28.7: Nail zinc (24c.7)

Fewer studies have examined nail zinc as a biomarker of zinc exposure than hair zinc even though the validity of nail selenium as a biomarker of selenium exposure in adults has been confirmed by several reports (Hunter et al., 1990). Nails, like hair, also incorporate zinc into their matrix when they are exposed to the blood supply within the germinal layer of the nail matrix, reflecting the quantity of zinc available in the blood supply at the time of the nail synthesis (He, 2011). The normal rate of growth for nails ranges from 1.6mm per month for toenails to about 3.5mm per month for fingernails (He, 2011). In cases where nail growth is arrested, as may occur in onychophagia (com­pulsive nail biting), nail zinc should not be used (He, 2011).

In healthy individuals not exposed to high levels of zinc in the atmosphere, nail zinc levels are similar to those in hair, with concen­trations ranging from 80 to 200µg/g (1.22 to 3.06µmol/g). For example, in German school children aged 3–7years, the average toenail zinc concen­tration was about 130µg/g (1.99µmol/g) (Wilhelm et al., 1991), although nail zinc concen­trations as high as 2000µg/g (30.6µmol/g)have been reported in galvanizers with a high exposure to zinc in the atmosphere (King et al., 2015). In a recent study of young children in Laos, nail zinc concen­trations were higher at endline in those children receiving a daily preventive zinc supple­ment (7–10mg Zn/d) for 32–40 weeks com­pared to those being given only a therapeutic zinc dose (20g) for 10 days (geometric mean, 95%CI) (115.8, 111.6–119.9 vs. 110.4, 106–104.8µg/g.; p=0.055)(Wessells et al., 2014). Nail zinc concen­trations can also be used as a longer-term retrospective measure of zinc exposure in case-control studies. For example, in a prospective study of US urban adults, toenail zinc was assessed in relation to the incidence of diabetes but no significant longitudinal association was found (Park et al., 2016).

Nails are exposed to exogenous surface contaminants so all specimens must always be washed using a recom­men­ded procedure. Chemicals introduced by nail polish can be removed by washing (He, 2011). Biological factors also affect nail zinc concen­trations and include age, possibly sex, rate of growth, and onychophagia (com­pulsive nail biting).

The BOND Zinc Expert Panel classified nail zinc as an “emerging” biomarker with a theoretical association with zinc intake or status, although the sensitivity and specificity to any changes in zinc nutrition need further study (King et al., 2015). No universally accepted reference values exist for fingernail or toenail zinc concen­trations exist and the reader is referred to Mikulewicz et al. (2013) for guidelines on their com­pilation.

24c.7.1 Measurement of nail zinc

Nails from all fingers or toes (at least 50mg) should be cleaned before clipping, and then stored in labeled T‑E free polyethylene bags prior to washing. Bank et al. (1981) recom­mend using an aqueous nonionic detergent, followed by vacuum drying for traditional analytical techniques. For non-destructive analytical methods such as INAA and the newer technique involving laser-induced breakdown spectroscopy (LIBS), cleaning fingernail clippings with acetone (analytical grade) in an ultrasonic bath for 10 minutes followed by drying in air for 20–30 minutes is recom­men­ded (Riberdy et al., 2017). All dried specimens should be stored in a desiccator after weighing, prior to analysis. The wet and dry ashing methods and the analytical techniques outlined above for hair zinc concen­trations can also be used for nail zinc. Alternatively, tetramethylammonium hydroxide (TMAH) can be used to solubilize nails at room temperature (Batista et al., 2008).

Laser-induced breakdown spectroscopy (LIBS) can be used and field-portable hand-held instruments operated from a battery have been developed. Investigations for measuring fingernail zinc in situ are underway (Riberdy et al., 2017). Preliminary results suggest that the in situmeasure­ment of fingernail zinc by LIBS has potential as a non-invasive, convenient screening tool for identifying zinc defi­ciency in popu­lations but may lack the precision required to generate absolute concen­trations to charac­terize individuals (Riberdy et al., 2017). Research is ongoing to enhance the precision of this in situ method, and confirm its sensitivity and specificity to changes in zinc nutrition.

Unlike for hair, no Certified Reference Material exists for nail trace element analysis. Instead, in-house controls prepared from homogenous samples of powdered fingernails and toenails should be prepared, digested if necessary, and spiked with several differ­ent known quantities of zinc and the recoveries measured. Alternatively, an aliquot of the in-house control can be sent to an external quality laboratory and the results com­pared.