## 4 Discussion  
This study found that commercially available EN formulas may not meet the daily polyamine requirements of critically ill patients, compared with a similar population's average dietary intake [8]. Daily intake may be associated with gastrointestinal effects. To the best of our knowledge, this is the first study to determine the composition of polyamines in EN formulas. Significant differences in polyamine composition were found among the 16 EN formulas analysed in this study. These differences were not associated with the nutritional characteristics of the formulas, such as the protein content, caloric density, osmolarity, or chemistry of the nutrients. Commercial preparation processes may be responsible for the variations in the polyamine content in the tested EN formulas. The polyamine content of the EN formulas was notably lower than that found in most foods of equivalent weight [8]. The analysis of eight patients admitted to the ICU for diverse pathologies revealed that the average daily polyamine intake from the EN formulas administered to the patients during their first seven days in the ICU was only 12-25 umol/day (considering the sum of putrescine, spermidine, and spermine). Even if patients were to ingest higher EN infusion rates--up to 63 mL/minute, supplying 1512 mL/day--the estimated polyamine intake would still be only 6-63 umol per day, depending on the EN formula administered. These values are considerably lower than the estimated daily dietary intake in the same population (around 400 umol/day) [8] and also for the polyamines considered individually. The average daily intake of putrescine (4.04 umol/day), spermidine (11.53 umol/day), and spermine (1.62 umol/day) were much lower than the average content in a regular diet, which was 273.05 umol/day, 87.71 umol/day, and 49.82 umol/day, respectively [8]. The reduced intake is not associated with the low-calorie consumption observed in the patients, which is approximately 1000 kcal/day, compared to 2200 kcal/day in individuals from the same population following a regular diet [8]. Adjusting the intake per 1000 kcal highlights that the EN formulas have a low polyamine content [8]. In the short term, the daily intake of polyamines may influence tolerance to EN formulas. This is suggested by the observed inverse correlation between total and individual polyamines--such as putrescine, spermidine, and \(N\)-acetyl spermine--and daily GRV [6]. The total polyamine and spermidine contents in the EN formulas were associated with contributing close to 20% of the variations in GRV in these patients. Additionally, tyramine also showed an inverse correlation, while \(\beta\)-phenylethylamine and isoamylamine were associated with worse EN tolerance. The association between polyamines and gastrointestinal tolerance aligns with their reported effects of modulating gastrointestinal motility [14; 15] and maintaining gastrointestinal homeostasis, as described in experimental models [46; 47]. Some of these gastrointestinal effects have been attributed to spermidine [48]. This study focused on determining the content of polyamines in EN formulas and their local effect on the gastrointestinal tract. It cannot be ruled out that additional clinical effects could arise if there is a prolonged reduction in the administration of polyamines due to their role as an energy source for the epithelium [11], their involvement in epithelial growth [49], their stimulation of the gut immune system [50], and their contribution to microbiota homeostasis [51]. Moreover, such a reduced intake could have a broader impact on cellular polyamine homeostasis, potentially influencing the protective roles attributed to polyamines, especially spermidine, in vital organs such as the kidneys [32], lungs [35; 36; 37], heart [33; 34], brain [39], and immune system [38]. These changes may be particularly significant for critically ill patients who are on mechanical ventilation and may experience persistent organ dysfunction after intensive care [31].