## 4 Discussion  
The principal purpose of the study was to assess the differences in the relative concentrations of brain metabolites between veterans and healthy, age-matched controls, as well as to compare the same between the brain hemispheres of the military personnel. Our paper provides evidence of significant differences in brain chemical composition between the two groups and between brain hemispheres, which has further implications on the health outcomes of former soldiers. To date, this is the first study on European soldiers with the use of \({}^{1}\)HMRS of the brain. The first important results of this study are the presence of a notably increased mI/Cr ratio in the frontal lobes, and to the lesser extent in the occipital lobes, as well as a decreased NAA/Cr ratio in the frontal and occipital lobes of soldiers, in comparison to healthy, age-matched controls. The comparison of our results with other studies is hindered by the inconsistency in their methods of results' presentation--as a ratio to water [8], choline [1], or as absolute units [10]. Moreover, the majority of studies focus on the comparison of brain metabolites between veterans with and without post-traumatic stress disorder diagnoses [10; 11; 12; 13; 14] or the history of mild TBI [8; 9; 10], without the comparison to healthy controls. Nonetheless, some tendencies are visible and they are consistent with the results of our study. [PERSON] et al. discovered an increased mI/H\({}_{2}\)O ratio in veterans with a history of mild traumatic brain injury, especially in the group with suicidal thoughts and attempts [8]. Similar results regarding an increased mI/H\({}_{2}\)O ratio were also observed in other groups with frequent exposure to brain trauma, such as ice hockey players or American football players [15]. Myosinositol is an osmolyte and an astrocyte marker and its elevated concentration is linked with glial proliferation, secondary to microglial activation and astrocytosis [16]. Such a phenomenon was also discovered histologically in the frontal cortex of rats exposed to repetitive blast overpressure injury, where an increase in gliosis marker GFAP and microglia activation marker Iba-1 immunoreactivity was observed, together with signs of astrocyte and microglia hypertrophy [17]. In turn, a decrease in the relative concentration of NAA, similar to that observed in our study, was previously detected in veterans' hippocampi [1; 9; 13; 14], especially in groups with PTSD [13; 14] and with a history of mild traumatic brain injury [9]. NAA is known as a marker of neuronal viability and its decline is associated with neuronal loss [15]. Interestingly, the long-term decrease of the NAA/Cr ratio was observed only in groups with chronic exposure to brain trauma, after a single brain trauma the NAA/Cr ratio recovers to the initial level after a few days [16]. Unfortunately, these molecular changes have their health implications. A reduction in the relative concentration of NAA correlates with poor long-term cognitive outcomes, lower neuropsychological measures of psychomotor speed, motor scanning and attention, impaired long-term memory, and attention deficits [16]. Moreover, lower NAA/Cr and NAA/ml ratios predict a higher hyperphosphorylated tau (\(p\)-tau) burden, which is a pathognomic lesion of chronic traumatic encephalopathy [15]. Therefore, \({}^{1}\)HMRS examination in the population of military workers may become a useful tool for the monitoring of the probability of chronic traumatic encephalopathy and its cognitive consequences. This may help to undertake adequate measures such as regular neurorehabilitation and neuropsychological care before irreversible changes in the brain occur. Regarding variations in brain metabolites between sides in military personnel, statistically significant differences were observed for both the mI/Cr and NAA/Cr ratios. Insoldiers preferring a right shooting position, the ml/Cr ratio was higher and the NAA/Cr ratio was lower in the right hemisphere compared to the left.