Tracing Zinc’s Role in Preterm Infants’ Health: A Narrative Review

Abstract

Zinc (Zn) is a trace element involved in numerous physiological processes, including enzyme function, gene transcription, and cell signaling. Its importance is especially pronounced in preterm infants, who are at high risk of Zn deficiency due to disrupted transplacental transfer, high nutrient demands, and medical complications. The inherent risk of Zn deficiency in this population is further increased by poor Zn dietary intake. Human milk from preterm mothers contains low concentrations of Zn, although it is highly bioavailable. Additionally, the Zn content steadily declines from colostrum (first few days postpartum) to mature milk (>10–14 d postpartum). Formula milk contains higher Zn concentrations to compensate for nutrient losses during production and storage, and lower bioavailability compared with human milk, which is further decreased in case of high phytate content, such as in soy milk-based formulas. Zn supplements may prove useful in meeting the preterm infant’s needs, although caution is warranted regarding potential interactions with other nutrients within multinutrient supplements. Early detection of Zn deficiency is challenging due to the lack of reliable Zn status biomarkers, necessitating a high index of suspicion. Clinical signs of Zn deficiency can range from mild, nonspecific symptoms to severe, multisystem involvement. Chronic deficiency may lead to failure to thrive. Zn supplementation can support growth and mitigate comorbidities in preterm infants, although variability across studies complicates efforts to establish optimal dosing, and define safety and long-term effects. Although rare, Zn toxicity in preterm infants should not be overlooked, especially in infants on long-term parenteral nutrition. This narrative review aimed to consolidate existing knowledge and identify research gaps, highlighting the critical role of Zn in supporting preterm infants’ health. Further research is needed to establish evidence-based practices to improve health outcomes in this vulnerable population.

Statement of Significance.

This review summarizes and consolidates current knowledge on the critical role of zinc in preterm infants’ health, highlighting the specific challenges unique to this population and underscoring the need for further studies to establish optimal zinc supplementation strategies. By providing an in-depth exploration of this important topic, this review aimed to support pediatricians and neonatologists in their clinical practice, hopefully improving health outcomes for these vulnerable infants.

Introduction

Zinc (Zn) is the second most abundant trace element in the body, after iron. It is ubiquitous in all tissues and body fluids, and predominates in skeletal muscles and bones, followed by skin and liver. The continuous turnover of Zn makes it available for use elsewhere. Accordingly, Zn plasma concentrations are regulated within a dynamic equilibrium [1].

Zn is classified as a micromineral, required in amounts <100 mg/d, similarly to iron and selenium. Nevertheless, Zn plays crucial catalytic, structural, and regulatory roles. Zn serves as an integral component of enzymes essential for the metabolism of carbohydrates, fats, and proteins, as well as for the clearance of reactive oxygen species. Zn supports the structural integrity of proteins, cell membranes, nucleic acids, and ribosomes. Furthermore, Zn plays a crucial role in facilitating anabolic processes, including growth, tissue maintenance, and wound healing. Additionally, Zn regulates gene transcription, as well as cell signaling, hormone release, and apoptosis [[1], [2], [3]].

Because of these fundamental biological roles, Zn is a recognized important nutrient in the pediatric age [4], especially for preterm infants (i.e., those born before the 37th week of gestation) [5].

Preterm infants are particularly vulnerable to Zn deficiency due to many factors, including their physiological immaturity and increased nutritional requirements [5]. The nutritional standard of care for preterm infants follows a biological hierarchy: mother’s own milk (appropriately fortified), donor human milk (DHM) (appropriately fortified), and formula milk. These feeding options vary in Zn content and bioavailability, so it is important to consider the feeding method when estimating daily Zn intake and establishing the optimal supplementation strategy.

Despite its significance, Zn supplementation practices and optimal dosages for preterm infants remain areas of ongoing debate and investigation [6]. Indeed, there is a notable gap in the evidence regarding the impact of supplemental Zn on preterm infant health that needs to be bridged to better understand its potential benefits and risks [7].

Aim of this narrative review was to consolidate existing knowledge, identify gaps in research, and raise awareness on the critical role of Zn in supporting preterm infants’ health, in order to inform clinical practice and ultimately improve outcomes for this vulnerable population.

Zn Requirements in Preterm Infants

The optimal micronutrient requirements for preterm infants are poorly defined [6]. Although breast milk serves as a benchmark for nutrient adequacy in healthy full-term infants, the application of such a standard to preterm infants is not as straightforward [6]. Moreover, questions have been raised concerning the extent to which the micronutrient content of breast milk reflects maternal intake rather than actual infants’ requirements [8]. In the absence of reliable longitudinal data, the nutrient content of human milk can only provide a rough estimate of preterm infants’ nutritional requirements.

In estimating mineral requirements for preterm infants, intrauterine accretion rates are often utilized, reflecting the pace of fetal mineral accumulation following transplacental transfer during gestation. Initial estimations suggested an intrauterine Zn accretion rate of ∼250 μg/kg/d during the third trimester [9]. However, subsequent research proposed higher accretion rates, ranging from 850 μg/kg/d at 28 wk to 354 μg/kg/d at 34 wk postconceptional age [2]. Using a factorial method considering fetal accretion and hepatic storage release, dietary Zn requirements have been estimated at ∼500 μg/kg/d at 27 wk, 400 μg/kg/d at 30–32 wk, and 200–300 μg/kg/d at 35–40 wk postconceptional age [10]. These estimates reflect the minimum intake necessary to replace endogenous Zn losses and meet tissue needs.

On the basis of these theoretical bases, international nutritional recommendations for parenteral and enteral Zn supplementation for preterm infants have been defined (Table 1) [[11], [12], [13], [14], [15], [16], [17], [18], [19]].

TABLE 1.

Recommendations on enteral and parenteral Zn supplementation in preterm infants across the world.

Recommendations	Enteral	Parenteral
WHO [11]	1–3 mg/kg/d	–
ESPGHAN [12,13]	2–3 mg/kg/d	0.4–0.5 mg/kg/d
ASPEN [14]	1–3 mg/kg/d	0.4–0.5 mg/kg/d
AAP [15]	1.4–2.5 mg/kg/d	0.4 mg/kg/d
AuSPEN [16]	–	0.4 mg/kg/d
Turkish Neonatal Society [17,18]	1–2 mg/kg/d ELBW: 3 mg/kg/d	–
CSPEN [12,19]	–	0.4–0.45 mg/kg/d

Abbreviations: AAP, American Academy of Pediatrics; ASPEN, American Society for Parenteral and Enteral Nutrition; AuSPEN, Australasian Society of Parenteral and Enteral Nutrition; CSPEN, Chinese Society of Parenteral and Enteral Nutrition; ELBW, Extremely Low Birth Weight; ESPGHAN, European Society for Paediatric Gastroenterology Hepatology and Nutrition.

Assessment of Preterm Infants’ Zn Status in Clinical Practice

Assessing the Zn status of preterm infants is essential to prevent potential deficiencies.

Zn can be measured on different matrices in preterm infants (Table 2) [[20], [21], [22], [23]].

TABLE 2.

Pros and cons of different sample matrices for the assessment of Zn status in preterm infants.

Sample matrix	Pros	Cons
Blood
Serum/plasma	-Responds to Zn intake - Established reference ranges for premature infants [20,21]	-Influenced by inflammation, albumin concentrations, hemolysis, metabolic state (anabolic or catabolic), hemodilution, time of day -Possible Zn contamination from contact with gel separators, rubber, and heparin (i.e., specific pre-approved collection tubes must be used) -Unable to detect marginal Zn deficiency [1]
Cells (erythrocytes, leukocytes)	-Erythrocytes have Zn concentrations 5–10-fold higher than plasma -Leukocytes contain ≤25 times more Zn than erythrocytes and have a shorter lifespan, making them potentially more sensitive to changes in Zn intake	-Large blood volumes required for analysis -Leukocyte subpopulations have varying Zn contents (monocytes > lymphocytes > neutrophils) -Difficult separation of specific leukocytic components from other white blood cell types [22]
Urine	-Noninvasive -During deficiency, urinary Zn concentration decreases due to adaptation. An inappropriately increased concentration suggests a urinary route of loss	-Not a sensitive biomarker of low Zn intakes -Affected by catabolic state, drugs, and amino acid intake -24-h urine collection necessary but difficult in preterm infants [1,22]
Hair	-Higher Zn concentrations than in blood and urine, facilitating measurement - Not influenced by inflammation or circadian rhythms	-Does not reflect recent Zn intake, but rather the quantity of Zn available to the hair follicles at the time of growth (not at the time of sampling) -Inconsistent response to supplemental Zn intake [22] -Difficult interpretation due to varying hair growth rate and environmental contamination
Meconium/stool	-Noninvasive -The gastrointestinal tract is the major site of Zn losses -Endogenous fecal Zn plays a crucial role in regulating Zn homeostasis -Zn concentrations in meconium increase with gestational age [23]	-Challenges in collection -Lack of validated and standardized measurement methods

Although only 1% of the total body Zn is present in circulating blood, plasma or serum Zn concentration is the most widely used biomarker in clinical practice and the only laboratory test recommended for the evaluation of Zn status [22]. The 2 different blood matrices display excellent concordance, despite serum Zn concentrations often resulting higher due to differences in time between collection and separation [24,25]. Although it serves as a good indicator of Zn intake in the absence of systemic inflammation, serum/plasma Zn does not reflect the total body Zn content, which is mostly involved in metabolic activities and not readily available for exchange. Unlike other biomarkers, a reference range for serum Zn concentrations in preterm infants is available (74–146 μg/dL) [20,21]. Despite the aforementioned limitations of serum/plasma Zn, this reference range still provides a standardized baseline for assessing and comparing Zn concentrations in preterm infants, guiding supplementation strategies, and monitoring their effectiveness. Its clinical utility is further enhanced when combined with clinical observations (e.g., growth and skin lesions), and other biomarkers, as part of a comprehensive evaluation.

Surrogate measures of Zn status have been proposed.

Alkaline phosphatase (ALP) is a Zn-dependent metalloenzyme typically used in preterm infants as a biomarker of metabolic bone disease. A reduction in ALP activity may indicate Zn deficiency [26,27]. No consistent effect of Zn intake on ALP activity has been demonstrated in preterm infants [28,29]. To date, ALP activity is not considered a useful marker of Zn status [22,30].

Because of Zn deficiency resulting in increased copper absorption, hypercupremia and an elevated Cu:Zn ratio have been suggested as biochemical markers of Zn deficiency [1,31].

Previous studies have shown that insulin-like growth factor-1 (IGF-1) concentrations decrease in Zn deficiency [32] and increase upon Zn supplementation [33]. This suggests that IGF-1 could serve as a marker for Zn status. The fact that lower IGF-1 concentrations are also seen in other conditions, such as growth hormone deficiency and severe illness, may limit its diagnostic specificity.

Stunting may be considered a proxy indicator of Zn status in preterm infants. Because Zn does not have a pharmacologic effect on growth (i.e., it does not directly stimulate or promote growth in non-Zn-deficient individuals), a growth response to Zn supplementation may indicate a pre-existing Zn deficiency, albeit not excluding other growth-limiting etiologies [22].

Currently, no single test reliably reflects whole-body Zn status in preterm infants. A combination of different approaches may be necessary to obtain a comprehensive understanding of an individual infant’s Zn status. Further research is needed to establish reliable and practical methods for assessing Zn status in preterm infants.

Non-dietary Causes of Zn Deficiency in Preterm Infants

Several studies have shown that preterm infants may experience a negative Zn balance during the first months of life [34,35]. Numerous factors could contribute to this phenomenon (Figure 1).

FIGURE 1.

Graphical representation of the main causes that may lead to Zn deficiency in preterm infants.

First, Zn accretion rates peak during the third trimester of gestation [2,9]; thus, a premature delivery inevitably reduces Zn body pool. Additionally, preterm infants’ rapid growth and heightened metabolic demands necessitate increased Zn intake [9,36]. However, preterm infants’ short small intestine length poses challenges in meeting these augmented needs, due to reduced absorptive capacity [37]. Moreover, urinary excretion of Zn in preterm infants has been observed to be notably high, gradually decreasing with postmenstrual and postnatal age [35]. Given its importance, Zn provision is recommended from the first day of parenteral nutrition (PN) in preterm infants requiring it, even in cases of supplemental (i.e., non total) PN and/or short-term courses [[1], [15], [17]]. However, the prolonged utilization of total PN may further aggravate the risk of Zn deficiency in preterm infants [38,39] due to insufficient Zn inclusion in parenteral formulations, inadequate provision relative to growth and utilization rates, and heightened urinary losses. Of note, most commercial intravenous trace element packages provide insufficient Zn for the preterm population and necessitate the separate addition of intravenous Zn to meet the recommended intakes. Unfortunately, the commercially available multitrace element mixtures have limited flexibility for meeting individual requirements. Consequently, in some patients at higher risk of Zn deficiency (e.g., extremely preterm or surgical newborns), sufficient provision of Zn may result in excessive delivery of other minerals such as manganese [40]. Although under optimal conditions bioavailability of parenteral Zn should be close to 100%, only ∼60% of parenterally infused Zn is retained [5,41]. A substantial portion of infused Zn is excreted in urine, with augmented losses attributed to specific amino acids supplemented in PN [1,12]. Additionally, the possibility of adherence to lines or formation of precipitates in the bag should not be overlooked.

Finally, comorbidities commonly observed in preterm infants can exacerbate the risk of Zn deficiency.

Sepsis may increase Zn urinary losses due to the associated muscle catabolism [1]. During acute phase responses or overt sepsis, Zn redistribution from the vascular compartment into cells may result in low plasmatic Zn concentrations, although not necessarily indicating an altered whole-body Zn status [1,42].

Preterm infants who underwent gastrointestinal surgery may face heightened risk of Zn deficiency, particularly if enteral losses are elevated, such as in the case of high-output ileostomies [12,43].

A recent prospective observational study identified bronchopulmonary displasia (BPD) as a risk factor for low serum Zn concentrations at term age in preterm infants [44].

Iatrogenic effects from prolonged use of medications should also be considered: thiazide diuretics may increase Zn urinary excretion [1], whereas glucocorticoids may interfere with Zn absorption [41].

Dietary Intake of Zn in Preterm Infants

Zn homeostasis is finely regulated through the absorption of dietary Zn, secretion and reabsorption of endogenous Zn, and excretion.

In humans, Zn is absorbed throughout the intestines, primarily in the duodenum and jejunum [45,46]. Studies have shown that the mechanisms regulating Zn absorption mature early, as both preterm and term infants demonstrate absorptive capacities comparable with adults when adjusted for length of the small intestine [37]. Zn absorption is a saturable process [47]. It is enhanced by specific amino acids and organic acids, and is closely related to fat absorption. Conversely, absorption is inhibited by phytates, calcium, and iron [1,5,41,48] (Table 3) [[48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70]].

TABLE 3.

Overview of the impact of different nutrients on Zn absorption.

Nutrient	Nutrient impact on zinc absorption		Zinc impact on nutrient metabolism
Iron		Iron at high Fe:Zn ratio and in water solutions limits Zn absorption [48].	High Zn doses in water solutions may reduce iron absorption [4].
Copper		Modestly increased copper intake does not interfere with Zn absorption if Zn intake is adequate [49], although competitive inhibition at intestinal binding sites is possible.	Excess Zn can interfere with the absorption of copper, although its clinical impact in premature infants is debated [50].
Calcium		Calcium forms insoluble complexes with phytates and Zn, increasing the inhibition of Zn absorption [48,51].	A high Zn intake can competitively decrease calcium absorption in the case of low calcium intake [52].
Phosphorus		Phosphorus reduces Zn absorption in animals but not in humans [53].	–
Vitamin A		Severe vitamin A deficiency may hinder Zn absorption due to reduced synthesis of an ileal Zn-binding protein [54].	Poor Zn status affects vitamin A utilization, as Zn-containing proteins are needed for its release from the liver and tissue metabolism [4]
Vitamin B9 (folate)		Despite previous studies suggesting that folate could impair Zn absorption due to insoluble chelate formation [55,56], more recent evidence seems to indicate otherwise [57].	Although a Zn-dependent enzyme is required for folate absorption, a negative effect of Zn deficiency on folate absorption has not been demonstrated [57].
Vitamin C		Vitamin C has no effect on Zn absorption [58,59]: unlike iron, Zn does not need to change its oxidation state for intestinal uptake.	–
Vitamin D		In vitro studies showed that vitamin D can enhance Zn absorption by regulating its transporters [60].	Zn is needed for vitamin D to exert its biological activity [61].
Lactose		Lactose in formula milk does affect Zn absorption [6,62], despite previous reports [63].	–
Fibers (cellulose)		Cellulose does not impact Zn absorption [64]. The previously reported negative impact of fibers on Zn absorption was likely due to the co-presence of phytates [48].	–
Proteins		Total protein intake enhances Zn absorption [48,65], although individual proteins may act differently (see below).	–
Casein		Casein tightly binds Zn reducing its absorption [41].	–
Free amino acids (histidine, methionine, and cysteine)		These amino acids chelate Zn and are able to dissociate it from Zn-calcium-phytate complexes, facilitating its absorption [66,67].	–
Medium-chain triglycerides		MCTs enhance Zn absorption [68].	-
EFAs (linoleic acid)		EFAs enhance Zn absorption, probably after conversion to PGs [69,70].	Zn may be necessary for processes involved in EFA metabolism [69].
Citrate		Citrate is an efficient Zn chelator [4] increasing Zn solubility. Its high concentration in HM is thought to contribute to Zn higher bioavailability.	–
Phytates		Phytates form strong insoluble intraluminal complexes with Zn, especially at high phytate:Zn ratios. The complexes are then excreted in stool due to the low phytase activity in the human gastrointestinal tract [48].	–

Abbreviations: EFAs, essential fatty acids; HM, human milk; MCT, medium-chain triglycerides; PGs, prostaglandins; Zn, zinc.

Red, inhibition of absorption; green, enhancement of absorption; yellow, no effect on absorption.

The fractional absorption of Zn is inversely related to Zn intake [71,72], meaning that lower intakes result in greater absorptive efficiency. Conversely, fractional absorption does not seem to be related to concurrent Zn status [73,74], although conflicting results have been reported in the literature [[75], [76], [77]].

The primary route of Zn excretion is intestinal, whereas Zn urinary losses are minimal except during the first 5 wk after birth, in preterm infants and in infants receiving Zn-supplemented PN [1,5,41]. Zn is efficiently excreted into bile and partially reabsorbed through enterohepatic circulation [1]. The amount of endogenous Zn (i.e., originating from the body and not the diet) excreted via the intestine correlates positively with the amount of absorbed Zn and the host’s Zn status, with endogenous fecal losses increasing with higher total absorbed Zn over prolonged periods [71,73,78]. Gestational age does not seem to affect the capacity to retain endogenous Zn [10,78].

The dietary intake of Zn is often poor in preterm infants, further increasing the inherent risk of Zn deficiency in this population. The different sources of dietary Zn for preterm infants are detailed below.

Human milk

Human milk, when properly fortified, is the gold standard for preterm infant nutrition.

Several studies have investigated the differences between preterm and term human milk in terms of Zn concentrations, although reporting conflicting results. In 2014, a comprehensive literature review on human milk micronutrient composition conducted on behalf of the European Food Safety Authority (EFSA) reported that Zn concentrations of human milk from mothers of term infants were 5 times higher than those from mothers of preterm infants [79]. More recently, a prospective cohort study on Swiss mothers found that Zn concentrations in human milk from mothers of preterm infants were on average 62% lower compared with those of term mothers, at equivalent postmenstrual age [80]. Such difference disappeared when correcting for postnatal age (i.e., stage of lactation) [80]. Another recent study confirmed that preterm human milk had lower Zn concentrations compared with full-term human milk [81]. The samples from preterm mothers were of mature milk [81].

The stage of lactation is a crucial factor to consider when analyzing and comparing Zn content in human milk [82]. A systematic review has recently reported no significant difference in Zn concentrations between preterm and term human milk, unless the time because birth was considered as a covariate, in which case preterm human milk showed higher Zn concentrations [83]. The higher Zn concentrations in preterm human milk compared with term human milk at the same stage of lactation may be a physiological adaptation to support the higher growth rates of preterm infants, while also compensating for the reduced milk volumes and potentially lower intakes compared with term infants at the same postnatal age [84,85]. Because of the conflicting results reported in the literature, further research is needed to clarify this aspect.

The decline in Zn concentrations from colostrum to mature milk is substantial (8.6 mg/L compared with 1.5 mg/L) [79], both in term and preterm human milk [83,86]. Considering human milk volume, it was estimated that exclusively breastfed infants receive ∼4 mg Zn per day during the first few days of life, which rapidly declines to ∼1.75 mg/d by 1 mo and further drops to <1 mg/d from 3 to 6 mo [82]. Consequently, human milk alone does not meet the Zn requirements for infants weighing <1500–2000 g at birth even when full enteral feeding (i.e., 150 ml/kg/die) is reached [13].

Genetic factors may also play a role in low Zn concentrations in human milk. Indeed, mutations in the mother’s SLC30A2 gene may affect the function of the Zn transporter ZnT2 responsible for Zn transfer into human milk, resulting in severe Zn deficiency in infants fed such milk [87].

Conversely, Zn content in human milk has been repeatedly shown to be independent from maternal Zn dietary intake [88,89] or status [90], thus limiting the possibility of intervening on the mother to enhance delivery of Zn to the newborn. However, a recent study by Han et al. [20] demonstrated that the daily administration of an intervention supplement containing 10 mg Zn glycinate chelate initiated before conception and continued throughout pregnancy increased subsequent human milk Zn concentrations compared with a control supplement without Zn. This effect was especially pronounced at 6 wk and persisted throughout the first 3 mo of lactation. Nevertheless, Zn concentrations in human milk declined over time, reaching a nadir at 12 mo (end of follow-up) in both the Zn-supplemented group and the control group.

In the absence of mother’s own milk, DHM is the second-best option [91] for preterm infants. DHM is typically pooled from samples of mature milk from mothers who delivered at term, and subsequently pasteurized. A recent study that analyzed the nutrient content of 15 DHM samples provided by 7 commercial milk banks found low Zn concentrations in DHM (2.3 ± 0.7 mg/L) [92]. Moreover, it should be considered that pasteurization of human milk significantly decreases fat absorption in preterm infants [93], which, in turn, may limit endogenous Zn reabsorption [45].

Fortification of human milk, both mother’s own milk and DHM, is a frequently employed measure to promote adequate development of preterm infants [94,95]. In a study by Bhatia et al. [96], fortification of preterm human milk (2–4 wk of lactation) with a liquid fortifier resulted in mean increases of 278% in milk Zn concentration, whereas fortification with a powder fortifier resulted in mean increases of 149%. However, losses due to delivery by continuous infusion were considerable, ranging from 8% for the liquid fortifier to 26% for the powder fortifier. Unless efficient agitation is provided, the lengthy administration via syringe-pump feedings may contribute to the loss of nutrients by precipitation or adherence to the walls of the delivery system [96].

Venkataraman et al. [97] conducted a prospective study to assess the impact of a human milk fortifier providing an additional 0.8 mg/dL of Zn on serum Zn concentrations in preterm infants. The study found no significant differences in serum Zn concentrations between preterm infants fed fortified compared with unfortified human milk. Similarly, another study [98] observed that, although feeding very low birth weight (VLBW) newborns human milk fortified with a Zn-containing fortifier doubled their Zn intake, there was no difference in serum Zn concentrations compared with newborns who received human milk fortified with a fortifier that did not contain Zn.

Formula milk

Zn concentrations in formula milk are ∼10 times higher than those in mature human milk to account for nutrient losses during production and storage, as well as reduced bioavailability [99].

Zn in cow milk-based formulas is poorly absorbed (30% [100]) likely due to its tight binding to casein [101]. As for soy milk-based formulas, the high phytate content delays Zn absorption (14% [100]) due to the formation of insoluble complexes with calcium [41,102].

As for all divalent cations, concern exists about potential interactions with formula milk iron fortification. Friel et al. [103] showed that in preterm formula a Zn:Fe ratio of 4:1 (i.e., much higher than most infant formulas) did not interfere with iron incorporation in red blood cells, nor, presumably, its intestinal absorption. However, competitive inhibition of Zn uptake by excess iron (Fe:Zn ≥ 2:1) has been documented [104], thus underscoring the importance of balanced mineral supplementation in infant formula.

Increased Zn net absorption has been reported in very preterm infants consuming medium-chain triglycerides-enriched formulas [68].

Zn supplements

Zn for preterm infants is also available in oral supplements containing Zn alone, Zn in combination with other ingredients, or within multivitamin/mineral products. Different Zn chemical forms have different bioavailabilities [[105], [106], [107]]: Zn sulfate, acetate, citrate, and gluconate are generally well absorbed, whereas Zn carbonate and Zn oxide are relatively insoluble in aqueous solutions, thus poorly absorbed. Zn citrate has a better palatability than Zn sulfate and Zn acetate (which have a strong metallic, bitter, and astringent taste) and is less expensive than Zn gluconate, while being comparably well absorbed [106]. Recent research on in vitro and animal models showed greater bioavailability of Zn bys-glycinate, an amino acid chelate consisting of 2 equivalents of glycine bound to 1 equivalent of Zn, compared with Zn sulfate [107,108].

Reciprocal interactions within multimicronutrient supplements are to be considered (Table 3). Similar physical and chemical properties may lead to competitive interactions with other minerals, potentially diminishing Zn absorption [48] with a dose-dependent effect. Despite earlier reports suggesting that high folate intakes may affect Zn status in preterm infants [55], possibly by reducing Zn absorption through the formation of insoluble chelates [56], more recent evidence seems to indicate otherwise [57].

Dietary Zn bioavailability

Despite the low concentrations [87,00], Zn fractional absorption from preterm human milk is ∼50%–60% [41,78,100], thus higher than that from fortified preterm human milk (36%), term formula milk (24%), and preterm formula milk (14%) [41,78,100]. The higher concentration of Zn-citrate complexes in human milk compared with cow milk has been suggested as a factor contributing to its greater bioavailability [100].

Preterm infants fed human milk exhibit significantly lower Zn fecal excretion and endogenous fecal losses compared with those fed preterm formula, term formula, or fortified preterm human milk [78].

Despite substantial differences in Zn concentration across different dietary sources, the net retention of Zn remains similar due to the varying bioavailability [41] (Table 4) [68,78,80,92,100,107,109].

TABLE 4.

Overview of the main dietary sources of Zn for preterm infants

Dietary source	Zn content	Fractional absorption	Comments
Human milk
Mother’s own milk	Term: 3.2 mg/L Preterm: 2.4 mg/L [80]	50%–60% [78,100]	Stage of lactation greatly influences Zn content. Maternal SLC30A2 mutations may reduce Zn transfer. Citrate binding may facilitate Zn absorption.
DHM	2.3 mg/L [92]	–	Pasteurization may indirectly affect Zn absorption.
Fortified preterm HM	10.8 mg/L [109]	36% [78]	Caution if administered via lengthy infusions due to adherence to lines or precipitation.
Formula milk (Cow milk-based)	Term: 4–5 mg/L Preterm: 10–11 mg/L	Term: 24% Preterm: 14% [78]	Greater Zn content compared with HM, but lower absorption and greater excretion. Casein binding may reduce Zn absorption. Cow milk’s casein has more Zn-binding phosphate groups than HM.
Soy-milk-based	3.8 mg/L [100]	14% [100]	Phytate binding may reduce Zn absorption.
MCT-enriched	–	–	Increased Zn absorption [68].
Supplements	1100–1500 mg/L	1%–9%1 [107]	Always check chemical form. Consider competitive interactions in multimineral supplements (especially iron and copper).

Abbreviations: DHM, donor human milk; HM, human milk; MCT, medium-chain triglycerides; Zn, zinc.

If not otherwise specified, the data are derived from the main infant formulas and supplement formulations available on the market.

¹Bioaccessibility.

Consequences of Zn Deficiency in Preterm Infants

Early detection of Zn deficiency can represent a diagnostic challenge. Clinical signs of Zn deficiency generally appear ∼3 mo of age [36]. Increasing evidence indicates that clinical manifestations may result from both severe and marginal Zn deficiency, a condition often overlooked due to the lack of reliable Zn status biomarkers [1,110]. Homeostatic mechanisms initially maintain plasma Zn concentrations within the reference range, further limiting early detection [1]. The coexistence of other nutrient deficiencies in the same patient may complicate the clinical picture [45].

Zn deficiency may manifest across a wide spectrum of severity, ranging from mild, nonspecific, symptoms to overt clinical manifestations. The diverse clinical manifestations of Zn deficiency depend on the pleiotropic role of Zn in the human body. If the condition is left untreated, it may progress to a multisystemic involvement.

In preterm as well as term infants, the main symptoms associated with Zn deficiency are skin lesions, diarrhea, and hair loss [26,[111], [112], [113]]. Cutaneous manifestations, mainly acrodermatitis (i.e., skin changes primarily affecting the extremities, such as hands, feet, and diaper area), are caused by compromised skin integrity due to decreased keratinocyte proliferation and survival [114]. Rapid resolution of symptoms following Zn supplementation underscores its etiologic role. Zn supports keratin production and is therefore partly responsible for the integrity and thickness of hair fibers. Consequently, its deficiency is also associated with hair thinning, increased shedding [115], and sparse eyebrows [26], even in preterm infants. Specific electron microscopic hair features have also been described [116].

Diarrhea is caused by disrupted turnover of epithelial cells, leading to impaired barrier function and increased intestinal permeability. Importantly, diarrhea increases Zn losses, resulting in a vicious cycle of worsening deficiency.

Chronic Zn deficiency can lead to failure to thrive, allegedly as a consequence of decreased IGF-1 concentrations [117].

Zn contributes significantly to immune regulation, with deficiency linked to reduced humoral and cellular immune responses [118], potentially leading to increased susceptibility to infections. A direct association between Zn deficiency and increased incidence of sepsis in preterm newborns is yet to be confirmed [119].

In a recent observational cross-sectional study on 360 preterm newborns born at >28 wk gestational age, serum Zn concentrations <70 μg/dL were found to be an independent risk factor for retinopathy of prematurity (ROP) [120], although further studies are needed to confirm these results.

Benefits of Zn Supplementation in Preterm Infants

Several studies have highlighted the potential benefits of Zn supplementation in preterm infants.

A recent systematic review [121] reported that enteral Zn supplementation in preterm or low birth weight (LBW, i.e., with a birth weight <2500 g) infants was associated with increases in weight (mean difference of 378.6 g), length (mean difference of 2.9 cm), and head circumference (mean difference of 0.56 cm). A separate systematic review [122] concluded that Zn supplementation may enhance weight gain and linear growth in preterm infants ≤6 mo corrected age, with pooled estimates showing a mean difference of 0.50 z-score for weight and 1.12 z-score for linear growth. The effect of Zn supplementation on weight was more pronounced in studies where doses exceeded 3 mg/kg/d, the maximal recommended daily dose for preterm infants. When supplementing preterm newborns with doses that significantly exceed the estimated physiological needs for tissue accumulation (as indicated by the recommended daily doses), potential toxicity must be carefully considered [6]. Although studies using Zn doses >3 mg/kg/d have not reported adverse effects [123,124], the risk:benefit ratio of administering Zn doses higher than internationally recommended should be thoroughly evaluated in adequately powered randomized controlled trials to ensure both safety and efficacy.

An effect of Zn supplementation on head circumference was only found to be small for gestational age preterm infants [122].

Regarding preterm-associated comorbidities, Terrin et al. [123] reported a lower incidence of a composite outcome measure of sepsis, BPD, periventricular leukomalacia, and ROP, and a lower incidence of necrotizing enterocolitis in VLBW infants (i.e., with a birth weight <1500 g) receiving high-dose Zn supplementation (10–11 mg/d, i.e., approximately 6–8 mg/kg/d) from day 7 of life to discharge or 42 wk postconceptional age compared with the control group, who received 1 mg/kg/d. Concerns have been raised [6] with regards to the high doses of Zn supplemented to the intervention group and the validity of comparing outcomes between infants receiving insufficient compared with pharmacologic doses of Zn.

Zn supplementation has shown promise in reducing serum bilirubin concentrations in preterm infants, despite conflicting results [125,126]. A reduced incidence of diarrhea has been reported in preterm and LBW infants [121].

Regarding neurodevelopmental outcomes, Zn supplementation has shown mixed effects on mental and psychomotor development scores, with some studies reporting improvements in motor development scores, whereas others showing no significant effects or even a decrease in mental development scores [121,122]. Comparability between studies is often impaired by the use of different instruments and outcome measures.

Evidence on other outcomes such as mortality, acute respiratory infections, sepsis, and hospitalization has been found to be of low certainty [121].

Despite the promising findings, further large-scale randomized controlled trials are needed to confirm the benefits of Zn supplementation in preterm infants, define optimal dosages, timing, duration, and safety of supplementation.

Zn Toxicity in Preterm Infants

Because of the limited data on excessive intake in children, tolerable upper intake levels (ULs) of Zn for this age group have been either estimated through extrapolation from adult data using metabolic rate or relative surface area (WHO and EFSA), or based on a No Observed Adverse Effects Level derived from few pediatric studies [United States Institute of Medicine (IOM) and International Zinc Nutrition Consultative Group (IZiNCG)] [127].

EFSA has not yet established ULs for infants [128], whereas WHO [129,130] and IZiNCG [131] have not provided ULs for children under 6 mo. IOM has set a UL of 4 mg/d for infants in the first 6 mo of life [132]. No ULs have been established for preterm infants.

Adverse effects from excessive Zn intake in infants are infrequently reported [133]. Zn is not stored in the body, and excess intake results in reduced absorption and increased excretion [45]. Efficient excretion into bile and intestinal secretions mitigates toxicity risks.

In adults, the symptoms of acute toxicity typically mimic those of food poisoning (i.e., vomiting, epigastric pain, abdominal cramps, and diarrhea). The literature documents only 1 reported case of acute Zn toxicity in a preterm newborn [134]. A 26-wk gestational age newborn received a lethal intravenous Zn dose due to a clerical error, where Zn was dosed in milligrams instead of micrograms, resulting in a 1000-fold overdose. Despite receiving edetate calcium disodium (calcium EDTA) chelation therapy, the newborn did not survive. The cause of death was determined to be cardiac failure due to Zn intoxication [134].

Long-term home PN with high-dose Zn supplementation may pose a risk for chronic Zn toxicity in infants. Concerns arise from the potential adverse effect of Zn on copper absorption, with a recommended Zn:Cu ratio <20:1 for preterm infants [135]. Zn supplementation can reduce serum copper concentrations by inducing intestinal metallothionein, which binds copper and inhibits its absorption [136]. This can potentially lead to copper deficiency, resulting in pancytopenia and neuropathy [137]. The impact of Zn supplementation on copper concentrations in preterm infants, particularly those with hypozincemia [50], remains unclear. Regular monitoring of Zn concentrations alongside copper concentrations, and complete blood count assessments, is advised for early detection of toxicity in patients on long-term PN.

Further research is needed to enhance our understanding of the potential toxic effects of high Zn doses in preterm infants, which may lead to a reevaluation of current ULs.

In conclusion, the combination of disrupted transplacental transfer, heightened nutrient demands, frequent medical complications, and feeding difficulties predisposes preterm infants to Zn deficiency. Although severe manifestations are rare, subtle clinical signs due to marginal Zn deficiency often go undiagnosed. Given the absence of a reliable single marker for Zn status, a comprehensive assessment of the preterm infant is recommended, integrating clinical observations (e.g., growth pattern and skin lesions) with biological markers (serum Zn, copper, and ALP). Evaluating the mode of feeding should be an integral part of assessing preterm infants’ Zn needs, considering the varying Zn content and bioavailability in different dietary sources (i.e., human milk, DHM, and formula milk). Although human milk remains the gold standard for preterm infant nutrition, clinicians should be aware of the potential need for Zn fortification and oral supplements, particularly given the low and rapidly declining Zn content in human milk. Although formula milk contains higher Zn concentrations than human milk, its lower bioavailability must be considered. Clinicians should not assume that total PN eliminates the risk of Zn deficiency in preterm newborns. Conversely, they should be vigilant for the apparently paradoxical risk of Zn deficiency associated with PN. Monitoring of Zn status is essential for preterm infants on long-term PN to ensure that adequate Zn concentrations are maintained.

Zn supplementation has shown promise in supporting optimal growth and preventing comorbidities in preterm infants. However, further research is needed to establish the optimal dosages and evaluate the efficacy of Zn supplementation, thus facilitating the adoption of evidence-based practices in neonatal intensive care units. Although Zn toxicity is rare, it is important to adhere to current dosage recommendations until reevaluations of the ULs are informed by a growing body of adequately powered randomized controlled trials.

Author contributions

The authors’ responsibilities were as follows – AC: designed and conducted the research, and wrote the paper; AC, CA: had primary responsibility for final content; RC, RO, MLG: supervised the research; and all authors read and approved the final manuscript.

Conflict of interest

The authors report no conflicts of interest.

Funding

This study was partially funded by Italian Ministry of Health, Current research IRCCS.

Data availability

N/A.

Declaration of Generative AI and AI-assisted technologies in the writing process

N/A.