Characterizing Variability in Non-Invasive Hydration Monitoring Using Raman Spectroscopy

Abstract

Significant dehydration can increase thermoregulatory and cardiovascular strain and impair physical and cognitive performance. Despite these negative effects, there are currently no objective, non-invasive tools to monitor systemic hydration. Raman spectroscopy is an optical modality with the potential to fill this gap because it is sensitive to water, provides results quickly, and can be applied non-invasively. In this work, high wavenumber Raman spectroscopy has been developed toward detection of systemic hydration via validation with tissue-mimicking phantoms, followed by three in vivo feasibility studies to investigate the relationship between spectral features and systemic hydration. The area under the curve (AUC) of the water bands and the ratio of water bands to CH bands are Raman-derived metrics that can be used to describe systemic hydration. Here, we determined a trend in decreasing water bands AUC after exercise, although the magnitude of the change was highly variable. In investigating the sources of variability, we identified significant inter-subject variability and a failure of current clinical standards to benchmark our developed technique against. Despite the high variability, we found that multiple anatomical locations were suitable for collecting the spectral measurements. While the high degree of variability may confound the use of Raman spectroscopy for non-invasive hydration monitoring, when implementing additional study standardization, significant differences (p <.05) in spectral metrics can be identified before and after exercise. Raman spectroscopy can allow for rapid, non-invasive detection of systemic hydration, which would improve routine hydration monitoring and reduce the incidence of negative side effects associated with dehydration.

Keywords: Raman spectroscopy; dehydration; hydration; in vivo Raman spectroscopy.

Graphical abstract

This is a visual representation of the abstract.

Figure 1.

(a) HW spectra of gelatin phantoms with water content from 65 to 90%. (b) The water AUC (mean ± SD) for each water concentration showing a significant increase in AUC as water content increases. (c) Water-to-CH ratio (mean ± SD) showing a significant increase in ratio as water content increases (****p < .0001).

Figure 2.

Percent change of the water AUC for each location demonstrating a general trend of negative percent change values, indicating a trend in decreasing water AUC after exercise, but high degrees of variability in magnitude are seen.

Figure 3.

Violin plot of the percent change of the water AUC for the four locations measured and the range and SD of the percent change values for the four locations.

Figure 4.

Violin plots of the percent change of the water-to-CH ratio (left) and water AUC (right) and the range and SD of percent change values showing the effect of increasing exercise standardization from Phases 1 to 3. (P1, unstandardized exercise; P2, standardized moderate exercise; P3, standardized intense exercise).

Figure 5.

Scatter plots of the water AUC and uSG percent change (a) and weight loss (WL) percent change (b) with the Pearson correlation coefficient showing weak or no correlation between the metrics. (c) Scatter plot of uSG percent change and WL percent change with the corresponding Pearson correlation coefficient showing weak correlation.

Figure 6.

(a) Clinical standards separated by sex highlighting the significant difference in distribution between the weight loss (WL) percentage of males and females. (b) Water AUC grouped by sex displaying a similar distribution as the percent WL for each sex. (c) Pearson correlation coefficient calculated for the total group (left side) and when data is separated by sex (right side), showing an increase in magnitude in correlation when grouped by sex (F-test significance: ***p < .001).

Figure 7.

(a) Violin plots of repetitive measurements for P2–P5 showing a high degree of variability between repetitive measurements. (b) Analysis of variability results shows significant inter-trial variability. (c) Bar plots (mean ± SD) representing the trend in decreasing water AUC after exercise.