The present study represents the unique exploration of the HS on aeronautical military personnel in Spain, as well as the association of this variable with body composition and anxiety status.
The main findings of our study indicate that TWI was adequate for the majority of the population, in accordance with recommendations established by WHO (3.7 L/day) , the National Health and Medical Research Council (3.4 L/day from males)  and the Institute of Medicine of the United States of America (IOM) suggest 3.7 L/day for males . Nevertheless, they consumed approximately 40% more of TWI than the EFSA WI reference values for men (2,5 L/day)  and the Spanish Society of Community Nutrition (SENC) recommendations . Many studies take into account only the water from beverages but no water intake from all sources (beverages plus food water intake) [39,40]. Previous studies in general European population found that TWI ranges 2310 mL/day in Ireland to 3254 mL/day in Greece [41-45]; and findings in the Spanish population (ANIBES Study) showed intakes of 1625 mL/day , much lower than our results. Worldwide, TWI ranges from 2230 mL/day in Japan to 3563 mL/day in the USA [46,47]. Deviation in water intake may reflect country differences in dietary habits, lifestyle choices, and environmental conditions; but also differences in the selection of method used to evaluate total water intake .
On the other hand, mean daily WE was 3402.4 mL/day for the whole sample. WE data were in accordance to current literature, which is 1500-3100 mL/day for adults [13,15]. In addition, the WB was positive for the total sample. WI is usually expressed in absolute units (L/day) even though a given L/day intake of water can be expected to have different effects on the hydration status of small versus large individuals . Nevertheless, recent NHANES studies suggest that not accounting for body weight to express WI (mL/kg of weight), may introduce confounding and/or effect modification into hydration studies [48-50]. In addition, the Spanish Guidelines of Hydration suggest that the standard requirements of WI are 30-35 mL/kg of weight , data that the present studied sample accomplished.
Laboratory data for urine (USG, urine color, osmolality) are considered good markers to evaluate HS . In the present study urine was measured via color chart in an afternoon urine spot, since USG findings of Bottin et al.  suggest that an afternoon sample may be an accurate and practical tool for hydration monitoring.
Blood parameters (hematocrit and hemoglobin) were analyzed due to their potential as HS markers ; however, no correlation was observed with any other HS variables in the present study. Francesconi et al.  investigated military personnel over field training and reported that even when the subjects had lost more than 3% of their body mass an had a high urine specific gravity, there was no change in hematocrit or serum osmolality measurements. They concluded that plasma volume is defended by the body in an attempt to maintain cardiovascular stability, and therefore plasma variables are not affected by mild hypohydration until a certain degree of body water loss (at least 3% of body mass loss). Similar findings were reported by Armstrong et al. .
In our population, 81% of the participants met the established hydration criteria and 19% did not meet the hydration criteria. A US military study showed that 31% of the total sample met the criteria for hypohydration (using USG of first urine of the morning) . Malisova et al.  concluded that 60% were euhydrated, 20% hyperhydrated and another 20% dehydrated on average over a seven-day period (using cutoffs of 24 h urine osmolality) in an adult free-living population. Furthermore, they observed that euhydrated subjects had a higher total water intake and water intake from beverages, but lower specific gravity and lighter color. These results are mostly in agreement with our study.
Regarding to anthropometric parameters, BMI suggests overweight values in the total sample. Conversely, the % body fat was considered in the range of healthy values. Several studies observed similar results, and the inability to distinguish the different contributions to body weight, fat and non-fat tissue, which explains why the BMI might overestimate adiposity in muscular and lean body in active people [54,55]. Body water content is an accepted indicator of HS . In this study, this parameter was assessed though BIA, and showed normal values . Although no significance differences were observed according to HS, subjects that did not meet the hydration criteria had higher weight, BMI, body fat (%) and waist/hip ratio than subjects that met the hydration criteria. In addition, total water intake per kg of weight is positively correlated with percentage of body water and negatively with body fat (kg), percentage of body fat and waist/hip ratio for the total sample, demonstrating a relation between hydration status and body composition; following data were noticed in recent studies . Likewise, linear and logistic binary regression analysis showed that percentage of body fat and percentage of body water were independent predictors of water intake normalized by body weight in the study population, as well as the percentage of body fat was independent predictor of Hydration Status. These findings suggest that an adequate water intake could improve body composition. Nevertheless age, co-morbidity and medication was not found to be a significant predictor of water intake normalized by body weight and of HS in the linear and logistic regression analysis. Thus, in addition, body composition parameters should be taken into account in hydration status monitoring.
In our study, subjects that did not meet the hydration criteria had significantly higher scores, in STAI-State, than subjects that met the hydration criteria, indicating a possible relationship between these two variables that should be carefully further evaluated. However, in other studies the relationship between hypohydration status and degraded mood was consistent [8,57]. In additional studies, measuring self-reported changes in mental state have consistently found associations between dehydration and mood, in conjunction with changes in performance [58,59]. Neave et al.  tested young adults on a range of cognitive tasks, including attention and working memory, showed that mood ratings significantly change when individuals were given water. Individuals reported feeling more ‘calm’ and ‘alert’ immediately after water consumption. These results are in line with those of other young adult studies that found similar reports of ‘alertness’ after water consumption .
The strengths of this study include the novelty in a vulnerable and high-risk group such as the aeronautical military personnel as well as the careful protocol and methodology used.
Even if the present study’s protocol was well controlled, some limitations should be acknowledged. Regarding the HS, the selection of hydration markers may have limitations. The most important limitation refers to the impossibility of directly assessing water consumption, since it has been estimated thought a validated questionnaire. Other limitation refers to the measurement of USG through reagent strips instead of refractometry and the non-availability of 24 hours urine samples.