Polyamines are produced primarily from amino acid precursors arginine, ornithine, and methionine (42). Diamine putrescine, triamine spermine, and tetraamine spermidine are the most abundant natural polyamines found in mammals (43). The total polyamine reservoir in mammals is regulated endogenously by host cells (43) and exogenously by gut bacteria (44–47) and diet (48, 49). Putrescine, spermidine, and spermine exist in the micromolar to millimolar range in various foods (48, 49) and across body sites of healthy adults as summarized in Figure 1 . The use of radiolabeled polyamines demonstrated that polyamines are readily absorbed and enter the circulation (48), where they are distributed across different organs and metabolic fluids, including the brain (50), kidneys (51), breastmilk (52–54), urine (55–58), and serum/plasma (55, 56, 59–62). Notably, the largest accumulation of polyamines occurs in the intestinal lumen as determined with fecal samples from healthy donors where putrescine is the most abundant followed by spermidine and spermine (63, 64). Fecal levels of polyamines have been shown to correlate with gut microbiota composition in humans (63–65) and rodent models (66, 67). Oral supplementation in rodents with arginine led to increased polyamine concentrations in feces in a dose-dependent manner, which was eliminated when these animals were treated with antibiotics prior to arginine administration (66). This implicates the role of the gut microbiota in amino acid metabolism that results in synthesis of polyamines in mammals. Additionally, both Gram-positive and Gram-negative bacteria isolated from human fecal samples, including Bifidobacterium, Clostridium, Enterococcus, and Lactobacillus, are able to produce polyamines in vitro (68). Of these genera, the abundance of Bifidobacterium animalis is decreased and Lactobacillus reuteri increased in obese individuals (69, 70). Together, these studies demonstrate that the gut microbiota is a major contributor to the total polyamine pool in vivo. Moreover, with the direct link between obesity/T2D and gut dysbiosis (71, 72), there is a compelling and urgent need to study host-microbiota-polyamine interactions and the contribution of polyamines in obesity/T2D.

Thus far, there are only a handful of studies that have reported polyamine levels in obese individuals, all of which examined circulating polyamines in serum or plasma (73–75). In a study of 114 overweight/obese adults (BMI=27-40 kg/m²) with and without type 2 diabetes, it was concluded that individuals with obesity/T2D had elevated levels of putrescine as compared to obese, non-diabetic participants (73). This study did not have an additional control group of healthy adults but levels of putrescine (0.076μM), spermidine (0.15μM), and spermine (0.027μM) were in range when compared to previously reported levels in plasma/serum of healthy adults (putrescine = 0.053-0.49μM, spermidine = 0.004-11μM, spermine = 0.009-5.9μM) (55, 56, 59–62). In a second cohort of morbidly obese adults, the same group of authors noted increases in serum putrescine that was associated with failure to ameliorate metabolic syndrome six months after bariatric surgery (74). These results differ from a third study of 102 obese adults that demonstrated there were no differences circulating levels of putrescine, spermine, and spermidine in male vs female obese adults (76). Unfortunately, this study also did not have healthy adult controls but when compared to studies in healthy adults, only putrescine was elevated whereas spermine and spermidine were in the range of previously reported levels (55, 56, 59–62). More recently, spermidine was found to be positively associated with obesity in rural Chinese residents (75). However, these individuals were more likely to decrease their BMI over two years in a follow-up study (75). This suggests that spermidine exert protective effects against weight gain but of unknown mechanism. Large ranges can be seen across these studies illustrated in Figure 1 . Variation in these studies likely originates from: i) different methods used to extract and analyze polyamines; e.g., HPLC was used in all obese/T2D studies whereas LCMS is considered to be more accurate and sensitive, ii) only including male participants (59), iii) not considering potential correlation between age and polyamines that has been previously identified (40, 63, 77). Therefore, although the majority of existing studies suggest polyamines are elevated in obesity/T2D, it is difficult to make substantial conclusions without control non-obese, non-diabetic healthy adults because exogenous polyamines have anti-obesity effects in preclinical studies (78–81). Additionally, patients with type 2 diabetes expressed reduced levels of polyamine synthetic enzymes such as ornithine decarboxylase that is necessary for putrescine production (82). Because these studies suggest an elevation in only putrescine when compared to healthy adults of other studies, it is important to identify changes in individual polyamine metabolites in future works. Note that these trends were also concluded from studies primarily in European or rural Chinese residents (59, 73–76). Additional studies are required in countries where obesity/T2D affect over 20% of the population including the United States, Mexico, Russia, and Brazil (83, 84).

Results from animal models of obesity-related diseases are also difficult to interpret due to the use of heterogenous models of obesity/T2D and sampling of tissues. Adipocytes from obese Zucker rats demonstrated a 4-fold increase in concentration of spermine and spermidine that was positively associated with adipose triacylglycerol formation (85). However, two-month-old leptin-deficient obese/T2D mice exhibited 31% less spermidine but 24% higher spermine in whole pancreatic islets (86). Obesity/T2D decreased expression of polyamine synthetic genes Odc (ornithine decarboxylase), Srm (spermidine synthase), and Sms (spermine synthase) that concomitantly resulted in a ~30% reduction in spermidine in the colon (87). These results differed from another study in diet-induced obese/T2D murine model that demonstrated no deficiency in polyamines due to obesity/T2D locally in the gut or systemically in plasma (88). Despite the variations in both humans and animal models of obesity/T2D, polyamines are essential to white adipose tissue homeostasis by stimulating adipocyte lipolysis (81) and the deletion of a spermidine to spermine conversion enzyme, spermidine/spermine N1-acetyltransferase, leads to late-onset obesity and insulin resistance (89–91).

Polyamines regulate a variety of cellular functions in both innate and adaptive immune cells that are often described to be immunosuppressive (41). At neutral pH, polyamines exist as positively charged molecules that interact with negatively charged macromolecules such as DNA, RNA, and proteins (43). As a result, polyamines are necessary for molecular regulation of growth, autophagy, differentiation, and activation of lymphocytes. Polyamine depletion with enzyme inhibitors induced abnormal differentiation of cytolytic T lymphocytes (92) and caused defects in B-cell production of immunoglobulins (93). Both of these phenotypes were rescued upon exogenous addition of polyamines. T helper cells are the most well-studied when investigating the role of polyamines. Polyamines regulate CD4+ T cell differentiation as demonstrated by spermidine induction of Foxp3 expression to polarize naïve T cells to become regulatory T cells in an autophagy-dependent manner (94). In addition to Tregs, polyamines have been proposed to regulate transcription factors such as Tbx21 (T-Bet), Gata3, and Rorc that mediate CD4+ T cell differentiation into other subsets Th1, Th2, and Th17 epigenetically (95). This suggests that polyamines have significant impacts on adaptive immunity.

The role of polyamines in innate immune cells are less clearly defined. Polyamine synthesis is required for natural killer cell metabolism and effector function including the production of granzyme B and IFN-γ (96). In neutrophils, polyamines regulate effector functions by promoting superoxide and myeloperoxidase production. In healthy human neutrophils, depletion of polyamines treatment led to decreased production of superoxides ( O 2 − , H₂O₂) and release of myeloperoxidase (97). This is congruent with a second study demonstrating physiological concentrations of spermidine induced superoxide generation in human neutrophils stimulated with chemotactic peptide fMet-Leu-Phe (98). While the effects of polyamines are pro-inflammatory in natural killer cells and neutrophils, polyamines are immunosuppressive in monocytes/macrophages. Spermine inhibited synthesis of pro-inflammatory cytokines (TNF, IL-1, IL-6, MIP-1α, MIP-1β) in LPS-stimulated human peripheral blood mononuclear cells (99) and inhibited the production of nitric oxide in J774 murine macrophages stimulated with LPS or IFN-γ (100). Spermidine promoted hypusination of the eukaryotic translation factor eIF5A that switches macrophage metabolism to oxidative phosphorylation, a phenotype associated with M2-like anti-inflammatory macrophages (95, 101). However, depletion of polyamines in the same cell line induced nitric oxide synthesis when macrophages were stimulated with LPS (102). Thus, the exact role of polyamines in immune cells, particularly innate immune cells, remains unclear. It is particularly interesting to investigate the role of polyamines in innate immune cell dysfunction in obesity/T2D.

Several studies have demonstrated beneficial effects of increased polyamine concentrations in correcting the metabolic complications of obesity/T2D (78–81, 87, 103). The total host polyamine pool can be increased either with diet ( Figure 1 ) or by supplementation with probiotics or prebiotics which increase the abundance of gut bacteria that synthesize polyamines (44–47, 68). Bifidobacterium spp. are often regarded as beneficial bacteria due to their production of short-chain fatty acids, which has a myriad of physiological effects on the host (104). More recently, Bifidobacterium spp. have also been linked to polyamine synthesis in animal models of obesity/T2D and importantly, improved clinical outcomes (67, 87, 88). In a diet-induced model of obesity/T2D, mice fed Bifidobacterium animalis subsp. lactis for 12 weeks rescued polyamine production with a concomitant correction of lipid and glucose metabolism, reduction in metabolic endotoxemia, and strengthening of gut barrier function (87). Similarly, administration of B. lactis with arginine, an amino acid precursor to polyamine synthesis, resulted in increased levels of circulating and colonic levels of polyamines that correlated with reduced inflammation in senescent mice (67). Furthermore, in another diet-induced obese/T2D mouse model, supplementation with oligofructose, a bifidogenic indigestible carbohydrate, led to increases in abundance of B. pseudolongum that was associated with elevation of bacteria-specific polyamine precursor acetyl-ornithine and down-stream levels of spermine and spermidine (88).

Together, these studies demonstrated that restoration of polyamine synthesis by B. lactis and B. pseudolongum ameliorated complications associated with obesity/T2D. Moreover, direct administration of polyamines reduced body weight, adipocyte differentiation, and lipid accumulation in obese mice (78–81). Enhancement of polyamine synthetic enzyme, SAT1, by triethylenetetramine dihydrochloride was also associated with anti-obesity and anti-diabetic effects in mice (103). There is an ongoing need to study the direct effects of polyamines on development of diabetes. In summary, these studies provide evidence that polyamines are integral to regulating metabolic syndrome and illustrate its potential as a nutraceutical for metabolic complications of obesity/T2D ( Figure 2 ).

Obesity/T2D is associated with increased risk for adverse clinical outcomes post-infection likely due to deficits in the immune system that have been likened to that of aging (105). It is particularly interesting because aging has been linked to declining levels of polyamines (40, 77), chronic low-grade inflammation (termed “inflammaging”) (106), and higher morbidity and mortality during infections (107). In fact, spermidine levels are depleted in the elderly and polyamine treatment corrects autophagy and B-cell senescence (108, 109). Similar dysregulation of polyamines in obesity/T2D would affect bacteria-host immune interactions that govern infection outcomes. In preclinical studies, obesity/T2D exacerbates inflammation and infection severity in models of osteomyelitis (110, 111), bacteremia (112), and skin infections (113, 114). Although not in the context of infection, administration of polyamines or associated probiotic and prebiotics have alleviated inflammation in a variety of disease models. Treatment with B. lactis and arginine, an amino acid precursor to polyamine synthesis, resulted in increased levels of circulating and colonic levels of polyamines that was associated with reduced inflammatory signaling in serum of mice (67). In a model of T-cell transfer colitis, spermidine potentiated Treg differentiation and ameliorated disease pathology in the gut (94). Injection of spermine protected mice from developing acute footpad inflammation (99), which may be important for mitigating excessive inflammation in diabetic foot ulcers (114). Therefore, polyamines are emerging players in dictating bacterial-host immune interactions in obesity/T2D by regulating inflammation ( Figure 2 ).

More recently, polyamines have been directly shown to reduce osteomyelitis severity in a murine model of obesity/T2D (88). Supplementation with oligofructose, a bifidogenic prebiotic, decreased Staphylococcus aureus burden in infected bone and tissue in obese/T2D mice. Oligofructose dampened systemic inflammatory signaling that normally exacerbates infections in obesity/T2D (111, 114, 115), consistent with prior studies (99, 116). The authors determined a 6-log fold-change in B. pseudolongum in the gut microbiota of obese/T2D mice due to oligofructose treatment. This compositional change was associated with elevated polyamines in the cecum and plasma of obese/T2D mice. Remarkably, direct oral administration of spermine and spermidine led to a reduction in osteomyelitis severity similar to oligofructose. These results suggest that polyamines promote beneficial effects during infections in obesity/T2D and is an unexplored area that warrants further investigation.