Amino acids and lean mass accretion in preterm neonates

Abstract

Purpose of review

This review evaluates recent evidence on how total protein and energy intake, amino acid composition, and nutrient delivery modality influence protein synthesis in preterm neonates, with the goal of informing nutritional strategies to support optimal lean mass accretion and growth.

Recent findings

Preterm neonates exhibit anabolic resistance to feeding due to impaired insulin and amino acid signaling in skeletal muscle. Enteral nutrition, especially fortified human milk, supports better lean mass accretion and neurodevelopment than parenteral nutrition. Intermittent bolus feeding and pulsatile leucine supplementation during continuous feeding further enhance mTORC1-dependent translation initiation signaling and protein synthesis efficiency in skeletal muscle, offering promising strategies to optimize lean tissue growth.

Summary

Strategies that prioritize early enteral feeding, appropriate milk fortification, and nutrient delivery patterns that mimic feeding pulsatility may overcome anabolic resistance and enhance lean mass accretion, supporting healthy growth trajectories in preterm infants.

INTRODUCTION

Despite advancements in neonatal intensive care, optimizing postnatal growth in preterm infants, especially lean mass accretion, remains a significant clinical challenge. Current nutritional strategies for preterm infants aim to replicate intrauterine growth rates [15–20 g/(kg•day)] and body composition [1]. However, many preterm neonates fail to meet these targets. Even when weight gain approximates intrauterine rates, body composition at term-equivalent age is often suboptimal and is characterized by lower fat-free mass and higher fat percentages [2^▪]. This altered pattern of tissue accretion is associated with increased risk of long-term metabolic complications, including obesity, type 2 diabetes, and cardiovascular disease [3].

Amino acids function as the primary substrate for protein synthesis, a key determinant of lean tissue growth, and contribute to many other metabolic processes. For example, leucine stimulates protein synthesis by activating mechanistic target of rapamycin complex 1 (mTORC1)-dependent translation initiation, arginine serves as a precursor for nitric oxide, and methionine is a major contributor of methyl groups that are critical for epigenetic regulation. In preterm neonates, the capacity to synthesize conditionally indispensable amino acids is limited, which consequently increases reliance on exogenous sources to optimize protein synthesis. A shortage of one or more indispensable or conditionally indispensable amino acids restricts the utilization of others for protein synthesis in the body. Moreover, protein synthesis is energetically demanding, requiring adequate energy to support optimal nitrogen retention. When energy supply is inadequate, amino acids are preferentially oxidized, leading to increased urea production and a higher risk of metabolic complications.

Recent evidence from both clinical and preclinical studies has highlighted the importance of not only the quantity and quality of amino acid intake but also the mode of delivery (parenteral vs. enteral) and feeding modality (continuous vs. intermittent) in shaping protein metabolism and growth trajectories. Enteral protein intake, particularly in the early postnatal period, has been associated with improved fat-free mass accretion and neurodevelopmental outcomes [4–6]. Furthermore, feeding patterns that mimic the physiological nutrient pulses that occur with meal feeding can further enhance anabolic signaling and lean tissue growth [7^▪▪].

This review synthesizes current knowledge on the regulation of skeletal muscle protein synthesis in preterm neonates, with emphasis on the role of amino acids and the mode of nutrient delivery. We examine the mechanistic basis of skeletal muscle anabolic resistance, evaluate the impact of different feeding strategies, and discuss clinical implications of optimizing amino acids on growth and development. By integrating findings from both clinical and preclinical studies, we aim to identify actionable strategies to enhance protein synthesis and support healthy growth trajectories in preterm infants. 

Box 1.

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RECENT INSIGHTS ON PROTEIN METABOLISM IN PRETERM NEONATES

Evidence from a relevant preclinical model for preterm infants, the preterm piglet, suggests that skeletal muscle in premature neonates exhibits a blunted anabolic response to nutrients. This reduced response occurs even when plasma concentrations of both amino acids and insulin, which are key regulators of protein synthesis, are maintained at concentrations similar in the preterm and the neonate born at term [8–10]. This diminished responsiveness of skeletal muscle protein synthesis to feeding may contribute to the reduced lean mass accretion and body weight gain that are frequently observed in preterm neonates.

The independently blunted effects of insulin and amino acids on protein synthesis in the skeletal muscle of preterm compared to term piglets were demonstrated using a pancreatic-substrate clamp technique [10]. This impaired responsiveness, however, was not observed in all tissues of the preterm piglets; the brain, heart, and lungs exhibited anabolic responses comparable to those of term piglets, while the liver displayed a higher basal rate of protein synthesis. The diaphragm, also a skeletal muscle, did not show an attenuated anabolic response to insulin and amino acids like other muscles. These findings raise the question of whether such tissue-specific adaptations reflect a natural homeorhetic shift in nutrient partitioning to prioritize the development of vital organs during early postnatal life in preterm neonates as occurs in utero.

At the molecular level, the stimulation of protein synthesis following feeding is tightly regulated by two key pathways: the insulin–Ras homolog enriched in brain (Rheb)–mTORC1 pathway and the amino acid–Ras-related GTP-binding protein (Rag)–mTORC1 pathway [11] (Fig. 1). Activation of both pathways is diminished in the skeletal muscle of preterm neonates but remains intact in the diaphragm and other tissues examined in the clamp study [10], including the brain, heart, liver, and lungs. Suryawan et al. [8] showed that although upstream insulin signaling through the insulin receptor and insulin receptor substrate 1 (IRS1) is intact, insulin-stimulated phosphorylation of protein kinase B (Akt/PKB) is significantly blunted in the skeletal muscle of preterm piglets. This attenuation is associated with reduced activation of key intermediates such as 3-phosphoinositide-dependent kinase 1 (PDK1) and mechanistic target of rapamycin complex 2 (mTORC2), lower levels of the membrane trafficking protein ubiquitin-like protein 4A (Ubl4A), and elevated abundance of negative regulators, including PH domain leucine-rich repeat protein phosphatase (PHLPP).

FIGURE 1.

Mechanisms of mTORC1-dependent activation of protein synthesis by insulin and amino acids. Gray denotes impaired signaling in preterm skeletal muscle [8–10,12].

At the same time, intracellular amino acid signaling to mTORC1 in preterm skeletal muscle is also compromised [10]. Despite the preserved abundance of amino acid transporters such as L-type amino acid transporter 1 (LAT1) and sodium-coupled neutral amino acid transporter 2 (SNAT2), the abundance of key intracellular amino acid sensors is reduced. The ability of leucine to suppress Sestrin1–GATOR2 complex formation, but not Sestrin2–GATOR2, is also impaired (Sestrin 1/2, stress response protein 1/2; GATOR1/2, GAP activity toward RAGs 1/2) [10,12]. As a result, the assembly of RagA/B–mTORC1 and RagC/D–mTORC1 complexes is impaired [10], which is essential for mTORC1 translocation to the lysosomal surface and subsequent activation by Rheb in response to insulin signaling.

Given the impaired upstream signaling through both the insulin–Rheb–mTORC1 and amino acid–Rag–mTORC1 pathways, the downstream effectors of mTORC1 activity that regulate translation initiation are also compromised in preterm skeletal muscle [9,10]. Phosphorylation of ribosomal protein S6 kinase 1 (S6K1), a direct target of mTORC1, is consistently reduced, leading to diminished activation of ribosomal protein S6 (rpS6), which plays a role in ribosome biogenesis and translational efficiency. In parallel, mTORC1-dependent phosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) is reduced, limiting the release of eIF4E from the 4E-BP1 inhibitory protein and reducing the formation of the eIF4E•eIF4G complex required for cap-dependent translation initiation. These molecular impairments converge to blunt the postprandial stimulation of protein synthesis, contributing to reduced lean tissue accretion in preterm neonates despite adequate nutrient provision.

Although protein synthesis is impaired in preterm skeletal muscle, current evidence does not suggest that prematurity inherently alters protein degradation pathways. Markers of autophagy (LC3-II/total LC3 ratio) and ubiquitin-proteasome activity (Atrogin-1 and MuRF1 abundance) were regulated similarly in skeletal muscle of preterm and term piglets [10]. Insulin-induced phosphorylation of FoxO3, a transcription factor that regulates proteolytic gene expression, also remains unaffected. These findings indicate that catabolic machinery is not upregulated in the context of prematurity. While it remains important to prevent clinical conditions that may stimulate protein degradation, implementing nutritional strategies to sensitize the protein synthetic machinery to feeding stimuli is necessary.

This anabolic resistance in preterm neonates likely contributes to the relatively elevated amino acid and protein intake needed to achieve appropriate nitrogen retention and growth. Recent findings provide compelling evidence that this blunted anabolic response is not merely a function of low birth weight but rather an intrinsic feature of prematurity [9]. Using a birth weight stratified design in both preterm and term piglets, the study demonstrated that skeletal muscle protein synthesis remains significantly attenuated in preterm neonates following feeding, even when body weight and nutrient intake are matched. Given that skeletal muscle represents the largest component of lean body mass, impaired protein synthesis in this tissue has a great impact on whole-body lean mass accretion during early postnatal development.

AMINO ACID REQUIREMENTS IN PRETERM NEONATES: THE ROLE OF FORTIFIED HUMAN MILK AND INFANT FORMULAS

Current guidelines for parenterally fed preterm neonates recommend a total amino acid intake of 3.0–4.0 g/(kg•day) to achieve intrauterine growth rates [1,13^▪▪]. However, these high intakes may exceed the metabolic capacity of some preterm infants, increasing the risk of complications such as refeeding syndrome, elevated blood urea nitrogen, sepsis, and neurodevelopmental impairments [14,15^▪]. Nonetheless, there is broad agreement that amino acid administration should begin soon after birth, typically at 2.5–3.0 g/(kg•day) and advance to a maintenance target of 3.0–3.5 g/(kg•day) [13^▪▪,14,16^▪]. For infants who start parenteral nutrition later, directly advancing to maintenance levels is considered appropriate. Concurrently, nonprotein energy and micronutrients must be provided to minimize the risk of metabolic complications and to optimize amino acid utilization for protein accretion and overall growth. Nonprotein energy intake should be between 85 and 105–115 kcal/(kg•day), aiming for an energy-to-protein ratio of approximately 30 kcal/g of amino acid [13^▪▪].

For stable, growing preterm infants who are fully enterally fed, a protein intake of 3.5–4.0 g/(kg•day) is recommended [1,17]. Intakes of up to 4.5 g/(kg•day) may be considered in cases of growth faltering, after excluding other contributing factors such as suboptimal energy intake or electrolyte abnormalities [1,17]. A recent systematic review and meta-analysis [18^▪] of enterally fed preterm infants receiving fortified human milk demonstrated a significant linear relationship between protein intake and weight gain. Meta-regression analyses suggested that achieving intrauterine-like growth rates of 17–20 g/(kg•day) may require protein intakes closer to 4.0–4.5 g/(kg•day) and a total energy intake of 130–140 kcal/(kg•day).

Human milk, particularly mother's own milk, is the preferred source of enteral nutrition for preterm infants and is associated with multiple short- and long-term benefits. Despite its advantages, human milk alone does not meet the nutrient requirements of preterm infants, especially in terms of protein, energy, calcium, and phosphorus [17]. A recent systematic review and meta-analysis [19] of 66 studies encompassing over 30 000 milk samples from mothers who delivered between 24 and 36 weeks of gestation found that the true protein content declined from 2.32 g/100 ml in colostrum (<4 days postpartum), to 1.77 g/100 ml in transition milk (5–14 days postpartum), to 1.46 g/100 ml in mature milk (>14 days postpartum), with further reductions observed beyond 84 days postpartum. While colostrum may provide sufficient protein to meet early postnatal intake recommendations [3.5–4.5 g/(kg•day)], the protein content of transition and mature milk is insufficient to achieve target intakes at typical feeding volumes [150–180 ml/(kg•day)]. A formal subgroup analysis to determine the influence of gestational age at birth on milk composition was not performed in the meta-analysis. These findings reinforce the need for human milk fortification informed by lactation stage and gestational age, to ensure adequate protein and amino acid delivery to support optimal growth in preterm infants.

Recent clinical trials have explored the impact of fortifier composition on growth and metabolic outcomes. A multicenter randomized controlled trial [20] demonstrated that a human milk fortifier, enriched with protein and lipids (including docosahexaenoic acid and arachidonic acid), but with a lower carbohydrate content, was not inferior to a conventional multicomponent cow's milk-based human milk fortifier in supporting weight growth velocity [18.4 vs. 18.5 g/(kg•day)] in very preterm infants (<32 weeks of gestation). Complementary findings from Rasmussen et al. [21] showed that fortification with bovine colostrum led to significantly elevated plasma concentrations of 12 amino acids (Ala, Arg, Asn, Gln, Glu, Gly, His, Phe, Pro, Ser, Tyr, and Val) compared to a conventional fortifier in very preterm infants. These increases were positively associated with plasma IGF-1 levels and head circumference growth, although no significant differences in weight or length gain between groups were found. Notably, bovine colostrum fortification resulted in lower plasma lysine and threonine levels, two nutritionally indispensable amino acids critical for protein synthesis and growth, suggesting that targeted amino acid supplementation may be necessary when using bovine colostrum-based fortifiers.

Preterm infant formulas are recommended when human milk (mother's own milk and donor human milk) is unavailable or insufficient for preterm infants [22]. In this context, Kwinta et al. [23] evaluated a two-stage preterm formula system designed to meet the evolving nutritional needs of very-low-birth-weight infants (<1500 g) during hospitalization (Stage 1) and after discharge (Stage 2). The Stage 1 formula provided 3.6 g protein/100 kcal and supported a mean weight gain velocity of 23.0 g/(kg•day), exceeding intrauterine growth targets. The Stage 2 formula provided 2.8 g protein/100 kcal and maintained adequate growth postdischarge. Biomarkers of protein and bone health remained within normal ranges, and neurodevelopmental outcomes at 24 months were within age-appropriate norms. These findings underscore the efficacy and safety of staged, protein-enriched formulas in promoting growth and development when human milk is unavailable or insufficient.

A 2025 consensus report from the National Academies of Sciences, Engineering, and Medicine [24^▪], commissioned by the U.S. Food and Drug Administration (FDA), critically evaluated the scientific basis of protein quality metrics used in infant formula regulation. The committee recommended adopting the human milk amino acid profile as the reference pattern for protein quality assessment. However, it is important to note that while this standard is appropriate for the infant born at term, it may not reflect the needs of preterm neonates. Specifically, preterm infants have limited capacity to synthesize certain conditionally indispensable amino acids, such as arginine and glycine, that are present in low concentrations in human, bovine, and porcine milks [25,26^▪]. This highlights the need for refining amino acid requirements for preterm infants and for developing fortification strategies that ensure optimal delivery of both indispensable and conditionally indispensable amino acids to support optimal growth, lean tissue accretion, and neurodevelopment.

MODE OF DELIVERY OF AMINO ACIDS ON GROWTH AND CLINICAL OUTCOMES – PARENTERAL VS. ENTERAL NUTRITION

The route of nutrient delivery in preterm neonates plays a vital role in determining early growth and long-term developmental outcomes. Recent studies demonstrate that compared to parenteral feeding, enteral protein intake promotes greater fat-free mass accretion and supports brain maturation extending through 7 years of age. These findings add to a growing body of literature suggesting that adequate neonatal nutrition, including optimal protein intake via the enteral route, can have lasting, positive effects on somatic growth, body composition, brain development, and cognitive outcomes in preterm infants [4–6,18^▪].

In a prospective cohort of infants born less than 32 weeks of gestation, Jerome et al. [5] found that a higher proportion of protein delivered enterally during the first 2 weeks of life was associated with significantly greater fat-free mass at both 2 weeks postnatal and 36 weeks postmenstrual age. In contrast, parenteral protein (amino acid) intake correlated negatively with fat-free mass at both time points, even after adjusting for gestational age, energy intake, and illness severity. Notably, total protein intake was associated with weight gain but not with lean mass, underscoring the importance of delivery route over quantity alone. Henkel et al. [4] introduced the enteral-to-parenteral protein ratio as a metric to evaluate the impact of delivery mode on growth and neurodevelopment. In a cohort of 256 very low birth weight infants, a higher ratio during the first 28 days was independently associated with increased brain tissue volume on MRI at term-equivalent age and greater weight gain velocity. Importantly, enteral protein, not total protein, was positively linked to brain volume, suggesting enteral delivery may confer neurodevelopmental advantages beyond simple nutrient provision.

Extending into later childhood, Poppe et al. [6] reported that very preterm infants receiving higher early total protein intake exhibited more mature white matter microstructure and improved thalamocortical connectivity at 7 years of age, along with better visual processing and cognitive outcomes. These structural differences were not reflected in absolute brain volume but in the proportion of intracranial volume occupied by brain tissue. However, the potential benefits of high protein intake must be weighed against emerging concerns. A meta-analysis by Das et al. [15^▪] found that while intakes greater than or equal to 3.5 g/(kg•day) modestly improved early growth, they were also associated with increased risks of metabolic disturbances and possible neurodevelopmental impairment. Upon reviewing the individual studies contributing neurodevelopmental outcome data to this meta-analysis, three of the four studies contributing to the outcome “survival without neurodisability at or beyond 12 months’ corrected age” and both studies contributing to the outcome “cognitive impairment or delay during the toddler period” involved increased parenteral amino acid administration without a corresponding increase in nonprotein energy provision. Although this limitation was not explicitly addressed in the meta-analysis, it may have influenced the observed adverse outcomes. These observations highlight the need to optimize energy-to-protein ratios and prioritize enteral nutrition whenever feasible to support well tolerated and effective growth in preterm infants.

Collectively, these studies emphasize that the route of protein delivery, as well as the total amount of protein, play a pivotal role in shaping early growth and neurodevelopmental trajectories in preterm infants. Enteral protein intake is consistently associated with more favorable outcomes in weight gain velocity, body composition, and brain maturation. These benefits are likely mediated by gut-derived signals and the metabolic capacity of splanchnic tissues to synthesize conditionally indispensable amino acids from precursors delivered enterally. For instance, enterocytes in the small intestine can generate citrulline and arginine from glutamine, glutamate, and proline (abundant in milk proteins) [27], and glycine from 4-hydroxyproline (abundant in milk peptides) [28], thereby compensating for suboptimal amino acid profiles; the immature gut function in preterm infants may reduce this compensatory capacity [29]. In contrast, parenteral nutrition bypasses these regulatory systems, limiting the body's capacity to correct amino acid imbalances. These findings reinforce the clinical imperative to prioritize early initiation and advancement of enteral nutrition, even in the context of ongoing parenteral support, while carefully monitoring metabolic responses and tailoring protein delivery to individual tolerance and developmental needs.

FEEDING MODALITIES ON PROTEIN SYNTHESIS AND GROWTH IN NEONATES – INTERMITTENT VS. CONTINUOUS FEEDING

Preterm infants who are unable to coordinate sucking, swallowing, and breathing are often fed via orogastric tube, using either a continuous or intermittent bolus schedule. Intermittent bolus feeding more closely mimics physiological postprandial hormonal and substrate fluctuations that regulate anabolic signaling and growth in neonates. The clinical benefits of intermittent bolus vs. continuous feeding have been debated but have not been discerned from the current available human clinical trials [30]. In neonatal piglets born at term, intermittent bolus feeding induces pulsatile increases in circulating insulin and amino acids. These postprandial surges robustly, but transiently, activate the mTORC1 signaling pathway, enhancing translation initiation and protein synthesis in skeletal muscle and visceral tissues. In contrast, continuous feeding maintains relatively stable but subthreshold levels of anabolic stimuli, resulting in diminished activation of mTORC1 and reduced protein synthesis efficiency [31,32].

In a 21-day feeding study, El-Kadi et al. [31,32] demonstrated that full-term neonatal piglets receiving intermittent bolus feeds exhibited significantly greater gains in body weight, spine length, lean mass, and growth of skeletal muscle and visceral organs compared to those fed the same nutrient intake but delivered continuously. The greater stimulatory effect of intermittent bolus compared to continuous feeding on skeletal muscle growth exceeded that observed for whole-body growth, whereas organ and whole-body growth were largely proportional. In skeletal muscle, ileum, and liver, this enhanced growth was driven by increased fractional protein synthesis rates. These anabolic responses were associated with increased phosphorylation of mTORC1 downstream targets, including S6K1 and 4E-BP1, and greater formation of the eIF4E•eIF4G complex. Notably, the efficiency of protein translation (g protein/(day·g/RNA)) was also higher in bolus-fed neonatal piglets, underscoring the importance of nutrient-driven signaling dynamics in regulating muscle growth.

In preterm piglets, however, the response to intermittent bolus feeding has been less consistent than that of term piglets. In some studies, no significant benefits of this feeding modality were reported [33]. In others [7^▪▪], intermittent bolus feeding every four hours significantly increased skeletal muscle protein synthesis of preterm piglets, coinciding with postprandial surges in plasma insulin and amino acids. The reasons for the inconsistent results among studies with preterm piglets remain unclear but may relate to the ability – or inability – of the feeding method and milk replacer formula composition to create a distinct and physiologically meaningful pulsatile pattern of insulin and amino acids.

TARGETED AMINO ACID SUPPLEMENTATION – THE CASE FOR LEUCINE

Despite its anabolic limitations, continuous feeding remains essential for neonates with feeding intolerance. To address this issue, studies have explored the use of pulsatile leucine supplementation to mimic the postprandial rise in plasma amino acids [34]. In neonatal piglets, discrete leucine pulses administered during continuous feeding of a complete diet significantly enhance skeletal muscle protein synthesis, achieving rates comparable to those observed with intermittent bolus feeding. This anabolic response is mediated by increased phosphorylation of mTORC1 downstream targets, including S6K1 and 4E-BP1, and occurs independently of insulin signaling [34].

Leucine was selected for supplementation due to its unique role as a nutrient signal that activates mTORC1 and promotes translation initiation in neonatal skeletal muscle. Leucine stimulates mTORC1 by disrupting the inhibitory Sestrin1/2–GATOR2 complex and facilitating the formation of RagA–mTOR and RagC–mTOR complexes, which enhance downstream signaling and the assembly of the eIF4E•eIF4G complex [35]. Notably, leucine's metabolite β-hydroxy-β-methylbutyrate (HMB) also stimulates mTORC1-dependent protein synthesis in neonatal pigs, but through a mechanism that appears to bypass the canonical leucine-sensing pathway [36]. These findings underscore the specificity and potency of leucine and its metabolites in stimulating protein synthesis, even under conditions of continuous feeding.

When the pulsatile leucine supplementation was extended over 21 days during continuous enteral feeding, it resulted in greater lean mass accretion while reducing fat deposition [37]. These effects were mediated by upregulation of amino acid transporters such as SNAT2, activation of mTORC1-dependent translation initiation, increased formation of the eIF4E•eIF4G complex, and higher protein synthesis rates in skeletal muscle. Moreover, metabolomic and transcriptomic analyses showed that the leucine pulses not only increased protein synthetic activity but also reduced the expression of protein catabolic pathways [38]. Notably, these anabolic effects occurred without changes in blood insulin or IGF-I levels, underscoring the insulin-independent role of leucine in moduˌlating protein synthesis and accretion. The administration of isonitrogenous alanine pulses to continuously-fed piglets had no beneficial effects. Although elevated plasma leucine concentrations have been associated with insulin resistance and type 2 diabetes in humans [39], none of our leucine supplementation studies in neonatal piglets have demonstrated impairments in insulin signaling pathways or other indicators of reduced insulin sensitivity, such as elevated plasma insulin or glucose concentrations [7^▪▪,40].

Evidence for the beneficial effect of pulsatile leucine supplementation in neonatal piglets has been extended to preterm piglets. Intravenous leucine pulses administered every four hours during continuous feeding produced a comparable anabolic response to enteral bolus feeds in preterm piglets [7^▪▪]. This included increased phosphorylation of mTORC1 and its downstream targets, resulting in higher fractional protein synthesis rates in skeletal muscle. However, this response required a higher leucine dose for preterm piglets than for piglets born at term (1600 μmol vs. 800 μmol/(kg•h) for 1 h every 4 h) [7^▪▪,37], reflecting the anabolic-resistant characteristics of preterm muscle.

These findings emphasize the importance of generating physiologically meaningful pulses of nutrients and hormones to overcome the anabolic resistance of prematurity. They also suggest that bolus feeding, or intermittent leucine supplementation during continuous feeding, may contribute to optimizing the anabolic environment necessary for muscle growth in preterm infants.

CLINICAL IMPLICATIONS AND FUTURE DIRECTIONS

Optimizing protein synthesis in preterm neonates is a multifaceted challenge that extends beyond simply meeting recommended amino acid intakes. The phenomenon of anabolic resistance, particularly in skeletal muscle, emerges as a potential barrier to adequate lean mass accretion and growth in this vulnerable population. Evidence from clinical studies supports early initiation and advancement of enteral nutrition, particularly maternal milk fortified with protein, energy, and minerals, as the preferred approach to promote overall growth, lean mass development, and neurodevelopment. Intermittent bolus feeding and pulsatile leucine supplementation may further enhance anabolic signaling, especially in skeletal muscle. Table 1 summarizes the clinical trials cited in this review.

Table 1.

Summary of clinical trials cited in this review

Study reference	Population	Study objective	Study design	Sample size	Key findings
Henkel et al. [4]	VLBW infants (<1500 g)	Assess impact of protein delivery route on brain and growth.	Retrospective cohort	256	Higher enteral-to-parenteral protein ratio associated with increased brain volume and weight gain.
Jerome et al. [5]	Very preterm infants (<32 weeks GA)	Examine relationship between the route of protein delivery and lean mass.	Prospective cohort	78	Greater enteral protein intake linked to increased fat-free mass; parenteral intake negatively associated.
Poppe et al. [6]	Very preterm infants	Investigate long-term effects of early protein intake.	Prospective cohort	99	Early high protein intake associated with improved white matter and cognitive outcomes at 7 years.
Picaud et al. [20]	Infants <32 weeks GA fed human milk	Compare novel vs. conventional human milk fortifier.	Randomized controlled trial	155	Novel fortifier supported adequate growth; noninferior to conventional fortifier.
Rasmussen et al. [21]	Infants <32 weeks GA fed fortified human milk	Evaluate amino acid profiles and growth after colostrum fortification.	Secondary analysis of RCT	225	Bovine colostrum increased plasma amino acids; associated with higher plasma IGF-1 and head circumference growth.
Kwinta et al. [23]	VLBW infants (<1500 g)	Assess two-stage formula on growth and neurodevelopment.	Single-arm interventional	34	Two-stage formula supported growth and neurodevelopment; all infants scored >70 on BSID-III.

It highlights the study population, objectives, design, and key findings relevant to amino acid delivery and protein synthesis in preterm neonates.

BSID-III, Bayley Scales of Infant and Toddler Development, Third Edition; GA, gestational age; IGF-1, insulin-like growth factor 1; RCT, randomized controlled trial; VLBW, very low birth weight.

Despite these advances, several critical gaps remain for future research:

• (1)Persistence of anabolic resistance: Current data are limited to the first few days of life. Whether anabolic resistance persists and contributes to altered body composition at term-equivalent age or beyond remains unknown. Nutritional strategies to mitigate this anabolic resistance also warrant investigation.
• (2)Parenteral amino acid formulations: Optimal amino acid composition for early life, sustained growth, and periods of illness or physiological stress (e.g., sepsis, necrotizing enterocolitis, hypoxemia, and hyperammonemia) remains undefined. This is particularly important, given that parenteral nutrition bypasses splanchnic metabolism, thereby limiting the body's ability to offset amino acid imbalances.
• (3)Long-term clinical outcomes: While needed, studies linking neonatal nutrition to long-term growth, body composition, and neurodevelopment are challenged by clinical heterogeneity and postdischarge environmental confounders. Preclinical models – particularly preterm piglets which closely mimic preterm human neonates in their gastrointestinal development, nutrient metabolism, and body composition – offer a physiologically and clinically relevant animal model to evaluate the sustained effects of early nutritional interventions on growth and body composition, as well as the mechanisms that govern these responses.

Future research should also focus on refining the amino acid requirements for preterm and intrauterine growth-restricted infants, optimizing delivery strategies, and translating mechanistic insights from preclinical models into clinical protocols that support long-term growth and neurodevelopment in these compromised infants.

CONCLUSION

Despite advances in neonatal nutrition, optimizing protein synthesis and lean growth in preterm infants remains a clinical challenge. Emerging evidence shows that anabolic resistance in skeletal muscle is a primary barrier to lean mass accretion. This resistance is driven by impaired insulin and amino acid signaling, underscoring the need for strategies that go beyond meeting amino acid intake thresholds.

Enteral nutrition, particularly with fortified human milk, supports better body composition and neurodevelopmental outcomes than parenteral delivery. Moreover, feeding modalities that mimic physiological nutrient pulsatility, such as intermittent bolus feeding or pulsatile leucine supplementation during continuous feeding, may enhance anabolic signaling and support healthy growth trajectories in preterm infants.

Acknowledgements

The authors want to thank Dr Agus Suryawan for his contribution to this body of work.

Financial support and sponsorship

This work was supported by the National Institute of Child Health and Development Grants HD-085573 (T.A.D.) and HD-099080 (T.A.D. and M.L.F.), USDA Current Research Information System Grant 3092-51000-060 (M.L.F.), and the Texas A&M AgriLife Institute for Advancing Health Through Agriculture (T.A.D.).

Conflicts of interest

All authors report no conflicts of interest.

REFERENCES AND RECOMMENDED READING

Papers of particular interest, published within the annual period of review, have been highlighted as:

• ▪ of special interest

• ▪▪ of outstanding interest