Polyamines in Human Milk and Their Benefits for Infant Health

Abstract

Breastfeeding is the gold standard for infant nutrition in the first six months of life when feeding choices determine growth and development. However, human milk is a complex and highly variable fluid that, in addition to nutrients, contains several bioactive components, including polyamines (putrescine, spermidine, and spermine), and constitutes the first exogenous source of these compounds for infants. Active in various cellular processes, polyamines are involved in the growth and maturation of the gastrointestinal tract and the development of the immune system and therefore play an important role in the first year of life. This chapter reviews the impact of polyamines on infant growth and health, the polyamine content in human milk and how it is influenced by factors related to both the mother-child dyad and breastfeeding itself. In addition, a comparative analysis of human milk and infant formulas in terms of polyamine content and profile is presented.

1. Introduction

The first 1000 days of life, from conception to 23 months postpartum, constitute the most important period of growth and development for the human body and brain. Therefore, early food choices play a crucial role in health in infancy and later in life [1, 2]. Widely accepted as the gold standard for human nutrition in the first six months of life, breast milk satisfies all the infant’s nutritional requirements for optimal growth and development [3, 4, 5]. According to the World Health Organization (WHO) and United Nations Children’s Fund (UNICEF), breastfeeding should begin within the first hour after birth, continue as the exclusive source of nutrition during the first semester of life, and ideally be extended until the infant is at least two years of age [3].

Human milk is a complex and highly variable fluid that, in addition to nutrients, contains other substances such as nucleotides, hormones, growth factors, immunoglobulins, oligosaccharides, cytokines, and antibodies, which participate in the development of the immune system and provide protection against infectious diseases (Figure 1) [5, 6, 7, 8, 9]. Although human milk has been described as sterile, it is now known to harbor a complex community of bacteria that help establish the infant’s gut microbiota, with an impact on future health (e.g., by helping to prevent allergies, asthma, and obesity) [10, 11, 12]. Among the bioactive compounds found in human milk are the polyamines putrescine, spermidine, and spermine [9, 13, 14], which are synthesized in the mammary gland during pregnancy and lactation and are associated with hormonal regulation of lactogenic processes [15].

Figure 1.

Qualitative composition of human milk.

Several studies have shown that growth patterns and body composition differ between infants fed with human milk and infant formulas [16, 17, 18]. For example, infants that are exclusively breastfed accumulate more fat (both in grams and percentage) during the first four months of life, whereas those that are formula-fed acquire more lean mass and gain weight more rapidly, a tendency associated with a higher risk of developing overweight and obesity later in life [16, 18, 19, 20, 21]. Likewise, differences in growth have also been described between exclusive and partially breastfed infants [18, 22, 23]. It has been suggested that the unique composition of human milk in terms of nutrients, protein concentration, and qualitative characteristics may explain these differences in growth and the beneficial effects of breastfeeding [21].

2. Polyamines: biochemical properties and functions

Polyamines are low molecular weight nitrogenous compounds found in all living organisms [24, 25, 26]. Putrescine (1,4-butane diamine), spermidine (N-(3-aminopropyl)-1,4-butane diamine), and spermine (N,N-bis(3-aminopropyl)-1,4-butane diamine) are characterized by the presence of two or more amino groups (Figure 2). Due to their structure, they are relatively stable compounds, capable of withstanding both acidic and alkaline conditions and establishing hydrogen bonds with hydroxyl solvents such as water and alcohol. At physiological pH, polyamines are fully protonated in the organism and can bind strongly to biomolecules such as DNA, RNA, ATP, proteins, and phospholipids, stabilizing their negative charges and, in many cases, modulating their function [27, 28]. In the cell, polyamines are stored mainly in the cytosol and nucleus, which participate in DNA transcription and RNA translation [24, 29].

Figure 2.

Chemical structure of polyamines.

In humans, polyamines play an essential role in regulating several cellular processes, such as cell growth and differentiation, and protein and nucleic acid synthesis [25, 30]. In addition, polyamines participate in the modulation of the immune response, ionic channels (e.g., acting as blockers of potassium channels), and cell apoptosis [24, 25, 31, 32]. Another important feature of polyamines is their antioxidant capacity, described as comparable to that of recognized antioxidants such as α-tocopherol, ascorbyl palmitate, and octyl gallate [33]. Polyamines exert antioxidant effects mainly through the mechanism of metal chelation, which prevents the formation of hydroperoxides and secondary oxidation compounds [33]. Spermine is the polyamine with the highest antioxidant capacity, as its chemical structure has the largest number of positive charges, followed by spermidine [33]. Additionally, polyamines are reported to protect DNA against oxidative damage by eliminating free radicals, especially in lipophilic media [33, 34].

The three polyamines can be found in a wide variety of animal and plant foods, but with qualitative and quantitative variation [25, 35]. For newborns, human breastmilk is the first exogenous source of these compounds [9, 24, 26].

2.1 Cellular homeostasis of polyamines

Cellular homeostasis of polyamines in the body is strongly regulated and depends on the balance between their endogenous synthesis, dietary intake, and catabolism [25, 29, 36]. Figure 3 schematically depicts the metabolic routes of polyamine synthesis, interconversion, and catabolism.

Figure 3.

De novo synthesis, interconversion, and catabolism of polyamines in the body. ODC: ornithine decarboxylase, AdoMetDC: S-adenosyl-l-methionine-decarboxylase; dcSAM: decarboxylated S-adenosyl-methionine, 5′MTA: 5′-methylthioadenosine, SSAT: spermidine/spermine N1-acetyl-transferase, PAO: polyamine oxidase Acetyl-CoA: Acetyl coenzyme-A. Adapted from Munoz-Esparza et al. [26].

The de novo synthesis of polyamines begins with the formation of putrescine from the amino acid ornithine by the action of the enzyme ornithine decarboxylase (ODC). Next, spermidine is obtained from putrescine by the action of spermidine synthase, which catalyzes the addition of a propylamine group from the decarboxylation of S-adenosyl-methionine. Finally, spermine synthase transforms spermidine into spermine by adding a second propylamine group [29, 37]. In addition to de novo formation, polyamines may arise from interconversion reactions, a cyclical process that controls polyamine turnover and maintains intracellular homeostasis (Figure 3). The interconversion begins with the acetylation of spermine or spermidine by the action of spermidine/spermine N1-acetyltransferase (SSAT) and with the participation of acetyl coenzyme-A. Subsequently, polyamine oxidase (PAO) removes a propylamine group to generate putrescine from the acetylated metabolite of spermidine, or spermidine from the acetylated metabolite of spermine [24, 25, 26, 38].

The principal source of exogenous polyamines is food [24, 26]. Dietary polyamines are absorbed in the small intestine, mainly in the duodenum and the first segment of the jejunum, either through transcellular absorption mechanisms involving transporters or passively through the paracellular route, the latter being more common [24, 28, 30, 39]. Although not much information exists about polyamine transporters in mammalian cells, two transmembrane transporters from the family of solute carriers (SLC) have been identified [30, 40]. Exogenous polyamines are incorporated into the cell via the carnitine transporter SLC22A16, whereas SLC3A2 is responsible for the excretion of acetylated polyamines, especially putrescine and spermidine [30, 39].

3. The role of polyamines in infant health

Polyamines participate in different biological processes in the first years of life, promoting the growth and maturation of the gastrointestinal tract and the development of the immune system [7, 41, 42, 43, 44, 45]. Thus, it has been postulated that polyamine requirements may be higher during the neonatal and infant stages when the rate of cell growth is more rapid [46, 47].

Several studies have shown that oral administration of polyamines in animal models induces morphological and biochemical modifications in the intestine. For example, the administration of spermine and spermidine in mice increased protein expression and modified the activity of disaccharidases, markers of small intestinal maturation [42, 43, 48]. Biol-N’garagba et al. [49] proposed that postnatal changes in glycoprotein fucosylation in the intestinal maturation process largely depend on polyamine intake. They demonstrated in mice that oral administration of 10 μmol/day of spermine and spermidine increased the activity of α-1,2-fucosyltransferase, causing a higher concentration of α-1,2-fucoproteins in the brush border membrane [49]. Other authors have reported biochemical changes associated with the maturation of the small intestine, such as the increased activity of alkaline phosphatase and glutamyl-transferase gamma [44, 48].

At the morphological level, Sabater-Molina et al. [44] demonstrated that a physiological dose of polyamines significantly increased crypt depth in the small intestine of newborn piglets compared to the control group. In addition, Pérez-Cano et al. [7] reported that spermine and spermidine administered to newborn rats were associated with increased gut weight and length, indicating a more mature gut structure.

Regarding the immune response, animal model studies indicate that oral administration of spermine and spermidine in the postnatal period induces the maturation of intestinal immune cells and increases concentrations of immunoglobulin A [7, 42, 43, 50]. Ter Steege et al. [50] demonstrated that the intake of spermine increased the percentage of intraepithelial lymphocytes (CD4, CD5, and CD54) and the expression levels of these antibodies. Likewise, Pérez-Cano et al. [7] observed that daily supplementation with spermidine and spermine significantly increased the maturation of CD8 intraepithelial lymphocytes and the number of natural killer cells are related to greater innate immunity.

During the first months of life, the intestine is more permeable to macromolecules, a state associated with a higher propensity to develop allergies [43]. Dandrifosse et al. [51] reported that spermine administration in rats increased pancreatic protease activity, which improved protein digestion and reduced the likelihood of allergenicity. It is well established that breastfed infants have a lower risk of developing food allergies than those fed infant formula [52]. This tendency is mainly attributed to the presence of polyamines in human milk and other bioactive compounds, such as immunoglobulins, growth factors, nucleotides, and cytokines [7, 52]. In support of this effect, Peulen et al. [53] reported that a higher spermine content in human milk during the first month postpartum was associated with a lower incidence of allergy at five years of age. Furthermore, intestinal maturation, promoted by polyamines, involves a reduction in intestinal permeability, limiting the passage of antigens from the lumen into the bloodstream, thus lowering the risk of allergy in the infant [7, 41, 45]. Consequently, supplementing infant formulas with polyamines has been described as important for allergy prevention [7, 43, 48, 53].

Finally, in addition to their role in intestinal maturation and immunity, polyamines have also been associated with changes in the microbiota [15, 54]. For example, studies in newborn mice found that administering an infant formula supplemented with polyamines significantly impacted the composition and activity of the intestinal microbiota, resulting in a significant increase in the number of Bifidobacteria species compared to the control group. These bacteria have a biological role in the relationship between the host mucosa and the microbiota, immune regulation, and the control of inflammation. Also observed was an increase in the concentration of the bacterium Akkermansia muciniphila, a common species of intestinal microbiota that promotes the development of innate and adaptive immune responses [15, 54].

4. Polyamine content and profile in human milk

As mentioned, human milk has a substantial content of polyamines, serving as the first dietary source of these compounds for infants. However, in the last three decades, several studies have investigated polyamines in human milk, reporting a wide variability of contents, with total polyamine concentrations ranging between 25.7 nmol/dL and 948.5 nmol/dL (Table 1). Explanations for this broad divergence include cohort heterogeneity, the uniqueness of each mother-child dyad, and the different phases of lactation at which samples were taken (from the first week to six months of life) [9, 13, 15, 41, 47, 55, 56, 57].

	Lactation phase	Total polyamines	Putrescine	Spermidine	Spermine	References
Full-term human breast milk	1st week	396.1	20.5	185.4	190.2	[55]
		713.3	82.4	457.5	173.4	[14]
	1st month	861.5	61.5	351.2	448.8	[55]
		881.0	21.0	352.0	508.0	[15]
		25.7	2.9	12.4	10.4	[47]
		790.5	18.0	389.5	382.9	[57]
	2 months	819	90.0	385.0	344.0	[41]
		145.8	Nd	73.6	72.2	[55]
		659.1^+	85.3^+	414.0^+	159.8^+	[56]
		567.7*	73.0*	348.6*	146.1*	[56]
		506.9°	34.1°	243.7°	206.2°	[9]
		876.6^♦	50.0^♦	407.7^♦	418.9^♦	[9]
		756.1	30.0	381.0	345.1	[57]
	3 months	562.3	30.4	285.7	246.2	[57]
	4 months	745	79.0	316.0	350.0	[41]
		429.0°	30.3°	206.2°	192.5°	[9]
		788^♦	51.0^♦	357.8^♦	379.2^♦	[9]
		508.3	29.6	239.9	238.8	[57]
	5 months	448.2	19.0	229.7	199.5	[57]
	6 months	618	87.0	298.0	233.0	[41]
		405.6°	35.0°	192.5°	178.1°	[9]
		708.6^♦	49.8^♦	379.2^♦	279.6^♦	[9]
	Mean	557	24.0	220.0	313.0	[13]
Preterm human breast milk	1st week	948.5	165.6	615.2	167.7	[14]
	1st month	82.2	5.8	46.2	30.2	[47]

Table 1.

Mean contents of polyamines in human breast milk (nmol/dL) reported in the literature.

Nd: not detected. Milk from mothers with +normal weight and *obesity. Average: does not specify month of lactation. The milk sample corresponds to °foremilk and ^♦hindmilk.

Studies disagree about the main polyamine in human milk, some indicating spermidine [14, 41, 47, 56], and others spermine [13, 15, 55]. In two recent studies, Muñoz-Esparza et al. [9, 57] found that spermidine and spermine predominated, being detected in similar proportions. In contrast, putrescine is unanimously described as a minor polyamine in human milk (Table 1).

5. Influence of the mother-child dyad and lactation on the polyamine content in human milk

It has been suggested that the polyamine content in human milk may be influenced by multiple factors associated with the lactation process and the individuality of each mother–child dyad (Figure 4). Thus, concentrations may be affected by genetics, ethnicity, nutritional status, and maternal dietary intake [13, 15, 47, 56, 58]. Gomez-Gallego et al. [15] reported that the polyamine content in human milk varies according to the geographical region, being higher in Spanish and Finnish versus Chinese and South African mothers. Likewise, Muñoz-Esparza et al. [9, 57] found higher concentrations of polyamines in breast milk samples from Spanish compared to Mexican mothers (Table 1). These differences could potentially be explained by the link between geographical regions and factors such as ethnic origin, genetics, and maternal dietary patterns. The influence of the diet on polyamine levels in human milk has been investigated in only two studies. Atiya-Ali et al. [14] reported a positive association between the maternal intake of polyamines and concentrations in breast milk. Thus, higher consumption of vegetables in lactating mothers was significantly associated with higher spermidine contents in breast milk, whereas fruits, mainly oranges, were correlated with higher levels of putrescine. It was also observed that the polyamine content in breast milk increased in obese mothers participating in a nutritional intervention, correlating with higher consumption of vegetables and fruits [56]. It should be noted that spermidine is the major polyamine in most vegetables and fruits, although putrescine predominates in citrus fruits [25, 35]. Further research is needed to shed light on the association between maternal polyamine intake and the content in breast milk, for which information is still scarce. Finally, regarding the nutritional status of the mother, Atiya-Ali et al. [56] reported lower polyamine concentrations in the milk of mothers with obesity compared to those of normal weight without exploring the underlying reasons.

Figure 4.

Factors influencing the content of polyamines in human milk. Adapted from Muñoz-Esparza et al. [9].

The type of birth may also affect the constituents of breast milk. Two studies found a higher polyamine content in the milk of mothers of premature versus term newborns [14, 47]. Another potentially influential factor is the type of delivery. Gómez-Gallego et al. [15] reported that mothers who delivered their babies by cesarean section, as opposed to natural birth, produced milk with a significantly lower polyamine content, the same trend being observed by Muñoz-Esparza et al. [9]. Again, more research is needed, not only to confirm these results but also to analyze why the type of birth can affect polyamine concentrations.

Regarding breastfeeding, two studies carried out in 1992 by Romain et al. [41] and Pollack et al. [55] with breast milk samples from North American and Belgian mothers, respectively, found a progressive decrease in polyamines during the lactation period, with contents being up to 28% lower after six months. More recently, a reduction in polyamine content during the first semester of lactation was also described in milk samples from Mexican and Spanish mothers [9, 57]. In both studies, a significant decrease in spermidine (around 30%) and spermine (25%) was found, whereas putrescine remained practically unchanged during this period [9, 57]. According to Plaza-Zamora et al. [47], the higher polyamine content in human milk produced at the beginning of lactation could be related to the high cell growth rate and differentiation during the first months of life. In addition, the increasing catalytic activity of the PAO enzyme in breast milk along the lactation period could be responsible for the progressive reduction of polyamine concentrations [59]. On the other hand, the polyamine content in human milk may not only depend on the lactation phase but can also vary within a feed. Muñoz-Esparza et al. [9] found significantly higher concentrations (up to twofold), particularly of spermidine and spermine, in the hindmilk produced at the end of the feed compared to the foremilk, regardless of the month of lactation. The fact that hindmilk remains in the mammary gland for longer in the presence of active endo- and exopeptidases (enzymes responsible for breaking down milk peptides) could lead to a higher content of free amino acids [60, 61]. Sadelhoff et al. [61] reported that hindmilk has a higher content of arginine, an amino acid precursor of ornithine and, therefore, a key substrate for the endogenous synthesis of polyamines [37, 40]. The finding that the polyamine content is higher in hindmilk is further evidence of the importance of full feeds for infants so that they may benefit from all the nutrients available in human milk.

Finally, it has also been proposed that certain infections, such as mastitis, can potentially modify the polyamine content of breast milk and lead to the appearance of other biogenic amines [62]. Perez et al. [62] reported higher concentrations of putrescine and the presence of tyramine and histamine in the milk of mothers with mastitis compared to healthy mothers. Likewise, Muñoz-Esparza et al. [57] detected abnormally high putrescine concentrations (up to 183 nmol/dL) in a breast milk sample, as well as the unusual presence of histamine (170 nmol/dL) and cadaverine (2680 nmol/dL), which may have been due to mastitis in the mother, although the infection was not corroborated. Perez et al. [62] suggested that the increase in putrescine concentrations, and the appearance of other amines, could be largely attributed to the aminogenic activity of the infectious bacteria (e.g., staphylococcal, streptococcal, and/or corynebacterial species) responsible for the inflammatory process of the mammary gland that gives rise to mastitis.

6. Polyamine content and profile in infant formulas

The WHO recommends that infants be fed exclusively with human milk in the first six months of life and that breastfeeding should continue until two years of age with suitable complementary feeding [3]. However, feeding with infant formulas during the first year of life remains a common practice, either in combination with breastfeeding or as the sole food [63].

Infant formulas or human milk substitutes are designed to replace breast milk, either partially or totally [64]. In 1977, the nutritional composition of formulas was established by the Nutrition Committee of the European Society of Pediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) to achieve products that match human milk as closely as possible and cover all infant nutritional requirements [64]. To benefit infant health and nutrition, the formulation of these preparations has been modified over the years based on new knowledge about the composition of human milk and according to the updated standards of ESPGHAN and the American Academy of Pediatrics [65, 66, 67].

Table 2 shows the contents of putrescine, spermidine, and spermine in different types of infant formulas reported in the literature [13, 14, 41, 55, 57, 68, 69]. In general, both polyamine content and profile are highly variable among studies.

Infant formulas	Total polyamines	Putrescine	Spermidine	Spermine	References
First formula	136.0	94.0	27.0	15.0	[41]
	213.0	192.0	10.0	11.0	[41]
	20.5	1.8	18.7	Nd	[55]
	313.3	80.4	188.2	44.7	[55]
	78.4	23.9	39.4	15.1	[13]
	333.6	17.6	213.0	103.0	[14]
	441.3	359.6	51.6	30.1	[68]
	2599	964.0	923.0	712.0	[69]
	15.7	Nd	15.7	Nd	[57]
	59.5	36.9	22.6	Nd	[57]
	80.5	36.8	43.7	Nd	[57]
Follow-on formula	83.3	13.7	56.5	13.1	[13]
	354.1	11.0	280.0	63.1	[14]
	1970	749.0	482.0	739.0	[69]
	77.4	41.4	36.0	Nd	[57]
	67.6	35.2	32.4	Nd	[57]
	110.4	58.4	52.0	Nd	[57]
Preterm formula	144.4	105.7	21.5	17.2	[55]
	649.9	154.5	433.1	62.3	[14]
	128.9	40.8	27.0	61.1	[57]
	290.9	Nd	228.2	62.7	[57]
	72.5	39.1	33.4	Nd	[57]
Soy-based formula	361.0	84.0	233.0	44.0	[41]
	385.0	64.0	278.0	43.0	[41]
	381.9	74.0	307.9	Nd	[57]
	254.3	51.9	202.4	Nd	[57]
	378.5	74.9	303.6	Nd	[57]
Rice-based formula	206.5	190.3	16.2	Nd	[57]
	242.2	194.0	48.2	Nd	[57]
	360	306.3	53.7	Nd	[57]

Table 2.

Mean contents of polyamines in infant formulas (nmol/dL) reported in the literature.

Nd: not detected.

The polyamine contents of infant formulas for special medical use, made from vegetable proteins such as soy and rice, have only been analyzed in two studies [41, 57], detecting only putrescine and spermidine. Soy-based formulas stood out for their high concentrations of spermidine (202.4–307.9 nmol/dL), while putrescine was the major polyamine in the rice-based formulas (190.3–306.3 nmol/dL). As in formulas made from cow’s milk, the amount and type of polyamines in those based on vegetable protein are related to the content in the raw material. Accordingly, various authors have described spermidine as the major polyamine in soybeans and putrescine in rice [35, 70, 71, 72, 73]. In addition, various factors, such as the conditions of cultivation, the environment, and harvesting, can potentially modify the final polyamine content in vegetables and, consequently, in derived products such as infant formulas [35, 74, 75, 76].

Regardless of the great variability of polyamines in infant formulas and human milk, the polyamine content is much lower in the former (up to 60 times lower). Furthermore, the polyamine profile also differs between spermidine and spermine, predominating in human milk and putrescine in infant formulas. These data indicate that infant formulas should be improved in terms of qualitative and quantitative polyamine content to achieve a closer match with the composition of human milk.

7. Conclusion

Polyamines are relatively underrecognized as bioactive components of human milk, even though their physiological effects on the growth and maturation of the gastrointestinal tract and the development of the infant’s immune system have been extensively studied. Breast milk is the first exogenous source of polyamines in humans, and the content can be influenced by various factors related to the mother–child dyad and the lactation process itself. The geographical region, closely linked to maternal dietary patterns, seems to strongly influence the polyamine concentrations in human milk, mainly spermidine and spermine. If these findings are confirmed in further studies, promoting polyamine-rich foods in the maternal diet would be a useful strategy to increase concentrations in breast milk. On the other hand, polyamine levels clearly decrease along the lactation period, thought to be an adaptation to the decreasing requirements of the infant as the growth rate slows. It should also be highlighted that the levels of polyamines, mainly spermidine and spermine, change within a single feed, up to twofold higher in the hindmilk provided at the end of the feed compared to the foremilk. This information supports the importance of the infant fully emptying the breast to take advantage of all the nutrients available in human milk, including polyamines. Analysis of infant formulas reveals a clear difference in polyamine content and profile compared to human milk, with much lower levels, especially of spermidine and spermine. Therefore, these products need to be reformulated to achieve a profile that more closely resembles human breast milk.

Conflict of interest

The authors declare no conflict of interest.