## 1 Introduction  
Adequate nutrition is crucial to health at all stages of life [1]. Consequently, when a patient cannot consume food orally, alternatives, such as enteral nutrition (EN) formulas, are necessary, especially for critically ill patients [2]. Parenteral nutrition is only considered when EN is contraindicated, as occurs in certain pathological circumstances [3]. The importance of EN is determined by the fact that the intestinal epithelium does not have blood vessels and mainly relies on nutrients obtained from the intestinal lumen to maintain gastrointestinal homeostasis [4]. Therefore, a prolonged absence of EN can lead to gut atrophy, luminal starvation, dysbiosis of the microbiota, and impaired immune functions [5]. Several EN formulations are available to provide essential nutrients to hospitalised patients, which are administered via different methods [2; 6]. These EN formulas, with their particular characteristics, should meet patients' metabolic requirements and provide them with essential nutrients and trace elements tailored to different clinical situations [7]. EN formulas are expected to contain polyamines (putrescine, spermidine, and spermine), since they are present in all macronutrients [8]. These compounds are essential dietary components, which are also produced by the microbiota. They contribute to the development and growth of the gastrointestinal mucosa and facilitate wound healing [9; 10]. Additionally, they serve as a direct energy source for the intestinal cells [11], promote a healthy microbiota (which also produces polyamines as metabolites) [12], stimulate the intestinal immune system [13], and modulate gastrointestinal motility [14; 15], thereby facilitating the digestion and absorption of nutrients from birth onwards [16; 17]. Intestinal polyamines are absorbed, complementing the endogenous synthesis carried out by the enzyme ornithine decarboxylase. This enzyme converts ornithine into putrescine, which is subsequently transformed into spermidine and then into spermine [18]. The cellular homeostasis of polyamines is tightly regulated, and any disruption in this balance is associated with various pathologies. Polyamines' importance extends to their systemic effects, including DNA replication, RNA transcription, protein synthesis, and posttranslational modification. Consequently, polyamines are crucial for regulating cellular proliferation and differentiation [19]. Studies in experimental models [20; 21] and epidemiological research [22; 23] have found that a higher intake of polyamines is associated with a reduced risk of age-related diseases and a potential increase in longevity. Among the different polyamines, spermidine has garnered particular interest in relation to these effects by eliciting the autophagy [21] and hypusination of the translational regulator eukaryotic initiation factor 5A (eIF5A) [24]. Nonetheless, the requirements for and intake of polyamines remain the subject of debate. Depending on the study, and likely also on the country, the daily intake of total polyamines--the sum of putrescine, spermidine, and spermine--has been found to range from 200 to 410 umol per person [25; 26; 27; 28; 29]. These values may be below the optimal level. Based on an ideal diet, this has been estimated to be 541 umol from the Swedish Nutrition Recommendations Objectified [30]. Despite their essential biological functions, polyamine intake and its effects in critically ill patients fed with EN formulas have not been previously investigated. These patients may experience acute organ dysfunction and post-intensive care syndrome [31]. Maintaining adequate polyamine homeostasis, particularly spermidine, could produce beneficial effects in these patients, given their protective role on various organs, including the kidney [32], heart [33; 34], lungs [35; 36; 37], immune system [38], and brain [39], among others. In addition to polyamines, other biogenic amines are present in foods, distinguished by their chemical structure and the number of amino groups.