In vitro digestion of proteins in human milk fortifiers and in preterm formula

Assessment of milk protein digestion kinetics: effects of denaturation by heat and protein type used

Knowledge about how molecular properties of proteins affect their digestion kinetics is crucial to understand protein postprandial plasma amino acid (AA) responses. Previously it was found that a native whey protein isolate (NWPI) and heat denatured whey protein isolate (DWPI) elicit comparable postprandial plasma AA peak concentrations in neonatal piglets, while a protein base ingredient for infant formula (PBI, a β-casein-native whey protein mixture) caused a 39% higher peak AA concentration than NWPI. We hypothesized that both whey protein denaturation by heat as well as changing protein composition by including β-casein, increases the rate of intact protein loss, and that changing the protein composition (by including β-casein), but not whey protein denaturation, yields a faster absorbable product release. Therefore NWPI (91% native), DWPI (91% denatured) and PBI hydrolysis was investigated in a semi-dynamic in vitro digestion model (SIM). NWPI and DWPI hydrolysis were also compared in a dynamic digestion model with dialysis (TIM-1) to exclude potential product inhibition effects that may occur in a closed vessel digestion model as SIM. In both models, the degree of hydrolysis (DH), loss of intact protein, and release of absorbable products (SIM: <0.5 kDa peptides and free AA, TIM-1: bioaccessible AA) were monitored. Additionally, in SIM, intermediate product amounts and their characteristics were determined. DWPI showed considerably faster intact protein loss, but similar DH and absorbable product release kinetics compared with NWPI in both models. Furthermore, more, relatively large, intermediate products were released from DWPI than from NWPI. PBI showed increased intact protein loss, similar DH, and absorbable product release kinetics, but more, relatively small, intermediate products than NWPI. In conclusion, both whey protein denaturation and β-casein inclusion increased the rate of intact protein loss without affecting absorbable product release during in vitro digestion. Our results suggest that intermediate digestion product characteristics are important in relation to postprandial AA responses.

Gastrointestinal Protein Hydrolysis Kinetics: Opportunities for Further Infant Formula Improvement

The postprandial plasma essential amino acid (AA) peak concentrations of infant formula (IF) are higher than those of human milk (HM) in infants. In addition, several HM proteins have been recovered intact in infant stool and appeared digestion resistant in vitro. We, therefore, hypothesized that gastrointestinal protein hydrolysis of IF is faster than HM and leads to accelerated absorbable digestion product release. HM and IF protein hydrolysis kinetics were compared in a two-step semi-dynamic in vitro infant digestion model, and the time course of degree of protein hydrolysis (DH), loss of intact protein, and release of free AA and peptides was evaluated. Gastric DH increase was similar for IF and HM, but the rate of intestinal DH increase was 1.6 times higher for IF than HM. Intact protein loss in IF was higher than HM from 120 min gastric phase until 60 min intestinal phase. Intestinal phase total digestion product (free AA + peptides <5 kDa) concentrations increased ~2.5 times faster in IF than HM. IF gastrointestinal protein hydrolysis and absorbable product release are faster than HM, possibly due to the presence of digestion-resistant proteins in HM. This might present an opportunity to further improve IF bringing it closer to HM.

Effects of Glycated Glutenin Heat-Processing Conditions on Its Digestibility and Induced Inflammation Levels in Cells

Protein is one of the three major macronutrients and is essential for health. The reaction of α-dicarbonyl compounds (α-DCs) with glutenin during heat processing can modify its structure, thereby reducing its digestibility. Furthermore, advanced glycation end products (AGEs) formed by the Maillard reaction are associated with long-term diabetes-related complications. In this study, we established a heat processing reaction system for α-DCs and glutenin by simulating common food processing conditions. An in vitro digestion model was used to study the digestibility of glycated glutenin; whereupon the effects of the digestion products on macrophage inflammatory response were further investigated. It was found that reaction conditions, including temperature, treatment duration, pH, and reactant mass ratio, can significantly affect the digestibility of glycation glutenin, in which the mass ratio of reactants has the most significant influence. We demonstrated that when the mass ratio of glutenin to methylglyoxal (MGO) was 1:3, the level of inflammation induced by glycated glutenin was the highest. The mass ratio of reactants significantly affects the digestibility of glycation glutenin and the level of macrophage-induced inflammatory response. This suggests that it is possible to protect the nutritional value of protein and improve food safety by controlling the heat processing conditions of wheat products.

Degradation of Proteins From Colostrum and Mature Milk From Chinese Mothers Using an in vitro Infant Digestion Model

This study provided insights into the degradation of human milk proteins in an in vitro infant digestion model by comparing colostrum (week 1) and mature milk (week 4) of 7 Chinese mothers individually. In this study, we adapted the exiting INFOGEST in vitro model, to conditions representative to infants (0 to 3 month-old). The level of undigested proteins was analyzed by LC-MS/MS after gel-electrophoretic separation and in-gel digestion. The BCA protein assay showed that the total undigested milk protein content decreased from the start to the end of digestion with variations between mothers, especially in the gastric phase (25–80%). Undigested proteins could also still be found after the intestinal phase, ranging from 0.5 to 4.2% of initial protein content. Based on LC-MS/MS analysis, milk protein digestion varied between the mothers individually, especially during the gastric phase. No differences could be observed between protein digestion from colostrum and mature milk after the intestinal phase. The highest levels of proteins remaining after intestinal digestion can be linked to the group immune-active proteins, for all mothers. The level of protease inhibitors and total protein content in the milk did not correlate with the overall proteolysis during digestion. The results also showed that milk serum proteins partly remained after the gastric phase, with 33% remaining from colostrum and 37% remaining from mature milk. More than 40 milk serum proteins were detected after the intestinal phase. Some of the highly abundant milk serum proteins (lactoferrin, serum albumin, bile salt-activated lipase, immunoglobulins, α₁-antichymotrypsin) were still partially present intact after the intestinal phase, for all mothers. Caseins were also not completely digested in the gastric phase, with 35% remaining from colostrum and 13% remaining from mature milk. Caseins, on the other hand, were almost completely digested after the intestinal phase. The complete degradation of caseins into peptides might be related to their structural features. Overall, this study showed that digestion differed for the various human milk proteins by adapting an in vitro digestion model to infant physiological conditions, with the main differences between digestion of the milk from individual mothers being observed after gastric digestion.

Macronutrient Digestion and Absorption in the Preterm Infant

The human fetus receives oral nutrition through swallowed amniotic fluid and this makes a significant nutritional contribution to the fetus. Postnatally, macronutrient absorption and digestion appear to function well in the preterm infant. Although pancreatic function is relatively poor, the newborn infant has several mechanisms to overcome this. These include a range of digestive enzymes in human milk, novel digestive enzymes involved in fat and protein digestion that do not appear to be present in the older child or adult, and the presence of a Bifidobacterium-rich colonic microbiome that may "scavenge" unabsorbed macronutrients and make them available to the infant.

Effect of nonenzymatic deamidation on the structure stability of Camelus dromedarius α-lactalbumin

Camelid α-lactalbumin is the only known protein that can undergo nonenzymatic deamidation on two Asn residues. This leads to the generation of a mixture of unusual isoAsp and d-Asp residues that may impact health. The effect of deamidation on camel α-lactalbumin instability was investigated. Circular dichroism showed that the altered protein acquired secondary structure resulting in an increase in α-helix content. In good agreement, the 3D structure of camel α-lactalbumin determined by X-ray crystallography, displayed a short additional α-helix probably induced by deamidation, compared to the human and bovine counterparts. This α-helix was located in the C-terminal region and included residues 101-106. Differential scanning calorimetry together with the susceptibility to thermolysin showed that the deamidation process reinforced the structural stability of the α-lactalbumin at high temperature and its resistance toward proteolysis.

How processing may affect milk protein digestion and overall physiological outcomes: A systematic review

Dairy is one of the main sources for high quality protein in the human diet. Processing may, however, cause denaturation, aggregation, and chemical modifications of its amino acids, which may impact protein quality. This systematic review covers the effect of milk protein modifications as a result of heating, on protein digestion and its physiological impact. A total of 5363 records were retrieved through the Scopus database of which a total of 102 were included. Although the degree of modification highly depends on the exact processing conditions, heating of milk proteins can modify several amino acids. In vitro and animal studies demonstrate that glycation decreases protein digestibility, and hinders amino acid availability, especially for lysine. Other chemical modifications, including oxidation, racemization, dephosphorylation and cross-linking, are less well studied, but may also impact protein digestion, which may result in decreased amino acid bioavailability and functionality. On the other hand, protein denaturation does not affect overall digestibility, but can facilitate gastric hydrolysis, especially of β-lactoglobulin. Protein denaturation can also alter gastric emptying of the protein, consequently affecting digestive kinetics that can eventually result in different post-prandial plasma amino acid appearance. Apart from processing, the kinetics of protein digestion depend on the matrix in which the protein is heated. Altogether, protein modifications may be considered indicative for processing severity. Controlling dairy processing conditions can thus be a powerful way to preserve protein quality or to steer gastrointestinal digestion kinetics and subsequent release of amino acids. Related physiological consequences mainly point towards amino acid bioavailability and immunological consequences.

Protein hydrolysate versus standard formula for preterm infants

BACKGROUND

When human milk is not available for feeding preterm infants, protein hydrolysate, rather than standard cow's milk formulas (with intact proteins), is often used because it is perceived as being tolerated better and less likely to lead to complications. However, protein hydrolysate formulas are more expensive than standard formulas, and concern exists that their use in practice is not supported by high-quality evidence.

OBJECTIVES

To assess the effects of feeding preterm infants hydrolysed formula (vs standard cow's milk formula) on risk of feed intolerance, necrotising enterocolitis, and other morbidity and mortality.

SEARCH METHODS

We used the standard Cochrane Neonatal search strategy including electronic searches of the Cochrane Central Register of Controlled Trials (CENTRAL; 2019, Issue 1), in the Cochrane Library; Ovid MEDLINE (1966 to 28 January 2019); Ovid Embase (1980 to 28 January 2019); and the Cumulative Index to Nursing and Allied Health Literature (CINAHL) (28 January 2019), as well as conference proceedings and previous reviews.

SELECTION CRITERIA

Randomised and quasi-randomised controlled trials that compared feeding preterm infants protein hydrolysate versus standard (non-hydrolysed) cow's milk formula.

DATA COLLECTION AND ANALYSIS

Two review authors assessed trial eligibility and risk of bias and extracted data independently. We analysed treatment effects as described in the individual trials and reported risk ratios and risk differences for dichotomous data, and mean differences for continuous data, with respective 95% confidence intervals (CIs). We used a fixed-effect model in meta-analyses and explored potential causes of heterogeneity in sensitivity analyses. We assessed quality of evidence at the outcome level using the GRADE approach.

MAIN RESULTS

We identified 11 trials for inclusion in the review. All trials were small (total participants 665) and had various methodological limitations including uncertainty about methods to ensure allocation concealment and blinding. Most participants were clinically stable preterm infants of less than about 34 weeks' gestational age or with birth weight less than about 1750 g. Fewer participants were extremely preterm, extremely low birth weight, or growth restricted. Most trials found no effects on feed intolerance, assessed variously as mean pre-feed gastric residual volume, incidence of abdominal distension or other gastrointestinal signs of concern, or time taken to achieve full enteral feeds (meta-analysis was limited because studies used different measures). Meta-analysis showed no effect on the risk of necrotising enterocolitis (typical risk ratio 1.10, 95% CI 0.36 to 3.34; risk difference 0.00, 95% CI -0.03 to 0.04; 5 trials, 385 infants) (low-certainty evidence; downgraded for imprecision and design weaknesses).

AUTHORS' CONCLUSIONS

The identified trials provide only low-certainty evidence about the effects of feeding preterm infants protein hydrolysate versus standard formula. Existing data do not support conclusions that feeding protein hydrolysate affects the risk of feed intolerance or necrotising enterocolitis. Additional large, pragmatic trials are needed to provide more reliable and precise estimates of effectiveness and cost-effectiveness.

Postprandial Amino Acid Kinetics of Milk Protein Mixtures are Affected by Composition, But Not Denaturation, in Neonatal Piglets

BACKGROUND

Multiple studies have indicated that formula-fed infants show a different growth trajectory compared with breastfed infants. The observed growth rates are suggested to be linked to higher postprandial levels of branched chain amino acids (BCAAs) and insulin related to differences in protein quality.

OBJECTIVE

We evaluated the effects of milk protein denaturation and milk protein composition on postprandial plasma and hormone concentrations.

METHODS

Neonatal piglets were bolus-fed randomly, in an incomplete crossover design, 2 of 3 milk protein solutions: native whey protein isolate (NWPI), denatured whey protein isolate (DWPI), or protein base ingredient, comprising whey and casein (PBI). Postprandial plasma amino acids (AAs), insulin, glucagon-like peptide 1, glucose, and paracetamol concentrations were assayed. Plasma responses were fitted with a model of first-order absorption with linear elimination.

RESULTS

DWPI (91% denatured protein) compared with NWPI (91% native protein) showed lower essential amino acids (EAAs) (∼10%) and BCAA (13-19%) concentrations in the first 30-60 min. However, total amino acid (TAA) concentration per time-point and area under the curve (AUC), as well as EAA and BCAA AUC were not different. PBI induced a ∼30% lower postprandial insulin spike than NWPI, yet plasma TAA concentration at several time-points and AUC was higher in PBI than in NWPI. The TAA rate constant for absorption (k a) was twofold higher in PBI than in NWPI. Plasma BCAA levels from 60 to 180 min and AUC were higher in PBI than in NWPI. Plasma EAA concentrations and AUCs in PBI and NWPI were not different.

CONCLUSIONS

Denaturation of WPI had a minimal effect on postprandial plasma AA concentration. The differences between PBI and NWPI were partly explained by the difference in AA composition, but more likely differences in protein digestion and absorption kinetics. We conclude that modifying protein composition, but not denaturation, of milk protein solutions impacts the postprandial amino acid availability in neonatal piglets.

Release of functional peptides from mother's milk and fortifier proteins in the premature infant stomach

Digestion of milk proteins in the premature infant stomach releases functional peptides; however, which peptides are present has not been reported. Premature infants are often fed a combination of human milk and bovine milk fortifiers, but the variety of functional peptides released from both human and bovine milk proteins remains uncharacterized. This study applied peptidomics to investigate the peptides released in gastric digestion of mother’s milk proteins and supplemental bovine milk proteins in premature infants. Peptides were assessed for homology against a database of known functional peptides—Milk Bioactive Peptide Database. The peptidomic data were analyzed to interpret which proteases most likely released them from the parent protein. We identified 5,264 unique peptides from bovine and human milk proteins within human milk, fortifier or infant gastric samples. Plasmin was predicted to be the most active protease in milk, while pepsin or cathepsin D were predicted to be most active in the stomach. Alignment of the peptide distribution showed a different digestion pattern between human and bovine proteins. The number of peptides with high homology to known functional peptides (antimicrobial, angiotensin-converting enzyme-inhibitory, antioxidant, immunomodulatory, etc.) increased from milk to the premature infant stomach and was greater from bovine milk proteins than human milk proteins. The differential release of bioactive peptides from human and bovine milk proteins may impact overall health outcomes in premature infants.

Temporal changes of major protein concentrations in preterm and term human milk. A prospective cohort study

BACKGROUND

Proteins are major contributors to the beneficial effects of human milk (HM) on preterm infant health and development. Alpha-lactalbumin, lactoferrin, serum albumin and caseins represent approximately 85% of the total HM protein. The temporal changes of these proteins in preterm (PT) HM and its comparison with term (T) HM is poorly characterized.

AIMS

To quantify and compare the temporal changes of the major proteins in PT HM and T HM.

METHODS

HM was collected for 4 months postpartum at 12 time points for PT HM (gestational age 28 0/7-32 6/7 weeks; 280 samples) and for 2 months postpartum at 8 time points for T HM (gestational age 37 0/7-41 6/7 weeks; 220 samples). Proteins were measured with a micro-fluidic LabChip system.

RESULTS

Casein, alpha-lactalbumin and lactoferrin decreased with advancing stages of lactation in PT and T HM, whereas serum albumin remained stable. Only marginal differences between PT and T HM were observed for alpha-lactalbumin during postpartum weeks 3-5 and for serum albumin at the first week. However, a comparison of HM provided to preterm and term infants at the same postmenstrual ages revealed that alpha-lactalbumin contents were significantly lower in PT HM than in T HM during the 39-48 postmenstrual weeks.

CONCLUSIONS

This study provides comprehensive information of the longitudinal changes of major proteins in PT and T HM, and suggests limited availability of alpha-lactalbumin, a nutritionally important protein, in breastfed PT infants after reaching the term corrected age. This information may be important to optimize HM protein fortification, although its biological relevance needs to be confirmed by intervention studies.

CLINICAL TRIAL REGISTRY

ClinicalTrials.gov (NCT02052245), https://clinicaltrials.gov/ct2/show/NCT02052245.

Dietary Considerations in Autism Spectrum Disorders: The Potential Role of Protein Digestion and Microbial Putrefaction in the Gut-Brain Axis

Children with autism spectrum disorders (ASD), characterized by a range of behavioral abnormalities and social deficits, display high incidence of gastrointestinal (GI) co-morbidities including chronic constipation and diarrhea. Research is now increasingly able to characterize the “fragile gut” in these children and understand the role that impairment of specific GI functions plays in the GI symptoms associated with ASD. This mechanistic understanding is extending to the interactions between diet and ASD, including food structure and protein digestive capacity in exacerbating autistic symptoms. Children with ASD and gut co-morbidities exhibit low digestive enzyme activity, impaired gut barrier integrity and the presence of antibodies specific for dietary proteins in the peripheral circulation. These findings support the hypothesis that entry of dietary peptides from the gut lumen into the vasculature are associated with an aberrant immune response. Furthermore, a subset of children with ASD exhibit high concentrations of metabolites originating from microbial activity on proteinaceous substrates. Taken together, the combination of specific protein intakes poor digestion, gut barrier integrity, microbiota composition and function all on a background of ASD represents a phenotypic pattern. A potential consequence of this pattern of conditions is that the fragile gut of some children with ASD is at risk for GI symptoms that may be amenable to improvement with specific dietary changes. There is growing evidence that shows an association between gut dysfunction and dysbiosis and ASD symptoms. It is therefore urgent to perform more experimental and clinical research on the “fragile gut” in children with ASD in order to move toward advancements in clinical practice. Identifying those factors that are of clinical value will provide an evidence-based path to individual management and targeted solutions; from real time sensing to the design of diets with personalized protein source/processing, all to improve GI function in children with ASD.

Personalizing protein nourishment

Proteins are not equally digestible-their proteolytic susceptibility varies by their source and processing method. Incomplete digestion increases colonic microbial protein fermentation (putrefaction), which produces toxic metabolites that can induce inflammation in vitro and have been associated with inflammation in vivo. Individual humans differ in protein digestive capacity based on phenotypes, particularly disease states. To avoid putrefaction-induced intestinal inflammation, protein sources, and processing methods must be tailored to the consumer's digestive capacity. This review explores how food processing techniques alter protein digestibility and examines how physiological conditions alter digestive capacity. Possible solutions to improving digestive function or matching low digestive capacity with more digestible protein sources are explored. Beyond the ileal digestibility measurements of protein digestibility, less invasive, quicker and cheaper techniques for monitoring the extent of protein digestion and fermentation are needed to personalize protein nourishment. Biomarkers of protein digestive capacity and efficiency can be identified with the toolsets of peptidomics, metabolomics, microbial sequencing and multiplexed protein analysis of fecal and urine samples. By monitoring individual protein digestive function, the protein component of diets can be tailored via protein source and processing selection to match individual needs to minimize colonic putrefaction and, thus, optimize gut health.

Protein hydrolysate versus standard formula for preterm infants

BACKGROUND

When human milk is not available for feeding preterm infants, protein hydrolysate rather than standard cow's milk formulas (with intact proteins) are often used because they are perceived as being tolerated better and less likely to lead to complications. However, protein hydrolysate formulas are more expensive than standard formulas, and concern exists that their use in practice is not supported by high-quality evidence.

OBJECTIVES

To assess the effect of feeding preterm infants with hydrolysed formula (versus standard cow's milk formulas) on the risk of feed intolerance, necrotising enterocolitis, and other morbidity and mortality in preterm infants.

SEARCH METHODS

We used the standard Cochrane Neonatal search strategy including electronic searches of the Cochrane Central Register of Controlled Trials (CENTRAL; 2017, Issue 4), Ovid MEDLINE, Ovid Embase, and the Cumulative Index to Nursing and Allied Health Literature (CINAHL) (to April 2017), as well as conference proceedings and previous reviews.

SELECTION CRITERIA

Randomised and quasi-randomised controlled trials that compared feeding preterm infants with protein hydrolysate versus standard (non-hydrolysed) cow's milk formula.

DATA COLLECTION AND ANALYSIS

Two review authors assessed trial eligibility and risk of bias and extracted data independently. We analysed treatment effects as described in the individual trials and reported risk ratios and risk differences for dichotomous data, and mean differences for continuous data, with respective 95% confidence intervals (CI). We used a fixed-effect model in meta-analyses and explored potential causes of heterogeneity in sensitivity analyses. We assessed quality of evidence at the outcome level using the GRADE approach.

MAIN RESULTS

We identified 11 trials for inclusion in the review. All trials were small (total participants 665) and had various methodological limitations including uncertainty about methods to ensure allocation concealment and blinding. Most participants were clinically stable preterm infants of gestational age less than about 34 weeks or birth weight less than about 1750 g. Fewer participants were extremely preterm, extremely low birth weight, or growth-restricted. Most trials found no effects on feed intolerance assessed variously as mean prefeed gastric residual volume, incidence of abdominal distention or other concerning gastrointestinal signs, or time taken to achieve full enteral feeds (meta-analysis was limited because studies used different measures). Meta-analysis found no effect on the risk of necrotising enterocolitis (typical risk ratio 1.10, 95% CI 0.36 to 3.34; risk difference 0.00, 95% CI -0.03 to 0.04; 5 trials, 385 infants) (low quality evidence; downgraded for imprecision and design weaknesses).

AUTHORS' CONCLUSIONS

The identified trials provide only low quality evidence about the effects of feeding preterm infants with protein hydrolysate versus standard formula. The existing data did not support conclusions that feeding with protein hydrolysate affects the risk of feed intolerance or necrotising enterocolitis. Further large, pragmatic trials are needed to provide more reliable and precise estimates of effectiveness and cost-effectiveness.

Repeatability of gastric volume measurements and intragastric content using ultrasound in preterm infants

OBJECTIVES

The aim of the present study was to determine whether serial gastric volumes and intragastric curding are repeatable within individual preterm infants when given sequential feeds of the same volume and composition. Infant and feeding characteristics that may influence differences in measured gastric volumes, echogenicity, and intragastric curding were also explored.

METHODS

Ultrasound images were used to calculate gastric volumes and to rate echogenicity and intragastric curding for 20 infants. A total of 29 paired feeds of the same volume and composition were monitored prefeed and postfeed and at 30-minute intervals thereafter. Statistical comparisons of paired gastric volume measurements and agreement between echogenicity and curding ratings were made for each time point. Analyses of factors that influence discrepancies between volume measurements and between curding ratings were performed.

RESULTS

Paired gastric volume measurements were repeatable (intraclass correlation coefficient [ICC] = 0.971, 0.938 < ICC < 0.987). Most (75%) discrepancies were <2 mL and increased over time, although volume differences were small. Overall moderate levels of consistency were observed for ratings of echogenicity (κ = 0.44), and curd presence (κ ≤ 0.65), density (κ = 0.41), and volume (κ = 0.47). Gastric emptying during feed delivery is influenced with infant positioning, fortification of breast milk, and feeding frequency.

CONCLUSIONS

For preterm infants serial gastric volumes are repeatable and ratings of intragastric echogenicity and curding are moderately consistent when fed milk of the same volume and composition. Ultrasound has the potential to further explore factors that influence gastric emptying in the preterm infant.

Considerations in meeting protein needs of the human milk-fed preterm infant

Preterm infants provided with sufficient nutrition to achieve intrauterine growth rates have the greatest potential for optimal neurodevelopment. Although human milk is the preferred feeding for preterm infants, unfortified human milk provides insufficient nutrition for the very low-birth-weight infant. Even after fortification with human milk fortifier, human milk often fails to meet the high protein needs of the smallest preterm infants, and additional protein supplementation must be provided. Although substantial evidence exists to support quantitative protein goals for human milk-fed preterm infants, the optimal type of protein for use in human milk fortification remains uncertain. This question was addressed through a PubMed literature search of prospective clinical trials conducted since 1990 in preterm or low-birth-weight infant populations. The following 3 different aspects of protein quality were evaluated: whey-to-casein ratio, hydrolyzed versus intact protein, and bovine milk protein versus human milk protein. Because of a scarcity of current studies conducted with fortified human milk, studies examining protein quality using preterm infant formulas were included to address certain components of the clinical question. Twenty-six studies were included in the review study. No definite advantage was found for any specific whey-to-casein ratio. Protein hydrolyzate products with appropriate formulations can support adequate growth and biochemical indicators of nutrition status and may reduce gastrointestinal transit time, gastroesophageal reflux events, and later incidence of atopic dermatitis in some infants. Plasma amino acid levels similar to those of infants fed exclusive human milk-based diets can be achieved with products composed of a mixture of bovine proteins, peptides, and amino acids formulated to replicate the amino acid composition of human milk. Growth and biochemical indicators of nutrition status are similar for infants fed human milk fortified with human milk protein and bovine milk protein.

(14)C radiolabeling of proteins to monitor biodistribution of ingested proteins

The economical preparation of microgram quantities of (14)C-labeled proteins by in vacuo methylation with methyl iodide is described. The (14)C radiolabeling was achieved by the covalent attachment of [(14)C]methyl groups onto amino and imidazole groups by reaction in vacuo with [(14)C]methyl iodide. The method was tested by investigating the biodistribution of (14)C in rats that were fed (14)C-labeled human soluble cluster of differentiation 14 (CD14) protein, a receptor for bacterial lipopolysaccharide. Two other control proteins, bovine serum albumin (BSA) and casein, were also labeled with (14)C and used for comparative analysis to determine the following: (i) the efficacy and cost efficiency of the in vacuo radiolabeling procedure and (ii) the extent of incorporation of the (14)C label into the organs of orogastrically fed 10-day-old Sprague-Dawley rats. [(14)C]BSA, [(14)C]casein, and [(14)C]CD14 were individually prepared with specific radioactivities of 34,400, 18,800, and 163,000 disintegrations per minute (dpm)/microg, respectively. It was found that the accumulation of (14)C label in the organs of [(14)C]CD14-fed rats, most notably the persistence of (14)C in the stomach 480 min postgavage, was temporally and spatially distinct from [(14)C]BSA and [(14)C]casein-fed rats.

Design and characterization of a human milk product for the preterm infant

BACKGROUND

It is necessary to fortify human milk to promote optimal growth of the very preterm infant. However, the addition of non-human milk components to human milk is not ideal because of the risk of feeding intolerance and necrotizing enterocolitis. Human milk products (HMP) are an alternative to commercially available fortifiers, but their preparation is likely to result in modifications to the qualities of human milk.

METHODS

Ten batches of HMP were prepared with the aim of meeting a desired protein:energy ratio of 3.0 g of protein/100 kcal. Ultrafiltration was used to produce a skim milk concentrate, to which cream was then added to produce the final HMP. Characterization of HMP and human milk fortified with commercial human milk fortifiers (Nutriprem [Cow & Gate, Limerick, Ireland] and S-26 SMA human milk fortifier [Wyeth Nutrition, Baulkham Hills, NSW, Australia]) included quantifying macronutrient content, osmolality, microbial content, and particle distribution.

RESULTS

Average protein:energy ratio of the final batch was 2.93 +/- 0.10 g of protein/100 kcal, equating to an inaccuracy of 2.5% relative to the desired ratio of 3.0 g of protein/100 kcal. Significantly greater fat (P < 0.01), lower lactose (P < 0.001), and lower osmolality (P < 0.001) were characteristic of the HMP compared to human milk fortified with either commercial fortifier. Microbial growth occurred during preparation of HMP but did not exceed 10(5) colony-forming units/mL, and pasteurization of human milk prevented contamination in 80% of batches.

CONCLUSIONS

HMP can be designed to accurately target the protein and energy requirements of the preterm infant, but modifications of the macronutrient, biochemical, structural, and microbial characteristics of human milk may affect the quality of the final product.

Set up of a newin vitromodel to study dietary fructans fermentation in formula-fed babies

A newin vitrofermentation model with immobilised infant faecal microbiota simulating the proximal colon of a formula-fed baby was developed and used to test the effects of known prebiotic fructans. Intestinal fermentation, based on a previously developed colonic fermentation model, using a new feeding medium simulating a formula-fed infant ileal chyme, was carried out for seventy-one consecutive days divided into four stabilisation periods intercalated with four prebiotic treatment periods. At the end of the first stabilisation period, total bacterial concentration in colonised beads and in faecal sample was similar, metabolite concentrations returned to stabilisation values after each treatment period. As expected, the four prebiotic treatments significantly increased the bifidobacterial populations, whereas they decreased bacteroides and clostridia. No difference was observed in the prebiotic effect of these substrates selected. The treatments significantly increased total production of SCFA and decreased ammonia compared to stabilisation periods. Long-term stability of the system together with the reproducibility of the known prebiotic effects highlights the potential of the present model to quantify and compare the effects of different substrates in a formula-fed infant microbiota within the same fermentation experiment.

Gut responses to enteral nutrition in preterm infants and animals

Preterm birth is associated with immature digestive function that may require the use of total parenteral nutrition and special oral feeding regimens. Little is known about the responses to oral food in the preterm neonate and how enteral nutrients affect the immature gastrointestinal tract (GIT). In vivo studies are difficult to perform in laboratory rodents because of their small body size and that of immature organs at birth, and this makes the large farm animals (e.g., pigs, cattle, sheep) more attractive models in this field. In these species, preterm delivery at 88%-95% gestation is associated clinical complications and degrees of GIT immaturity similar to those in infants born at 70%-90% gestation. Studies in both animals and infants indicate that the immature GIT responds to the first enteral food with rapid increases in gut mass and surface area, blood flow, motility, digestive capacity, and nutrient absorption. To a large extent, the enteral food responses are birth independent, and can be elicited also in utero, at least during late gestation. Nevertheless, preterm neonates show compromised GIT structure, function, and immunology, particularly when delivered by caesarean section and fed diets other than mother's milk. Formula-fed preterm infants are thus at increased risk of developing diseases such as necrotizing enterocolitis, unless special care is taken to avoid excessive nutrient fermentation and bacterial overgrowth. The extent to which results obtained in preterm animals (most notably the pig) can be used to reflect similar conditions in preterm infants is discussed.

Immobilization of infant fecal microbiota and utilization in an in vitro colonic fermentation model

Bacteria isolated from infant feces were immobilized in polysaccharide gel beads (2.5% gellan gum, 0.25% xanthan gum) using a two-phase dispersion process. A 52-day continuous culture was carried out in a single-stage chemostat containing precolonized beads and fed with a medium formulated to approximate the composition of infant chyme. Different dilution rates and pH conditions were tested to simulate the proximal (PCS), transverse (TCS), and distal (DCS) colons. Immobilization preserved all nine bacterial groups tested with survival rates between 3 and 56%. After 1 week fermentation, beads were highly colonized with all populations tested (excepted Staphylococcus spp. present in low numbers), which remained stable throughout the 7.5 weeks of fermentation, with variations below 1 log unit. However, free-cell populations in the circulating liquid medium, produced by immobilized cell growth, cell-release activity from gel beads, and free-cell growth, were altered considerably by culture conditions. Compared to the stabilization period, PCS was characterized by a considerable and rapid increase in Bifidobacterium spp. concentrations (7.4 to 9.6 log CFU/mL), whereas Bifidobacterium spp., Lactobacillus spp., and Clostridium spp. concentrations decreased and Staphylococcus spp. and coliforms increased during TCS and DCS. Under pseudo-steady-state conditions, the community structure developed in the chemostat reflected the relative proportions of viable bacterial numbers and metabolites generally encountered in infant feces. This work showed that a complex microbiota such as infant fecal bacteria can be immobilized and used in a continuous in vitro intestinal fermentation model to reproduce the high bacterial concentration and bacterial diversity of the feces inoculum, at least at the genera level, with a high stability during long-term experiment.

Influence of components of infant formulas on in vitro iron, zinc, and calcium availability

The effect of casein content and Ca concentration on Fe, Zn, and Ca dialyzability was assessed using a response surface design. Tested casein levels were 5.31-13.75 g/L (34.8-90.2% of total protein). Whey protein was added to complete 15.25 g/L total protein. Calcium levels were adjusted with calcium citrate within a range between 417.4 and 804.9 mg/L. Through the experimental design utilized, we found that of both assessed factors, only the casein content significantly influenced Fe and Zn dialyzability. Protein composition did not influence calcium dialyzability, and calcium concentration did not affect either Fe or Zn dialyzability. No effect of casein-Ca on iron, zinc, and calcium dialyzability was found. According to these results, whey-dominant formulas are less prone to hamper mineral availability, and are therefore suitable in order to improve iron and zinc availability.

Development of an in vitro digestion method to assess carotenoid bioavailability from meals

The objective of this study was to develop a model for assessing the bioavailability of carotenoids from meals using an in vitro digestion procedure. A meal was prepared using baby food carrots, spinach, and a meat, plus tomato paste. The aqueous fraction was isolated from digesta to determine the quantity of carotenoids transferred from the food to micelles. The micellarization of lutein (25-40%) exceeded (p < 0.01) that of alpha- and beta-carotene (12-18%) and lycopene (<0.5%). Micellarization of carotenoids was not affected by elimination of the gastric phase of the digestive process. The absence of bile extract prevented the transfer of carotenoids from foods to micelles, whereas omission of pancreatin only reduced the micellarization of the carotenes. Differentiated cultures of Caco-2 human intestinal cells accumulated 28-46% of micellarized carotenoids from the medium after 6 h. These results support the usefulness of the in vitro digestion process as a rapid and cost-effective model for screening the bioavailability of carotenoids from meals.