Proteins are the main building blocks of the body. They are polymers, composed of 30 or more amino acids. Twenty different standard amino acids combine to form the proteins. Some amino acids are essential diet components, since they are not synthetized by human metabolic processes.

Proteins are present in and vital to every living cell. They are essential for healthy growth and development and also influence major functions of the body (Figure 1). They can influence metabolic parameters (weight gain and adipogenic activity), presence of beneficial bacteria in the gut, risk of developing atopic dermatitis, digestive functions and renal system.

The infant’s first year is a critical time of rapid growth and development; this rapid growth must be supported by a high rate of protein synthesis. Infant nutrition requirements are primarily satisfied by a single and highly specific food source: breast milk. The composition of breast milk is the gold standard for estimated total protein and essential amino acid requirements during infancy [1].

Both total protein content and concentrations of individual proteins in human milk change throughout the first year of lactation to fulfill the needs of the infant. Milk proteins are also a source of biologically active peptides, released by gastrointestinal digestion and having a positive impact on body functions and ultimately promoting health benefits [2].

Infant formulas have been designed for infants who cannot be breastfed. These infant formulas need to be similar to breast milk in their composition but also in functional outcomes, to insure appropriate growth, optimal development, maturation of immune system, programming of the metabolic system… [3]. They have been evolving throughout decades along with the scientific knowledge. Protein sources and processes have been optimized to provide very similar amino acid profiles as found in breast milk. More recently, clinical data showed that lower protein content in infant formula has a long-term preventive impact on BMI and obesity risk [4].

In addition to guaranteeing healthy growth and development of bottle-fed infants, specific infant formulas have also been designed for specific needs, by modifying their protein component.

As an example, three different types of infant formulas have been designed for management of cow’s milk allergy (affecting around 5% of infants): partially hydrolyzed, extensively hydrolyzed or amino acid based infant formulas (Figure 2). Allergy is triggered by protein components; one way to decrease allergenicity of proteins is to modify their conformation and/or structure responsible for the allergy, by cutting protein into peptides which are not able anymore to trigger an allergic reaction.

Some partially hydrolyzed whey based infant formulas have been clinically proven to prevent atopic dermatitis in infants [5] and a first ever FDA claim has been granted for their use in infants at risk of allergy. The specific process leading to partially hydrolyzed infant formulas leads to a reduction in allergenicity of the milk proteins and generates specific immunomodulatory peptides allowing the beneficial effect.

Extensively hydrolyzed infant formula are resulting from a different production process leading to very short peptides, having almost no allergenic properties anymore. These infant formulas have been designed for therapeutic application, offering a solution to reduce allergy symptoms in infants allergic to cow’s milk proteins.

Finally, in case of severe allergic reaction to cow’s milk proteins, only amino acid based infant formula are able to decrease symptoms because they are totally devoid of allergens.

In conclusion, proteins provided via breast milk or infant formula are essential components of infant’s diet and therefore the specific quality, quantity and conformation of these proteins are essential for a safe growth and development.

Hydrolyzed proteins are used worldwide in the therapeutic management of infants with allergic manifestations and have long been proposed as a dietetic measure to prevent allergy in at risk infants. Cow’s milk allergy (CMA) is the most frequent food allergy in early childhood and affects about 0.5-6% of infants with range dependent on type of feeding (breast or bottle fed) and criteria of diagnosis (self-report or challenge-proven). Cow’s milk presents two different fractions of proteins: casein and whey proteins, both with allergenic properties.

Immunoreactive epitopes and peptide fragments of both beta-lattoglobulin and casein have been well characterized. Milk allergenicity can be reduced by various processing methods and mainly by hydrolysis. Hydrolysed formulas are differentiated according to the method of hydrolysis (such as enzymatic, ultra-heating, ultra-filtration), the timing of hydrolysis, the degree of hydrolysis (which basically switch from intact protein to peptides of various molecular weight), the protein source (casein, whey, rice, soy) and other nutritional components.

There is no general agreement on unique standards to specifically define partial (p) or extensive (e) hydrolyzed formulas (HF), but the distinction is generally made by the molecular weight and percentage of the peptides fragments. A pHF contains peptides with a molecular weight generally <6 KDaltons (KD), ranging from 3 to 10 KD, with some commercial pHF containing 18% of peptides over 6 kD; an eHF usually has more than 90% of peptides <3 KD, with 1-5% of peptides >3.5 KD. In contrast, the molecular weight of whole cow’s milk protein ranges from 14 KD (alfa-lactalbumin) to 24 KD (casein) and up to 67 KD (bovine serum albumin).

The weight of peptides has immune and clinical relevance because the “bigger” the peptides the “more allergenic” can be. Peptides greater than 6 KD and predominantly greater than 10 KD frequently act as allergens, but already in the range between 970 and 1400 Dalton may bind IgE in vitro, whereas peptides with a molecular mass >1400 Dalton are needed for skin reactivity and may causes a type I reaction if in the range of 3-5 KD. Partial HF have been developed with the aim of minimizing the number of sensitizing epitopes within milk proteins, while at the same time retaining peptides with sufficient size and immunogenicity to possibly stimulate the induction of oral tolerance but not suitable for tretament. Extensive HF have been extensively hydrolyzed in order to destroy allergenic epitopes.

Molecular weight profile only enables to differentiate protein characteristics of formulas, but does not clearly determine the allergenic formula properties and clinical response. Although lower than pHF, residual allergenicity is present even in eHF whilst the only anallergic formulas are the elemental ones totally based on free amino-acids (AAF) that cannot determine an immune stimulation. Compared to eHF, AAF has, in most countries, an higher cost, a different taste and possible long term nutritional effect. A certain whey pHF offers a valid option for primary allergy prevention, mainly for atopic dermatitis, in high-risk infants who are not exclusively breastfed [6].

More studies are needed to determine their real benefit in the prevention of cow’s milk allergy and in the general population. The largest diet interventional study (GINI) reported a significant reduction of eczema at all study points (1,3,6,10 years) when using a pHF (OR 0.56; 95% CI, 0.32–0.99) or a casein-based eHF (OR 0.42, 95% CI, 0.22–0.79) but not a whey-based eHF or a standard CM formula. eHF cow’s milk protein based is the preferred treatment option in infant with CMA who is not breast fed except in the ones who refuse or do not tolerate eHF or in the most severe cases in which AAF should be started.

There is limited evidence that the addition of prebiotics or probiotics (L. rhamnosus GG, Bifidobacteria breve) to an eHF offer an additional benefit. A rice protein-based eHF can be a second choice to a cow’s milk protein based extensive hydrolyzed formulas. The maintenance of a well-balanced diet is not easy especially in more severe cases of CMA but is mandatory for each child. The choice of the eHF should be based on scientific evidence of efficacy, tolerance and nutritional adequacy.

Partially hydrolysed formulae (pHF) are increasingly used in the prevention of atopic disease and in the management of infants with functional gastro-intestinal (GI) manifestations. Nowadays, pHF are more likely to be used by primiparous women, those breastfeeding longer and in infants with family history of allergies. A Cochrane review concluded that no serious adverse events associated with pHF were reported. Adverse events were mentioned in three studies, but none was attributed to the pHF. There may be a theoretical concern that pHF is both absorbed and metabolized faster than intact protein. Whether this has any health outcome impact is not known.  Long-term safety data are non-existent.

A prospective double-blind, randomized cross-over trial in 115 regurgitating infants showed a significant decrease of the mean number and volume of regurgitation with two thickened formulas, with statistically better results for the pHF. No difference was reported in stool frequency and consistency between the two groups .

Data suggest that pHFs may reduce infant colic. However, dietary changes include often a reduction of lactose, supplementation with prebiotic oligosaccharides, structed lipids with a higher proportion of Sn-2 position beta-palmitate, decreasing the formation of calcium soaps.  No randomised clinical trials have been published demonstrating the efficacy of pHF in infantile colic. Experience has shown that pHF can be a useful option when cow's milk protein allergy is not a potential cause of the colic. In fact, various randomised controlled trials are published demonstrating the efficacy of pHFs. However, the role of (reduced) lactose can be questioned, as soy formula was not associated with a decrease in infantile colic. There are insufficient data to recommend a pHF as single dietary intervention in colicky infants as most studies included other dietary changes as well.

Constipation is more frequent in casein than in whey-predominant formula. pHFs result in  more frequent and softer stools in non-constipated infants. Significantly more stools were passed by breastfed and extensive HF-fed infants versus standard infant formula or soy-based formulas. Infants receiving breast milk or an extensive HF had twice as many stools as the other formula groups. GI transit time is shorter in preterm when a pHFis compared with standard preterm formula. The pHF had a markedly shorter GI transit time (9.8 h) than standard infant formula (19 h). pHF, fortified with prebiotics and/or probiotics, with high sn-2 palmitate in the fat blend or without palm oil as the main source of fat in the oil blend, have been tested lately and seem to offer a good alternative for managing functional constipation in infancy. There are no studies evaluating the efficacy of a pHF as single intervention in constipated infants.

Infants with minor GI problems such as infantile colic and/or regurgitation and/or constipation were fed for 14 days with a formula containing a mixture of oligosaccharides, partially hydrolysed whey protein, low levels of lactose and palmitic acid in the beta-position: a reduction in frequency of colic and of regurgitation was reported in 79%  and 70% of infants, respectively, whereas an increase of defecation was noticed. Testing the same formula in 267 infants with infantile colic, the authors demonstrated a statistically significant decrease in colic episodes after 1 and 2 weeks compared to standard formula and simethicone. 

Conclusion

Based on the limited available literature, pHF tend to have some beneficial effect on functional GI manifestation such as regurgitation and constipation, although the evidence in insufficient to formulate a recommendation.

Prematurity occurs during a critical period of development with the most rapid rate of growth in lifespan. The adequacy of nutritional support, in particular protein intakes, plays an important roles on many short- and long-term outcomes. Proteins are the major driving force of growth and represent the major functional and structural components of human body.
Their properties and functions depend on the structure of their polypeptide chain of amino acids (AA). Body proteins are constantly degraded and synthesized to and from AA and protein turnover rate is very high in preterm infant compare to older infants, children, and adults. It implies that body AA pool is in constant equilibrium with potential instabilities and adverse effect of either insufficient or excessive AA concentrations.

Postnatal enteral nutrition is essential to enhance gastrointestinal maturation and postnatal development but feeding problems are very frequent in preterm infants. These infants frequently suffer from postnatal feeding intolerance and sometimes develop severe gastrointestinal disease like necrotizing enterocolitis. Human milk (HM) is considered the preferred source of nutrients for preterm infants but is not always available. Thus, the industry has developed specially designed formula for preterm infants (PTF, preterm infant formula).

Most infants’ formulas are developed from cow’s milk after several adaptations to meet infants’ requirements. Most of them content intact cow’s milk proteins. Extensive or partial protein hydrolysates (PH) have been developed to treat cow’s milk protein allergy or to prevent allergic sensitization. These PH are also proposed and used when facing several digestive and behavioral problems in infants. Thus, for different kind reasons, term infants’ formulas with PH have also been used in preterm infants and the industry has developed specific PTF with PH.

Few studies have been published evaluating the use of PH in preterm infants. Most studies included varying source of protein, varying degree of hydrolysis, and varying nutrient content. These studies demonstrated that protein source play an important role in nutritional adequacy and that adequate source need to be used in PTF. Protein utilization and efficiency is generally lower for PH and poorer weight gain can be observed. Some differences in plasma AA profile frequently occurred with sometimes potential insufficient essential AA availability. Several studies also showed that mineral absorption could also be reduced with PH.

PH have also been proposed in preterm infants in order to improve their feeding tolerance. Several studies have shown that PH may accelerate gastric emptying and transit time in both term and preterm infants. However, the clinical benefit of PH for preterm infants in this indication is not clear and should require more studies. The question whether there might be an interest of feeding preterm neonates with PH to prevent the development of later atopic diseases was also suggested.

Indeed, preterm infants had higher intestinal permeability and immature immune system. However, preterm infants are not really at higher risk for later atopic disease and most studies were unable to demonstrate a reduction in cow’s milk protein sensitization or in allergic manifestations in these infants.

In conclusion, the quantity and the quality of protein intakes plays a major role in preterm infants. The use of PH might have significant consequences on gastrointestinal transit, on the intestinal absorption of proteins and minerals, and on plasma AA profile. Like for all PTF, the development and the use of PH in preterm infants requires adequate nutritional studies.

Hydrolyzed formulas (HF) are presently used primarily in infants that cannot be exclusively breastfed, those with documented cow’s milk allergy (CMA), and for primary prevention of allergic disease. HFs are increasingly being used worldwide, begging the question if HFs may be recommended as the optimal choice for all standard-risk, full-term infants who are not exclusively breastfed.

From a regulatory standpoint, HFs meet nutrient requirements to be considered as standard infant formulas (SIF). Though both partially (pHF) and extensively (eHF) HFs are considered hypoallergenic by European definitions, pHFs may be attributable to allergic reactions in one-third to one-half of children with CMA, and thus North American regulatory agencies consider only eHFs to be hypoallergenic. Data regarding the nutritional adequacy of modern-day HFs are scarce and lack long-term data suggesting that growth in infants fed HF versus SIF is equivalent.

There may be theoretical concern that partially hydrolyzed protein is both absorbed and metabolized faster than intact protein; whether this has any impact on the health outcomes of infants is unknown, but available data from eHFs are reassuring. Based on limited available literature, pHFs have some beneficial effect on functional GI disorders, such as regurgitation and constipation, but data are insufficient to make a formal recommendation for their utility.

Human breast milk is the optimal source of nutrition for multiple reasons, mainly including cost, secretory IgA, and iron absorption. A 2006 systematic review determined there were no comparable long-term studies regarding the prolonged use of HFs vs. breastfeeding.¹ There are studies, however, that have examined the use of various formulas as a primary source or supplement to reduce the risk of atopic disease. A meta-analysis of formula consumption and the risk of atopic dermatitis (AD) found that infants fed pHF had a 65% lower risk of AD than those fed cow’s milk (CM) formula (summary relative risk estimate 0.45, 95% CI 0.40-0.70).² Other studies have identified differences between feeding with pHFs and eHFs.

The German Infant Nutritional Interventional (GINI) Study followed 945 high-risk newborn infants in a randomized trial investigating the effects of breastfeeding supplemented with one of 4 formulas - CM, pHF-whey (pHF-W), eHF-W, or eHF-casein (eHF-C) - in the first four months of life. Feeding with pHF-W and eHF-C formulas had a preventive effect on the cumulative incidence of AD in high-risk children, lasting until 10 years. Feeding with a HF compared to a CM formula had no effect on asthma, allergic rhinitis, or on specific sensitization at 10 years.³ Additional trials are needed in high-risk infants to confirm these findings.

Cost should be considered in decision-making regarding choice of formula, but global comparison of this is difficult given large cost differences in different countries. Data suggest that pHF given to every formula-fed infant is a cost-effective intervention for the prevention of atopic disorders, such as AD. Questions arise, though, when the impact of allergy prevention in studies using HFs is limited to AD prevention, suggesting that these effects do not generalize to preventing the development of other allergic diseases in the atopic march.

Despite the issues raised here, the desire to provide concrete recommendations of widespread HF use need to be balanced carefully so as not to overstate claims of benefit. Long-term studies are needed to investigate the feasibility of HF as a routine feeding option for healthy, standard-risk infants. Because of the paucity of data, routine use of HF as an equivalent option to breastfeeding or SIF cannot be supported at present based on available scientific evidence.

Human milk is considered as the gold standard for infant feeding . Breast feeding advantages extend beyond the the properties of human milk itself. A complex of nutritional, environmental, socioeconomic, psychological as well as genetic interactions establish a massive list of benefits of breast feeding to the health outcomes of the breast fed infant and to the breastfeeding mother. For this reason exclusive breast feeding is recommended for about 6 months and should be continued as long as mutually desired by mother and child.

The evidence in the literature on the effect of breastfeeding on health outcomes is based on observational studies due to the fact that it is unethical and practically impossible to randomize children to be breast fed or not. As such, multiple confounders cloud the evidence and one must base conclusions on the accumulating evidence when not contradictory and on the one intervention study, the PROBIT (Promotion of Breastfeeding Intervention Trial) .

This review highlight some of the health outcomes related to breastfeeding such as the prevention of infections, the effect of breastfeeding on neurodevelopmental outcome, obesity, allergy and celiac disease. Available evidence as well as some of the contradictory results are discussed.

Given the documented short- and long-term advantages of breastfeeding, human milk as a sole source of nutrition for first few months of newborn life is considered a normative standard.

Each macro constituent of human milk plays a crucial role in growth and development of the baby. Lipids are largely responsible for providing more than 50% of the energy as well as providing essential fatty acids and minor lipids that are integral to all cell membranes. Carbohydrates can be broadly divided into lactose and oligosaccharides, which are readily digestible source of glucose and indigestible nonnutritive components, respectively.

Proteins in human milk provide essential amino acids indispensable for growth of the infants. What is more interesting is that protein concentration profoundly changes from colostrum to mature milk. In this report, we share data from an observatory, single center, longitudinal trial with HM collection at 30, 60, and 120 days postpartum from 50 mothers (singleton-deliveries of 25 male and 25 female infants).

The protein content decreased with evolving stages of lactation from an average of 1.45 g/100 mL to 1.38 g/100 mL. The data did not show any gender differences as it was reported for lipid content at 120 d postpartum by our group. Additionally we also share consolidated literature data on protein evolution of human milk during the first year of lactation.

GROWTH. Accelerated weight gain during infancy and early childhood is a strong predictor of childhood and adult obesity. Meta-analyses indicate that rapid weight gain in infancy explains between 20-30% of obesity risk in the adult population. Breastfeeding, in particular exclusive breastfeeding during the first 4-6 months and continuation of breastfeeding during the second half of infancy, seems to somehow protect from child- and adulthood obesity. WHO has published global growth standards which are based on longitudinal data of predominantly breastfed ( >6 months) children whose mothers were not malnourished (i.e. BMI 18-25 kg/m²). Weight from 4 months to 2 years of the WHO standards is lower than indicated by international growth references, which are based on data both from formula- and breastfed children The WHO growth standards are now used in most countries of the world.

Longitudinal randomized clinical trials now indicate that children who are fed infant- and follow-up formulas with protein concentrations >2,25 g/100 kcal (high protein formulas) during the first year of life grow faster than indicated by the WHO standards. How can we slow down accelerated growth in formula-fed infants? The best way is to promote breastfeeding. A meta-analysis now indicates that infants fed a modified whey based formula with 1,8g protein per 100 kcal during the first 4 months can grow according to the WHO standards – i.e. like breastfed infants. Three     longitudinal randomized trials show that infants receiving low protein formulas with modified protein (1,6-1,8g/100kcal) between 3 to 12 months have slower weight gain than infants fed high protein formulas.

METABOLIC BIOMARKERS. Biomarkers which are indicators of growth such as IGF-1, insulin, c-peptide, and branched-chained amino acids (BCAAs) are higher in infants receiving high protein formulas than in breastfed infants or infants fed low protein formulas . The IGF axis regulates early growth and influences adipose tissue differentiation and early adipogenesis. The BCAAs leucine, isoleucine, and valine are physiologic stimulators of insulin secretion. High protein intake of infants with formula stimulates the IGF axis and insulin release, which is associated with a higher weight-for-length and body mass index at the age of 2 years. Recently it has been shown that lower β-oxidation of fatty acids in infants who are fed high protein formulas results in higher early weight gain and increased body fat deposition.

BODY  COMPOSITION. Estimation of fat- (FM) and fat-free mass (FFM) allows more detailed insights both in quantitative and qualitative weight gain during or after feeding high or low protein formulas. A randomized controlled trial with infants who were fed high- or low protein formulas from birth indicates that FM at 6 months (isotope dilution) correlates with BMI and weight gain velocity. In infants of overweight and obese mothers who were fed high- or low protein formulas, weight gain between 3 and 12 months and weight at 12months were significantly higher in the group fed the high protein formula. Percentages of FM and FFM were similar at 12 months (DEXA). One randomized prospective study in an unselected US population reported longitudinal data of infants who were fed high- or low protein formulas. Children were followed until 60 months of age. Weight gain and composition of weight gain (FM and FFM in grams) were similar when the infants were exclusively fed the formulas fed between 3 and 6 months (PEA POD). Children who were fed the high protein formula gained significantly more fat between 6-36 months and 6-60 months (DEXA).

CONCLUSIONS Quantitative and qualitative growth indicators are among the most sensitive biomarkers to monitor long-term effects of early nutrition on health of our children. Several studies now indicate that growth of children can be influenced by early nutrition. Breastfeeding and the use of low protein formulas in those infants who cannot be breastfed can help to prevent accelerated growth during infancy and early childhood.  In addition, fat gain until 5 years is lower in children who had been breastfed or fed low protein formula. It is most important that the new low protein formulas are safe and adequate for the whole infant population. Based on new protein technologies, their essential- and branched-chained amino-acids are now close to breast milk.

Breast-feeding has been associated with many benefits both short-term and long-term. Infants being breast-fed generally have less illness and have better cognitive development at one year of age than formula-fed infants. Later in life, they have a lower risk of obesity, diabetes and cardiovascular disease. Several components in breast milk may be responsible for these different outcomes, but bioactive proteins/peptides likely play a major role. Some proteins in breast milk are comparatively resistant towards digestion and may therefore exert their functions in the gastrointestinal tract in intact form or as larger fragments.

Other milk proteins may be partially digested in the upper small intestine and the resulting peptides may exert functions in the lower small intestine. Lactoferrin, lysozyme and secretory IgA are examples of proteins that have been found intact in the stool of breast-fed infants and are therefore examples of proteins that are resistant against proteolytic degradation in the gut.

These proteins serve protective roles against infection and support immune function in the immature infant. Alpha-lactalbumin, beta-casein, kappa-casein and osteopontin are examples of proteins that are partially digested and the resulting peptides provide functions in the gut. Such functions include stimulation of immune function, mineral and trace element absorption and defense against infection.  

The World Health Organization, the American Academy of Pediatrics, ESPGHAN and others all support the feeding of human milk for all infants including preterm infants.  The benefits of human milk include immunologic, nutritional, developmental, psychology, social and economic advantages.  In preterm infants, feeding of human milk is associated with reduction in necrotizing enterocolitis and sepsis.  In the long-term, premature infants also demonstrate advantages in neurocognitive development.

However, there are several challenges in providing exclusive human milk feedings to meet the nutritional needs of very low birth weight infants.  These include inadequate milk supply, the variability of nutrient composition of human milk and the limitation of human milk itself.  Human milk varies in volume with method of milk expression, time of day, type of milk [fore or hind milk] and stage of lactation.  Reasons for low volumes include stress, lack of family or medical support, maternal illness, lack of a breast pump and difficulties in storing and transporting milk.

Preterm infants have higher requirements than term infants and after the milk transitions to mature milk in 2-3 weeks, the protein content is usually insufficient to meet the nutritional demands of a rapidly growing infant.  Similarly, human milk does not have sufficient quantities of calcium, phosphorus and Vitamin D to support bone health. 

Energy density of human milk also declines over time.  When mother’s own milk is not available, it is recommended that donor milk be fed to infants.  In the first place, donor milk is usually obtained from mothers who have been breastfeeding for several months and, therefore, protein and energy content are low.  In addition, pasteurization alters concentrations of some water soluble vitamins as well as altering the activity of several bioactive components of human milk.

Poor growth is thus seen both with the use of unfortified mother’s own milk as well as donor milk especially in very low birth weight infants making fortification of human milk a priority.  In this discussion, macronutrient requirements of preterm infants, composition of mother’s own milk and donor milk will be reviewed.  Fortifiers, bovine and human milk-based, fortification strategies and duration of fortification will also be discussed.  Thus, using appropriate fortification methods will assist in meeting the nutrient requirements of these infants, while protecting the beneficial effects of human milk itself.

Because of their exceedingly high rate of growth, premature infants have very high needs for all nutrients. But because protein is limiting for growth, requirements for protein are of particular importance. Requirements have been estimated by the factorial method based on the body composition of the fetus. In spite of using a somewhat different data base and different methods of data analysis, we (Ziegler et al., 1976) and Forbes (1983) arrive at very similar estimates of protein needs for growth of the premature infant. The Table summarizes our estimates.

Meeting these intakes is not particularly difficult if nutrients are provided by formulas. But human milk is the preferred feeding for premature infants because of its protective effects. When human milk is fed it must be fortified with nutrients, and then the variability of the composition of expressed milk seems to pose a problem. On the basis of studies carried out several decades ago using very high protein intakes, it is believed that high intakes of protein may be dangerous for premature babies. To prevent protein intakes from being too high when the protein content of expressed milk should be high, the protein content of fortifiers has been kept low. This has had the result that protein intakes of preterm infants have been too low most of the time.

The idea that protein intake in the preterm infant may influence, or programme, the long-term health of the infant born preterm has been strongly supported by several decades of research starting from the early 1980’s. 

At the time, it was recognised that a high protein intake was required in preterm infants to achieve a post-natal growth rate closer to the intra-uterine rate of growth of a normal fetus of the same post-conceptional age, a goal regarded optimal for short- and long-term health. Subsequently, long-term follow-up of preterm infants randomised to a high protein formula (for an average of only 4 weeks after birth) demonstrated beneficial effects up to 16 years later on brain structure and function including 10% greater volume of the caudate nucleus, higher IQ, and practical benefits for cognitive function (e.g. mathematical reasoning, numerical operations, reading comprehension).

Since this early research, numerous observational studies have demonstrated an association between sub-optimal nutrition in the early post-natal period (as measured by faltering growth, poor growth in head circumference, and inadequate protein intake) and impaired long-term neuro-cognitive development. Consequently, International recommendations for protein intake in infants born prematurely have increased progressively.

Nonetheless, despite the extensive observational evidence, the role of early protein intake in preterm infants for later neurodevelopment remains unclear.  For instance, Cochrane reviews of randomised trials have not shown evidence supporting early amino acid administration, or higher versus lower protein intake in formula preterm infants for improving later neurodevelopment.  Therefore, although there are strong associations between early post-natal protein intake and neonatal growth, and between growth faltering and impaired later neurodevelopment, whether high protein supplementation can improve cognitive function in preterm infants remains controversial.

In contrast to the benefits for neurodevelopment, longer-term follow-up the same infants in the preterm nutritional trials above have suggested that faster post-natal weight gain increased later risk factors for cardiovascular disease. Infants randomised to a higher protein formula for the first 4 weeks were shown to have increased adiposity, insulin resistance, dyslipidaemia, markers of inflammation and vascular endothelial dysfunction up to 16 years later. 

These programming effects of early growth, termed the growth acceleration hypothesis, have now been demonstrated in randomised and observational studies in several preterm populations, as well as in infants born at term with both low and appropriate weight for gestation^. Therefore, as is common in biological systems, faster infant weight gain appears to have both a benefit and cost on long-term health outcomes.

Current nutritional policy for preterm infants is based on the widely accepted consensus that supporting optimal neurodevelopment is the neonatologist’s highest priority. Therefore, on balance, this policy favours early administration of a higher protein intake in order to improve later cognitive function, irrespective of any increase in cardiovascular risk. 

However, this consensus is largely based on research that has focused on infants <31 weeks gestation and it is uncertain whether the risk-benefit of faster weight gain differs for the larger, more mature, healthy preterm infant than those with extreme prematurity. Furthermore, the critical window for these effects is unknown and whether the same nutritional policy should apply after discharge is controversial.

For instance, a randomised trial of post-discharge formulas, showed that a higher protein intake after hospital discharge, although increasing the rate of growth, did not have adverse effects on later body composition or cardiovascular risk factors.

This presentation will consider the role of protein intake on long-term health outcomes in infants born preterm, focusing on the risk-benefit for accelerated growth and emphasising the need for further research.

Amino acids and proteins form the main building blocks for fetal and neonatal growth. Despite improvements in neonatal care including postnatal nutrition, growth faltering and suboptimal outcome after premature birth are still frequently encountered. Nutrition can partly be held responsible. Over the years, there has been a trend in delivering amino acids earlier from birth onwards and in larger quantities.

Studies showed positive results on efficacy as usually measured in terms of nitrogen balance (figure 1) or stable isotope studies. Short term safety has been questioned as parameters are difficult to interpret, while long term effects are not frequently assessed. Besides, it is unlikely that we have achieved the optimal therapy with regards to protein for premature neonates yet.

It is therefore also important to gain insight in how the developing fetus is able to metabolize amino acids and proteins and map this to preterm infants of similar gestational age. Exploring fetal metabolism and growth could enhance our understanding of the challenges of postnatal development, even although the placenta cannot adjust and filter metabolites anymore postnatally. Unfortunately, very little is known about fetal protein metabolism also due to ethical and technical reasons. For one, we hardly know how much nutrients the human fetus actually receives.

Only a few studies have been performed, using stable isotope techniques, that gives us an indication of the uptakes and metabolic faith amino acid in human fetuses. These studies show that a relatively large proportion of amino acids taken up are utilized for oxidation rather than solely being used for protein synthesis.

Extrapolation of the individual amino acid uptakes to total amino acid intakes (that could serve as a base to determine total amino acid requirement of preterm infants of similar ages) are hampered by the different metabolic fates of the individual amino acids. In addition, these studies do show that the fetal liver is capable of synthesizing large quantities of albumin, possibly even higher than is currently seen in premature infants, fed at current recommended intakes.

Theoretically it would thus seem to be possible to improve certain aspects of postnatal metabolism as well, as the metabolic apparatus of a premature infant should ontogenetically be able to achieve a high hepatic rate of protein synthesis under optimal circumstances (as in utero). Nevertheless, we must acknowledge the complex interplay between the placenta and the fetus.

During the last decades several studies have been performed in premature neonates investigating amino acid metabolism, mostly comparing different nutritional regimes in terms of protein content. Protein metabolism is however influenced by many other factors. For example, the quality (individual amino acid composition) of the intravenous solution or enteral formula or concomitant energy intake could influence the efficacy of protein handling and therefore the total requirements as well.

Individual amino acid requirements during different stages of postnatal course are therefore to be determined, of which only a start has been made. Besides, non-nutritional factors will influence the requirements and tolerability of amino acids, although these are hardly ever studied. Efforts should be undertaken to study the effects of intra-uterine growth restriction, or needs during and following additional critical illnesses (besides prematurity itself).

Thus, although we might attempt to determine amino acid requirements for the stable preterm infant based upon improved knowledge on both fetal and neonatal physiology, we are far away being able to predict the requirements for specific subgroups such as being small for gestational age or stressed infants with additional diseases. Unfortunately, only few of these factors have been unraveled. Only by gaining more knowledge on both fetal and neonatal physiology and disease, we should be able to optimize growth and functional outcome in premature infants.

A large proportion of Extremely Low Birth Weight Infants require parenteral nutrition for variable lengths of time. Amino acids are the key ingredient of parenteral nutrition. The aim of appropriate amino acid administration is to promote anabolism and optimal cellular development with the final goal of reducing postnatal growth restriction, which is associated with neurodevelopmental delays.

The benefits of early amino acids commencement is compelling especially on nitrogen balance, while long-term outcome studies are lacking. An intake of 2.5 g/Kg/day of amino acids is to be preferred to lower amounts. Benefits of amino acid intakes above 2.5 g/kg/day without extra energy remain controversial. Two RCTs do not show benefits on short-term growth and at 2 year-neurodevelopment. Studies with amino acid intake above 2.5 g/kg/day with extra energy are warranted.