around 0.056 mm/d and the largest nail is 10 mm. No increase in toenail selenium occurred in the first three months, but appeared to be an accurate measure of selenium status from between 26 and 52 weeks, reflecting the slow growth rates from the nail beds. Selenium status may therefore be assessed in the long term using toenail selenium concentrations, a suggestion that is supported by other research. The selenium concentration of toenails may be useful in retrospective studies whereby a long term picture of selenium status is required, such as at childbirth to assess the selenium exposure of the foetus from the mother. Combining toenail selenium with other tissue markers of selenium (such as blood glutathione peroxidase or selenoprotein P) may allow improved estimations of selenium intakes.
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