ABILITY OF NAIL TRACE ELEMENT TO PREDICT CHRONIC DISEASES
The measurements of trace elements in nails have been used in clinical research for decades. For some hypotheses, nail measurements have been demonstrated to be useful in the study of trace element status as it relates to chronic diseases including cancer and CVD. For example, toenail Se level provides a time-integrated measure that is superior to other biomarkers in assessing Se status.8, 11, 12 A number of studies investigated nail Se in relation to cancer risk. A prospective cohort study examined toenail Se status and the risk of lung cancer,13 and the results indicated an inverse association between Se status and lung cancer and suggested a modification of the effect of Se by the antioxidants β-carotene and vitamin C. The findings were supported by a nested case-control study in men,14 but not another nested case-control study in women.15 Nevertheless, a meta-analysis quantitatively analyzed Se and lung cancer and reported an overall inverse association between toenail Se and lung cancer.16 The authors found that the evidence for the potential protective effect was greater in studies of toenail Se than in studies involving other measures of Se status. Studies also examined toenail Se in relation to risk of prostate cancer. The results were inconsistent and inconclusive based on the current evidence.17–23Some studies have related toenail Se to other cancers.24–26 In addition, nail specimens were used as biomarkers for other trace elements in cancer studies. For example, a case-control study examined iron (Fe), zinc (Zn), calcium (Ca), chromium (Cr) and cobalt (Co) in toenails in relation to cancer of upper aerodigestive tract.27 Although the evidence was weak, the study suggested that there might be differences in those mineral intake or metabolism between individuals who developed some carcinomas of the upper aerodigestive tract and those who did not. Another case-control study investigated toenail cadmium (Cd) and risk of prostate cancer and suggested that Cd exposure increased prostate cancer risk.28 A recent case-control study also found an inverse associated between toenail Zn level and gastric cancer risk.29
Associations between nail trace elements and CVD risk have been reported. Studies indicate that toenail Se concentrations predict risk of myocardial infarction (MI).30, 31 Also, an inverse association between toenail Cr levels and coronary heart disease (CHD) is observed among diabetics32 and non-diabetics.33In spite of sparse data, toenail Sc levels are suggested to predict acute MI.34 Although the findings are inconsistent, studies indicate toenail Mercury (Hg) levels predict risk of MI.31 In addition, studies find that toenail As concentrations are a reliable, long-term biomarker of total Arsenic exposure even for quantifying low level of As exposure.35–38 In fact, studies indicate nail As levels predict blackfoot disease,39 which is a unique peripheral vascular disease that is largely due to arteriosclerosis.