BACKGROUND: Hydration status in firefighters is commonly assessed by changes in body mass during working shift and/or urine specific gravity. However, these measures may not provide actual hydration status as beginning of shift body mass cannot indicate if a subject is currently dehydrated and urine tests are only accurate for the first morning urine, which usually occurs prior to the work shift. Saliva osmolarity is a measure of hydration status that can now be tested onsite with a meter, which will allow firefighters to know if they are dehydrated when they first arrive on shift, a common occurrence. Therefore, the purpose of this study was to compare body mass and saliva osmolarity assessments of daily hydration status in recruit firefighters over a 6-week period. METHODS: 10 recruit firefighters attending an intensive 12-week Fire school had measures of salivary osmolarity via a meter (MX3 Hydration) and body mass measured at the beginning and end of each working shift for the first 6 weeks (a total of 27 days). Salivary osmolarity is a measure of the concentration of particles in the saliva, with a higher value indicating a more dehydrated state. Acute body mass changes (i.e. within the same day) typically reflect fluid changes, with a decrease in body mass indicating a more dehydrated state. Change scores (end of day - beginning of day) in osmolarity and body mass were computed for each day and then averaged for the 6-week period. The correlation between the average change in saliva osmolarity and average change in body mass was analyzed via bivariate correlation in SPSS version 29. RESULTS: The relationship between average change in salivary osmolarity (-1.14±7.64 mOsm) and average change in body mass -0.39±1.11 was positive, moderate, and not significant (r(10)=.349, p=.324). CONCLUSION: Change in body mass and change in salivary osmolarity methods of measuring hydration status produce results which are not related to one another leading to different assessments of, and directions for achieving, hydration. The two measurements were expected to be inversely related because a decreased body mass should correspond to an increase in salivary osmolarity (i.e. reduced hydration status). The moderate positive relationship observed emphasizes the need to implement sophisticated and valid methods to test hydration status. Data collection is ongoing to further investigate this relationship.

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