Milk mimicry – Triglyceride mixtures that mimic lipid structuring during the digestion of bovine and human milk

Compartmentalised enzyme-induced phase transformations in self-assembling lipid systems

HYPOTHESIS

Understanding the digestion of lipid-based pharmaceutical formulations and food systems is necessary for optimising drug and nutrient delivery and has been extensively studied in bulk emulsion systems using the pH-stat method [1]. However, this approach is not suitable for investigation of individual lipid droplets, in particular the interface where the lipase acts. Microfluidic approaches to study digestion at lipid-water interfaces using droplet trapping have been proposed, however the aqueous phase in that case washes over the interface presenting uncertainty over the stoichiometry of interactions [2]. The internal interface of a Janus-like droplet, containing distinct aqueous and lipid compartments, mimics the interface of a lipid droplet in aqueous solution with controlled stoichiometry [3]. Hence, it was hypothesised that the internal interface of Janus droplets can offer a precise way to study the enzymatic digestion of lipids formulations.

EXPERIMENTS

Using microfluidic methods, Janus-like droplets were formed by coalescing emulsion droplets containing lipid formulation and pancreatic lipase. Polarised light microscopy (PLM) and in-situ small-angle X-ray scattering (SAXS) were used to investigate the droplets.

FINDINGS

PLM revealed the growth of an aligned inverse hexagonal phase (H2), and with SAXS showed that this phase transformation and alignment resulted from enzymatic digestion. A subsequent partial transformation from H2 to inverse bicontinuous cubic phase occurred when simulated intestinal fluid was used instead of Tris buffer. Suggesting that phospholipids and bile salts could diffuse across the internal interface to locally affect their surroundings.

Comparison of phospholipid composition and microstructure of milk fat globules contained in human milk and infant formulae

Phospholipids play key roles in infant nutrition and cognitive development. It is hypothesized that infant formula (IF) has lower phospholipid species, content and milk fat globule (MFG) structural integrity than human milk (HM). Herein, we performed qualitative and quantitative analyses of phospholipids in six classes of IF and HM using ultra-performance liquid chromatography with mass spectrometry. The contents of phosphatidylethanolamine (15.81 ± 7.20 mg/L) and sphingomyelin (35.84 ± 15.56 mg/L) in IF were significantly lower than those in HM (30.74 ± 17.38 mg/L, 45.53 ± 16.04 mg/L, respectively). Among the six IF classes, cow's milk-based IF had the highest number of phospholipid species, and IF containing milk fat globular membrane had the highest phospholipid content. The size, zeta potential, and amount of MFGs in IF were significantly lower than those in HM. These results may prove useful for designing better IF that mimic HM.

Construction of a Synthetic Colostrum Substitute and Its Protection of Intestinal Cells against Inflammation in an In Vitro Model of Necrotizing Enterocolitis

Colostrum provides bioactive components that are essential for the colonization of microbiota in the infant gut, while preventing infectious diseases such as necrotizing enterocolitis. As colostrum is not always available from the mother, particularly for premature infants, effective and safe substitutes are keenly sought after by neonatologists. The benefits of bioactive factors in colostrum are recognized; however, there have been no accounts of human colostrum being studied during digestion of the lipid components or their self-assembly in gastrointestinal environments. Due to the weaker bile pool in infants than adults, evaluating the lipid composition of human colostrum and linking it to structural self-assembly behavior is important in these settings and thus enabling the formulation of substitutes for colostrum. This study is aimed at the rational design of an appropriate lipid component for a colostrum substitute and determining the ability of this formulation to reduce inflammation in intestinal cells. Gas chromatography was utilized to map lipid composition. The self-assembly of lipid components occurring during digestion of colostrum was monitored using small-angle X-ray scattering for comparison with substitute mixtures containing pure triglyceride lipids based on their abundance in colostrum. The digestion profiles of human colostrum and the substitute mixtures were similar. Subtle differences in lipid self-assembly were evident, with the substitute mixtures exhibiting additional non-lamellar phases, which were not seen for human colostrum. The difference is attributable to the distribution of free fatty acids released during digestion. The biological markers of necrotizing enterocolitis were modulated in cells that were treated with bifidobacteria cultured on colostrum substitute mixtures, compared to those treated with infant formula. These findings provide an insight into a colostrum substitute mixture that resembles human colostrum in terms of composition and structural behavior during digestion and potentially reduces some of the characteristics associated with necrotizing enterocolitis.

Impact of pasteurization on the self-assembly of human milk lipids during digestion

Human milk is critical for the survival and development of infants. This source of nutrition contains components that protect against infections while stimulating immune maturation. In cases where the mother's own milk is unavailable, pasteurized donor milk is the preferred option. Although pasteurization has been shown to have minimal impact on the lipid and FA composition before digestion, no correlation has been made between the impact of pasteurization on the FFA composition and the self-assembly of lipids during digestion, which could act as delivery mechanisms for poorly water-soluble components. Pooled nonpasteurized and pasteurized human milk from a single donor was used in this study. The evolving FFA composition during digestion was determined using GC coupled to a flame ionization detector. In vitro digestion coupled to small-angle X-ray scattering was utilized to investigate the influence of different calcium levels, fat content, and the presence of bile salts on the extent of digestion and structural behavior of human milk lipids. Almost complete digestion was achieved when bile salts were added to the systems containing high calcium to milk fat ratio, with similar structural behavior of lipids during digestion of both types of human milk being apparent. In contrast, differences in the colloidal structures were formed during digestion in the absence of bile salt because of a greater amount of FFAs being released from the nonpasteurized than pasteurized milks. This difference in FFAs released from both types of human milk could result in varying nutritional implications for infants.

The influence of lipid digestion on the fate of orally administered drug delivery vehicles

This review will focus on orally administered lipid-based drug delivery vehicles and specifically the influence of lipid digestion on the structure of the carrier lipids and their entrained drug cargoes. Digestion of the formulation lipids, which are typically apolar triglycerides, generates amphiphilic monoglycerides and fatty acids that can self-assemble into a diverse array of liquid crystalline structures. Tracking the dynamic changes in self-assembly of the lipid digestion products during digestion has recently been made possible using synchrotron-based small angle X-ray scattering. The influence of lipid chain length and degree of unsaturation on the resulting lipid structuring will be described in the context of the critical packing parameter theory. The chemical and structural transformation of the formulation lipids can also have a dramatic impact on the physical state of drugs co-administered with the formulation. It is often assumed that the best strategy for drug development is to maximise drug solubility in the undigested formulation lipids and to incorporate additives to maintain drug solubility during digestion. However, it is possible to improve drug absorption using lipid digestion in cases where the solubility of the dosed drug or one of its polymorphic forms is greater in the digested lipids. Three different fates for drugs administered with digestible lipid-based formulations will be discussed: (1) where the drug is more soluble in the undigested formulation lipids; (2) where the drug undergoes a polymorphic transformation during lipid digestion; and (3) where the drug is more soluble in the digested formulation lipids.

Formation of Self-Assembled Mesophases During Lipid Digestion

Lipids play an important role in regulating bodily functions and providing a source of energy. Lipids enter the body primarily in the form of triglycerides in our diet. The gastrointestinal digestion of certain types of lipids has been shown to promote the self-assembly of lipid digestion products into highly ordered colloidal structures. The formation of these ordered colloidal structures, which often possess well-recognized liquid crystalline morphologies (or “mesophases”), is currently understood to impact the way nutrients are transported in the gut and absorbed. The formation of these liquid crystalline structures has also been of interest within the field of drug delivery, as it enables the encapsulation or solubilization of poorly water-soluble drugs in the aqueous environment of the gut enabling a means of absorption. This review summarizes the evidence for structure formation during the digestion of different lipid systems associated with foods, the techniques used to characterize them and provides areas of focus for advancing our understanding of this emerging field.

Emulsions containing optimum cow milk fat and canola oil mixtures replicate the lipid self-assembly of human breast milk during digestion

HYPOTHESIS

The digestion of different milks and milk substitutes leads to the formation of a variety of self-assembled lipid structures, with the structuring of human milk being paramount for infant nutrition. It was hypothesised that mixing cow milk fat rich in medium/long-chain lipids with canola oil rich in long-chain unsaturated lipids would replicate the structuring of human milk by balancing lipid chain lengths and saturation levels.

EXPERIMENTS

Emulsions of cow milk fat/canola oil mixtures were prepared in two ways - by pre-mixing ghee and canola oil before dispersing them and by dispersing canola oil directly into commercial cow milk. Small angle X-ray scattering combined with titration of the fatty acids produced during digestion allowed for the correlation of dynamic lipid self-assembly with the extent of lipid digestion. Laser light scattering was used to show that the particle sizes in the digesting mixtures were similar and coherent anti-Stokes Raman spectroscopy (CARS) microscopy was used to confirm the mixing of canola oil into cow milk fat globules.

FINDINGS

As the amount of long-chain unsaturated canola oil lipids in the mixtures increased, the lipid self-assembly tended towards colloidal structures of greater interfacial curvature. When the ratio of cow milk fat to canola oil lipids was 1:1 (w/w), the digesting lipids assembled themselves into the same liquid crystalline structures as human breast milk. This observation was independent of the method used to mix the lipids, with CARS microscopy indicating uniform mixing of the canola oil into cow milk upon ultrasonication.