Portable X-ray fluorescence of zinc applied to human toenail clippings

Could toenail zinc content assessed by portable X-ray fluorescence be a field friendly biomarker of zinc status? Professor David Fleming of Mount Allison University describes where the science is at.

Zinc deficiency is a widespread problem which has been associated with a variety of serious health effects. Diagnosing zinc deficiency at the individual level is, unfortunately, a somewhat thorny issue. Currently, there is no single method of determining zinc status which is both simple and reliable. The best option is probably the measurement of plasma (or alternatively serum) zinc concentration [1,2]. This approach, however, requires a sequence of non-trivial steps pertaining to the sample acquisition, storage, and analysis (usually at a distant lab). Other reliable or potentially useful biomarkers of zinc status have been noted, including zinc concentration in urine and hair [1,2]. These approaches also have drawbacks.

Zinc concentration in nail was recently described as an “emerging biomarker” by the Biomarkers of Nutrition for Development (BOND) zinc expert panel [2]. This designation indicates that nail zinc concentration has a theoretical association with zinc status, but requires further study. This need for additional development was reinforced in a Technical Brief from IZiNCG, which provided an overview of assessing zinc exposure using hair or nail zinc [3]. 

Toenail clippings ready for measurement by portable X-ray fluorescence (Photo: David Fleming)

We recently conducted a study involving the measurement of zinc concentration in toenail clippings using two different methods: portable X-ray fluorescence (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS) [4]. Of the two methods, portable XRF is a relatively new approach to zinc biomarker analysis, while ICP-MS may be considered a “gold standard”. We measured single toenail clippings from 60 different individuals living in Atlantic Canada (the population was drawn from the Atlantic PATH cohort [5]). The clippings were measured first by portable XRF, a non-destructive technique, and then by ICP-MS. The population average zinc concentration was found to be 85 µg/g from ICP-MS. The portable XRF technique was very sensitive to detecting zinc in the clippings and provided a reasonable estimate of zinc concentration. Using the XRF output spectrum to determine a normalized zinc signal, we found a correlation coefficient r = 0.68 between the XRF results and the ICP-MS zinc concentrations. Looking forward, correlation between the two methods might be improved by measuring multiple points on each clipping when using XRF – in our study, only a single point on each clipping was assessed by XRF.

Single toenail clipping in portable X-ray fluorescence instrument (Photo: David Fleming)

Single toenail clipping (Photo: David Fleming)

The portable XRF approach presents a number of potential advantages for the analysis of zinc concentration in nail clippings. Measurements could be made in a field setting and results provided quickly. Sample preparation is minimal, operating conditions are simple, and the technique is relatively inexpensive. If the correlation of XRF results with ICP-MS concentrations could be improved, portable XRF might therefore be an especially attractive approach for assessing zinc concentration in nail clippings. The other critical issue here, of course, is whether zinc concentration in nail is truly representative of an individual’s zinc status. That remains an open question which will require additional study to resolve. If zinc concentration in nail is eventually upgraded from “emerging biomarker” to “useful biomarker”, we anticipate that portable XRF will receive a great deal of attention and become a potentially important component of public health initiatives targeting zinc deficiency.

References

[1] N.M. Lowe, K. Fekete, T. Decsi, Methods of assessment of zinc status in humans: a systematic review, Am. J. Clin. Nutr. 89 (2009) 2040S-2051S.

[2] J.C. King, K.H. Brown, R.S. Gibson, N.F. Krebs, N.M. Lowe, J.H. Siekmann, D.J. Raiten, Biomarkers of nutrition for development (BOND) – zinc review, J. Nutr. 146 (2016) 858S-885S.

[3] International Zinc Nutrition Consultative Group (IZiNCG) Technical Brief No. 8, Assessing population zinc exposure with hair or nail zinc (2018), https://www.izincg.org/technical-briefs

[4] D.E.B. Fleming, S.L. Crook, C.T. Evans, M.N. Nader, M. Atia, J.M.T. Hicks, E. Sweeney, C.R. McFarlane, J.S. Kim, E. Keltie, A. Adisesh, Portable X-ray fluorescence of zinc applied to human toenail clippings, J. Trace Elem. Med. Biol. 62 (2020) 126603.

[5] E. Sweeney, Y. Cui, V. DeClercq, P. Devichand, C. Forbes, S. Grandy, J.M.T. Hicks, M. Keats, L. Parker, D. Thompson, M. Volodarsky, Z.M. Yu, T.J.B. Dummer, Cohort profile: the Atlantic partnership for tomorrow’s health (Atlantic PATH) study, Int. J. Epidemiol. 46 (2017) 1762-1763i.

OpeN-Global: More and better data.

February 27, 2019: OpeN-Global, live online

OpeN-Global is a freely available, online resource aimed at supporting the objective, accurate and detailed assessment of nutritional biomarkers from populations globally. In their guest blog for IZiNCG, Drs Jessica Farebrother and Sophie Moore share what OpeN-Global is all about.

OpeN-Global is specifically designed to support and enable work in populations from low- and middle-income countries (LMICs), but the analytical methods presented can be applied to samples from any population group. OpeN-Global additionally provides a network of experts to support the implementation of biomarker assays in laboratories globally, and to help interpret the data obtained. OpeN-Global is an open-access website that is free for all to use.

So what’s it all about?

Malnutrition, from either under- or over-nutrition, affects one in three people globally. This makes malnutrition, together with diet, the biggest risk factors for the global burden of disease - by far.

The recent 2018 Global Nutrition Report stated: “Micronutrient deficiencies are estimated to impact a significant number of people around the world, but there remains far too little information on micronutrient status and deficiencies. More essential information and surveillance need to be gathered to make substantial progress on global targets.”

What’s basically needed, are more and better data.

One of the key barriers to the development of both nutrition-specific and nutrition-sensitive programmes and policies, particularly in LMICs, is the lack of robust evidence on nutritional status in the most vulnerable groups.

Though many health and nutrition related activities in LMICs are underpinned by well-designed national surveys (e.g. the UNICEF Multiple Indicator Cluster Survey), data from the most vulnerable population groups, i.e. infants, children, adolescents, pregnant and lactating women, are often limited to anthropometry and easy-to-assay parameters e.g. haemoglobin, or single micronutrients e.g. vitamin A, iodine.

Other available information may be based on prevalence data, e.g. stunting in under 5’s to assess population risk of zinc deficiency, or household coverage of adequately iodised salt in salt fortification monitoring.

Whilst these proxy measures remain recommended strategies for population nutrition monitoring, a more accurate picture of population nutrient status can be given by biochemical data using human biomarkers.

Stunting prevalence (data) vs Plasma zinc concentration?

Household coverage of iodised salt (salt sample) vs Urinary iodine concentration?

These biochemical assays provide a real picture of nutrient status. Such assays could feature as routine, however are rarely included in demographic and health surveys in LMICs.

What are the barriers to biochemical assessment in population nutrition surveillance?

With the help of our global partner network including leading scientists based in LMICs, we uncovered some of the key barriers to assessment of biochemical samples in LMIC nutrition surveys. And funding was not the only barrier! Our kick-off workshop highlighted several key themes, including:

1. A clear need for guidance and technical support to facilitate the laboratory capabilities in many LMIC settings to support the assessment of a wider panel of nutritional biomarkers than is currently available and routinely used;

2. Excellent research infrastructure and laboratory platforms are in place in many LMIC settings, but several barriers, including staff training opportunities and technical know-how prevents full utilisation.

Support in these key areas would start a positive cycle of quality control and assurance technical accreditation, investment and increased trust – leading to better outputs. More and better data.

Supporting the generation of quality data in-country will support evidence-based policy, and transform nutritional outcomes.

Our response: OpeN-Global

We have established a global network of expertise in nutritional research methodologies. Our 37-strong global partner network of committed academic, technology and industry experts spans 20 institutions across 10 countries on 5 continents.

Through this partnership, we have created OpeN-Global a hub of resource tools to assess nutritional status in global health settings in the form of an open-access website that provides laboratory SOPs to download or signposting to published nutrition assay methods, details on quality control and accreditation, technical support, and fully-referenced general information for over 20 nutrition biomarkers, including key nutrients such as zinc and iodine, to newer technologies and -omics approaches.

And the list is growing.

Our aim: to support the objective, detailed, accurate and high-quality assessment of nutrition biomarkers used in Global Health population surveys and research.

For more information, please contact the project lead, Dr Sophie Moore, Department of Women and Children’s Health, King’s College London on sophie.moore@kcl.ac.uk, or head to www.open-global.kcl.ac.uk from February 27th, and contact us directly from the website.

Al images taken at Medical Research Council The Gambia. Photo credits: Ian Farrell.

Written by Jessica Farebrother and Sophie Moore.

Footnotes:

1 Prevalence of stunting in children under 5 years is a recommended strategy to assess population risk of zinc deficiency. Recommendations of Biomarkers of Nutrition for Development – Zinc, and IZiNCG

2 Assessment of household coverage of iodised salt is a recommended strategy in monitoring and surveillance of relevant salt fortification strategies. Biomarkers of Nutrition for Development - Iodine, and the Iodine Global Network (www.ign.org).