What can nails and saliva testify? Chemists at St Petersburg University investigate methods for studying human biomaterials

What do nails testify?

Based on the results of determining macro- and micronutrients in the human body, conclusions can be drawn about: their professional characteristics and state of health; and the impact of the environmental situation in the area of their residence on the body. Blood is most often used for such analyses. However, other biomaterials, such as nails and saliva, have a number of advantages and are also able to tell a lot about a person.

So, nails, like hair, record the composition and ratio of substances and trace elements accumulated during their growth phase. Like bioassays but unlike blood, they are easy to transport and can be stored for a long time without special conditions. Additionally, samples can be collected in a non-invasive way, that is, without penetrating the human body.

The donors for the experiment on studying the composition of nails were 73 residents of St Petersburg aged 9 to 68. Some of them gave samples repeatedly over the course of 6–10 months, so that scientists could determine the dynamics of changes in their trace element composition.

‘The main analysis method used in the study is atomic emission spectrometry with an arc discharge. It has a relatively low cost compared to other multi-element methods. At the first stage, the sample digestion scheme (the decomposition of a sample into liquid form) and the spectrum registration conditions were optimised. Then, we tested validity. And only at the last stage did we start to determine the composition of nail samples of various donors with subsequent statistical processing of the obtained results,’ says Sergey Savinov, one of the authors of the study, Senior Lecturer at the Department of Analytical Chemistry at St Petersburg University, Candidate of Chemistry.

The experiment lasted about two years. During this time, we managed to optimise the analysis scheme and to determine the composition of elements in the samples of donor nails.

Sergey Savinov, one of the authors of the study, Senior Lecturer at the Department of Analytical Chemistry at St Petersburg University, Candidate of Chemistry

During the study, the researchers managed to specify the list of elements that are found in the nails: aluminium, silver, copper, manganese, nickel, iron, calcium, magnesium, zinc, cadmium, chromium, and lead. However, due to their low concentrations in the nails and not very low detection limits, the scientists were not able to detect a complete set in all samples. For donors, whose samples were monitored for several months, the relative constancy of the trace element composition of nails was registered. 

There was a short-term increase in the cadmium content of one donor, which in time coincided with a short-term change in location. Thus, the possibility of using nails as a source of information about past exposure has been confirmed.

Sergey Savinov, one of the authors of the study, Senior Lecturer at the Department of Analytical Chemistry at St Petersburg University, Candidate of Chemistry

‘Additionally, the influence of the donors’ sex on the content of copper in the nails was found. Also, smoking turned out to influence the content of lead. In the latter case, the statistical significance is still insufficient.  This might be due to the small number of smoking donors and the inability to determine the concentration in all samples in the experiment,’ says Sergey Savinov.

He also added that there is information in the literature that the content of nickel and cadmium differ in the nails of smoking and non-smoking donors, and the content of cadmium and lead are higher in samples of urban residents than of those who live in rural areas. Interestingly, a high concentration of copper and zinc was registered in the nails of vegetarians.

The scientists also identified certain correlations between the content of certain elements. For example, magnesium and calcium, copper and zinc, copper and aluminium, copper and magnesium, as well as cadmium and lead are positively correlated. Calcium and cadmium are negatively correlated. As Sergey Savinov noted, the significance of these correlations remains to be determined in future research.

‘Only a more major study with the analysis of supplementary influence factors and the involvement of specialists from other areas will make it possible to say what exactly these correlations mean. If speak about our work, it was focused on studying the characteristics of nails as bioassays in determining the elements in them. And it is already clear that these characteristics exist and they need to be taken into account: for example, when comparing data obtained by various researchers,’ explained Sergey Savinov.

Secrets of saliva

Many factors can influence the results of saliva analysis: from the human circadian rhythm to bad habits. However, it is important for scientists and doctors to get the most ‘clean’ samples. St Petersburg chemists have been trying to find out how to achieve this for several years, and this experiment is only one of the stages of a large-scale study. The objective of this work is to understand how the conditions for saliva sampling affect the results of determining its macro- and micronutrient composition.

The research findings have been published in the Journal of Analytical Chemistry: Determination of the Trace Element Composition of Human Nails; and  Effect of Conditions for Sampling of Human Saliva on the Results of Determination of Macro- and Micronutrients.

‘At present, the “traditional” biofluid in determining the trace element composition for the human body is blood. However, the benefits of saliva are obvious. This is primarily a non-invasive sampling. Additionally, the sampling process is quite simple. It is easy to frequently take samples. Academic papers have shown that: the content of some elements in saliva correlates to their content in the blood; there is a connection between the concentrations of trace elements in saliva and various diseases; and these concentrations are also associated with the environmental situation in the region of residence. However, researchers have different approaches to the sampling processes, their storage and subsequent sample preparation. As a result, the validity of the results obtained and the correctness of their comparison remain to be seen. These are the issues that our published work is devoted to,’ stated Sergey Savinov, one of the authors of the study, Senior Lecturer at the Department of Analytical Chemistry at St Petersburg University, Candidate of Chemistry.

Like in the experiment with nail samples, the main method for the analysis of saliva used in the work was atomic emission spectrometry with an arc discharge. ‘In addition to the above advantages, in the case of saliva analysis, there is one more thing. It is the possibility of direct analysis of the selected sample without special manipulations. This, in particular, made it possible to establish the influence of sample preparation conditions on the analysis results,’ emphasises Sergey Savinov.

While studying each individual factor, samples were taken from a different number of donors: from four to ten people. ‘Of course, this sample is not sufficient. However, it already makes it possible to identify certain trends,’ says the scientist.

So, to identify the effect of circadian rhythm on the content of elements in saliva, samples were taken from five donors during the day from 10am to 6pm with a two-hour interval. It has been found out that in order to minimise the influence of this factor on the analysis results, it is better to collect samples in the afternoon (after 3pm), since at this time the composition of trace elements reaches a constant value. Smoking turns out not to significantly affect the results of the saliva analysis. However, smokers show an increased content of lead, cadmium and other metals not only in the nails, but also in saliva.

The research can make it possible to develop general approaches to sampling, processing and analysing non-invasive bioassays for the most accurate determination of the micro- and macronutrient composition of saliva and nails. ‘A large number of factors influence the content of trace elements in biological samples. Among them are: the sex of donors; their age; the region of residence; diet; the presence of bad habits; some diseases; and their areas of activities. These features are manifested in different ways both for various elements and for various samples. It is important therefore to establish these influence factors in order to be able to correctly compare the analysis results for various donors and identify possible pathologies,’ concludes Sergey Savinov.