## 4 Discussion
Thus, FFM is another important parameter to consider for the nutritional monitoring of ALS patients. Hydration plays a crucial role in maintaining good health and preventing diseases [58]. Compared to healthy controls, we observed dehydration in ALS patients, according to low levels of TBW and the placements of the vectors. In agreement, [PERSON] et al. (2011) identified significantly lower TBW values in ALS patients [42]. Risk factors for dehydration in ALS include bulbar-onset, dysphagia, old age, and advanced disease. These authors have found that water intake and hydration status are prognostic indicators of survival in ALS, independent of age, site of onset, BMI, body size, or disease severity [59]. Water intake and TBW parameters should stand out in monitoring and guiding nutrition intervention in ALS patients. Regarding the PhA, our study observed an average value of 3.4, indicating low cellularity. In agreement, [PERSON] et al. (2015) found means of 3.8 and 4.5 for women and men with ALS, respectively [18]. [PERSON] et al. (2008) demonstrated that the PhA was greatly reduced in ALS patients, with a mean value of 3.2. Moreover, a PhA below 2.5 was associated with a significantly poor survival rate [60]. PhA values ranging from 5 to 7 signify healthy cellular structure and robust membrane integrity, typically observed in a good population. Conversely, lower PhA values are associated with alterations in fluid balance and compromised membrane integrity [21; 61]. A large body of evidence confirms that PhA is related to cellular health and hydration, muscle mass [62; 63], muscle strength [64], and quality of soft tissues [20; 61], reflecting cellular integrity [18]. Also, PhA has been useful as an indicator of muscle strength and muscle quality [64]. Low PhA in ALS can be a reflex of malnutrition and alterations in cell membrane integrity [60], loss of muscle strength [65], inflammation, and oxidative stress [66]. This can compromise cell functions and increase the risk of cell apoptosis [67]. PhA relates to metabolically active tissue such as FFM. In a representative study with healthy subjects, FFM emerged as one of the key determinants of PhA [66]. PhA has a relevant role in ALS because it is an independent prognostic factor [18; 68]. The decrease in PhA during ALS reflects changes in BC, overall health, and functional status [42], and is associated with shorter survival [18]. PhA has been used as a prognostic factor not only in ALS [41; 60] but in many clinical conditions such as DMD [26], systemic sclerosis [29], Alzheimer's [27; 69], Parkinson [28], and psychogeriatric patients [70]. PhA has a good correlation with other nutritional assessment methods [60]. Thus, the use of the PhA should be encouraged in the nutrition care of ALS patients. Regarding the BIVA, we observed that most patients, regardless of sex, were outside the 95% tolerance ellipse. ALS patients had lower R/H and Xc/H values when compared to the controls, which can be seen by the case group vector shift to the right. This confirms lower PhA and less soft tissue in ALS patients. Because of the lack of studies relating to BIVA and ALS patients, we discussed this point considering other neurological conditions. In systemic sclerosis patients, authors observed significant differences in several BIVA variables when compared to the control group. In addition, BIVA had a predictive association with mortality in these patients [29]. In another study comparing BIVA between healthy individuals and patients with Parkinson's disease (PD), the authors found a significant displacement of the average impedance vector in PD male patients aged 60 to 79 years, compared with healthy controls. Also, BIVA was useful in detecting cachexia and dehydration in individual patients with PD [71]. The literature points to BIVA as a useful measure for detecting changes in muscle mass and sarcopenia in elderly people [64].