Emulsion Stability Modulates Gastric Secretion and Its Mixing with Emulsified Fat in Healthy Adults in a Randomized Magnetic Resonance Imaging Study

Understanding of formation, gastrointestinal breakdown, and application of whey protein emulsion gels: Insights from intermolecular interactions

Whey protein emulsion gel is an ideal model food for revealing how the multilength scale food structures affect food digestion, as their structure and mechanical properties can be precisely manipulated by controlling the type and intensity of intermolecular interactions between protein molecules. However, there are still significant understanding gaps among intermolecular interactions, protein aggregation and gelation, emulsion gel formation, gel breakdown in the gastrointestinal tract (GIT), and the practical use of whey protein emulsion gels, which limits their GIT-targeted applications. In this regard, the relationship between the structure and digestion behavior of heat-set whey protein emulsion gels is reviewed and discussed mainly from the following aspects: (1) structural characteristics of whey protein molecules; (2) how different types of intermolecular interactions influence heat-induced aggregation and gelation of whey protein in the aqueous solutions and the oil-in-water emulsions, and the mechanical properties of the final gels; (3) functions of the mouth, the stomach, and the small intestine in processing of solid foods, and how different types of intermolecular interactions influence the breakdown properties of heat-set whey protein emulsion gels in GIT (i.e., their respective role in controlling gel digestion). Finally, the implications of knowledge derived from the formation and gastrointestinal breakdown of heat-set whey protein emulsion gels for developing controlled delivery vehicles, human satiety enhancers, and sensory modifiers are highlighted.

Delivery of encapsulated bioactive compounds within food matrices to the digestive tract: recent trends and future perspectives

Encapsulation technologies have achieved encouraging results improving the stability, bioaccessibility and absorption of bioactive compounds post-consumption. There is a bulk of published research on the gastrointestinal behavior of encapsulated bioactive food materials alone using in vitro and in vivo digestion models, but an aspect often overlooked is the impact of the food structure, which is much more complex to unravel and still not well understood. This review focuses on discussing the recent findings in the application of encapsulated bioactive components in fabricated food matrices. Studies have suggested that the integration of encapsulated bioactive compounds has been proven to have an impact on the physicochemical characteristics of the finished product in addition to the protective effect of encapsulation on the fortified bioactive compound. These products containing bioactive compounds undergo further structural reorganization during digestion, impacting the release and emptying rates of fortified bioactive compounds. Thus, by manipulation of various food structures and matrices, the release and delivery of these bioactive compounds can be altered. This knowledge provides new opportunities for designing specialized foods for specific populations.

Strategies for modulating the lipid digestion of emulsions in the gastrointestinal tract

The structural changes in emulsion products can be used to control the bioavailability of fatty acids and lipophilic compounds. After ingestion, lipid droplets undergo breakdown and structural changes as they pass through the gastrointestinal tract. The oil-water interface plays a critical role in modulating the digestive behavior of lipid droplets because changes in the interfacial layer control the adsorption of lipase and bile salts and determine the overall rate and extent of lipid digestion. Therefore, lipid digestibility can be tuned by selecting the appropriate types and levels of stabilizers. The stabilizer can change the lipase accessibility and exposure of lipid substrates, resulting in variable digestion rates. However, emulsified lipids are not only added to food matrixes but are also co-ingested from other dietary components. Therefore, overall consumption behaviors can affect the digestion rate and digestibility of emulsified lipids. Although designing an emulsion structure is challenging, controlling lipid digestion can improve the health benefits of products. Therefore, a thorough understanding of the process of emulsified lipid digestion is required to develop food products that enable specific physiological responses. The targeted or delayed release of lipophilic molecules and fatty acids through emulsion systems has significant applications in healthcare and pharmaceuticals.

Lipid emulsion interfacial design modulates human in vivo digestion and satiation hormone response

Lipid emulsions (LEs) with tailored digestibility have the potential to modulate satiation or act as delivery systems for lipophilic nutrients and drugs. The digestion of LEs is governed by their interfacial emulsifier layer which determines their gastric structuring and accessibility for lipases. A plethora of LEs that potentially modulate digestion have been proposed in recent years, however, in vivo validations of altered LE digestion remain scarce. Here, we report on the in vivo digestion and satiation of three novel LEs stabilized by whey protein isolate (WPI), thermo-gelling methylcellulose (MC), or cellulose nanocrystals (CNCs) in comparison to an extensively studied surfactant-stabilized LE. LE digestion and satiation were determined in terms of gastric emptying, postprandial plasma hormone and metabolite levels characteristic for lipid digestion, perceived hunger/fullness sensations, and postprandial food intake. No major variations in gastric fat emptying were observed despite distinct gastric structuring of the LEs. The plasma satiation hormone and metabolite response was fastest and highest for WPI-stabilized LEs, indicating a limited capability of proteins to prevent lipolysis due to fast hydrolysis under gastric conditions and displacement by lipases. MC-stabilized LEs show a similar gastric structuring as surfactant-stabilized LEs but slightly reduced hormone and metabolite responses, suggesting that thermo-gelling MC prevents lipase adsorption more effectively. Ultimately, CNC-stabilized LEs showed a drastic reduction (>70%) in plasma hormone and metabolite responses. This confirms the efficiency of particle (Pickering) stabilized LEs to prevent lipolysis proposed in literature based on in vitro experiments. Subjects reported more hunger and less fullness after consumption of LEs stabilized with MC and CNCs which were able to limit satiation responses. We do not find evidence for the widely postulated ileal brake, i.e. that delivery of undigested nutrients to the ileum triggers increased satiation. On the contrary, we find decreased satiation for LEs that are able to delay lipolysis. No differences in food intake were observed 5 h after LE consumption. In conclusion, LE interfacial design modulates in vivo digestion and satiation response in humans. In particular, Pickering LEs show extraordinary capability to prevent lipolysis and qualify as oral delivery systems for lipophilic nutrients and drugs.

Physiological fluid interfaces: Functional microenvironments, drug delivery targets, and first line of defense

Fluid interfaces, i.e. the boundary layer of two liquids or a liquid and a gas, play a vital role in physiological processes as diverse as visual perception, oral health and taste, lipid metabolism, and pulmonary breathing. These fluid interfaces exhibit a complex composition, structure, and rheology tailored to their individual physiological functions. Advances in interfacial thin film techniques have facilitated the analysis of such complex interfaces under physiologically relevant conditions. This allowed new insights on the origin of their physiological functionality, how deviations may cause disease, and has revealed new therapy strategies. Furthermore, the interactions of physiological fluid interfaces with exogenous substances is crucial for understanding certain disorders and exploiting drug delivery routes to or across fluid interfaces. Here, we provide an overview on fluid interfaces with physiological relevance, namely tear films, interfacial aspects of saliva, lipid droplet digestion and storage in the cell, and the functioning of lung surfactant. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe therapies and drug delivery approaches targeted at fluid interfaces. STATEMENT OF SIGNIFICANCE: Fluid interfaces are inherent to all living organisms and play a vital role in various physiological processes. Examples are the eye tear film, saliva, lipid digestion & storage in cells, and pulmonary breathing. These fluid interfaces exhibit complex interfacial compositions and structures to meet their specific physiological function. We provide an overview on physiological fluid interfaces with a focus on interfacial phenomena. We elucidate their structure-function relationship, discuss diseases associated with interfacial composition, and describe novel therapies and drug delivery approaches targeted at fluid interfaces. This sets the scene for ocular, oral, or pulmonary surface engineering and drug delivery approaches.

New insights into in vivo gastroduodenal digestion of oil-in-water emulsions: gastric stability and in vitro digestion modeling

In this paper, effect of emulsion stability on gastroduodenal emptying/secretion was reviewed and differentiated. Moreover, novel perspectives on physiology of gastric lumen, duodenum, and gall bladder were achieved using mathematical models, being useful for designing artificial digestive systems. In this regard, numerical data for dynamic gastric emptying/secretion were offered for gastric-stable and gastric-unstable emulsion intakes. It was shown that alterations in human gastric and duodenal volume follow, respectively, linear and sinusoidal curves, with high correlation coefficients (r² > 0.93). For both emulsions, about 30-40 mL ingesta discharged rapidly from stomach upon ingestion; However, further gastric emptying was regulated for the rest of digestion period, so that 0.1 mL/min oil was passing through duodenum. Intragastric evacuation of both emulsions started with a lag phase during which stomach stored secretions incrementally by slow gastric discharge. Lag phase ended with fat layering, when emptying considerably enhanced. This reduction was gradual for stable emulsion while unstable emulsion experienced a rapid emptying before slow declining trend. Along with initial gastric emptying, 87% of gallbladder content discharged into duodenum, prolonged up to the gradual reduction phase of stomach. Supplementary investigations are needed to quantify gastroduodenal secretions, particularly pepsin and pancreas in response to emulsion ingesta.

A Rat Model of Human Lipid Emulsion Digestion

A better understanding of how dietary lipids are processed by the human body is necessary to allow for the control of satiation and energy intake by tailored lipid systems. To examine whether rats are a valid model of human dietary lipid processing and therefore useful for further mechanistic studies in this context, we tested in rats three lipid emulsions of different stability, which alter satiety responses in humans. Different sets of 15 adult male Sprague Dawley rats, equipped with gastric catheters alone or combined with hepatic portal vein (HPV) and vena cava (VC) catheters were maintained on a medium-fat diet and adapted to an 8 h deprivation/16 h feeding schedule. Experiments were performed in a randomized cross-over study design. After gastric infusion of the lipid emulsions, we assessed gastric emptying by the paracetamol absorption test and recorded in separate experiments food intake and plasma levels of gastrointestinal hormones and metabolites in the HPV. For an acid stable emulsion, slower gastric emptying and an enhanced release of satiating gastrointestinal (GI) hormones were observed and were associated with lower short-term energy intake in rats and less hunger in humans, respectively. The magnitude of hormonal responses was related to the acid stability and redispersibility of the emulsions and thus seems to depend on the availability of lipids for digestion. Plasma metabolite levels were unaffected by the emulsion induced changes in lipolysis. The results support that structured lipid systems are digested similarly in rats and humans. Thus unstable emulsions undergo the same intragastric destabilization in both species, i.e., increased droplet size and creaming. This work establishes the rat as a viable animal model for in vivo studies on the control of satiation and energy intake by tailored lipid systems.

Intraluminal Impact of Food: New Insights from MRI

Understanding how the gut responds to food has always been limited by the available investigatory techniques. Previous methods involving intubation and aspiration are largely limited to liquid-only meals. The aim of this review is to describe how MRI has allowed analysis of the processing of complex multiphase meals. This has demonstrated the role of physical factors such as viscosity, fat and fibre content in controlling gastric secretion and motility. It has also allowed the description of changes induced in small bowel water content and the role of osmotic effects of poorly absorbed carbohydrates such as fructose, sorbitol and mannitol. Intestinal secretions can be shown to be stimulated by a range of fruit and vegetables and the effect of this on colonic water content can also be measured. This has been used to demonstrate the mode of action of commonly used laxatives including bran and psyllium. The wealth of data which can be obtained together with its non-invasive nature and safety makes the technique ideal for the serial evaluation of the impact of different nutrients and drugs in both health and disease.

Rheology of Swiss Cheese Fondue

Cheese fondue is a popular Swiss dish prepared by melting cheese under the addition of wine, starch, and seasoning. The flow behavior or rheology of fondue is crucial for mouthfeel, flavor release, and the tendency of fondue to cling to the bread. Fondue is a complex multiphase system whose rheology is determined by the interactions of its colloidal ingredients. We establish cheese fondue as a water-continuous system with dispersed fat droplets, charged caseins, and starch granules. Irreversible phase separation, a common issue in fondue preparation, may be prevented by addition of a critical minimum starch concentration. Fondue was found to be a shear-thinning yield stress fluid, which is desirable for mouthfeel and facilitates fondue to cling to the bread for consumption. Fondue showed a viscoelastic stress response around the gel point (G' ≈ G″), which is proposed as crucial for the balance of orally perceived gumminess (G') and liquidity (G″). Ethanol addition and lowering pH toward the isoelectric point of casein, as associated with wine addition, decrease fondue viscosity due to a decrease in casein micelle size. Below the isoelectric point of casein, fondue is unstable and phase separates, potentially impeding fondue digestion. Thus, fondue rheology is governed by the complex colloidal interactions within its ingredients, and ultimately determines fondue eating experience.

Gastric motor and sensory function in health assessed by magnetic resonance imaging: Establishment of reference intervals for the Nottingham test meal in healthy subjects

BACKGROUND

Current investigations of gastric emptying rarely identify the cause of symptoms or provide a definitive diagnosis in patients with dyspepsia. This study assessed gastric function by magnetic resonance imaging (MRI) using the modular "Nottingham test meal" (NTM) in healthy volunteers (HVs).

METHODS

The NTM comprises (a) 400 mL liquid nutrient (0.75 kcal/mL) labeled with Gadolinium-DOTA and (b) an optional solid component (12 agar-beads [0 kcal]). Filling sensations were documented. MRI measurements of gastric volume, emptying, contraction wave frequency, and secretion were obtained using validated methods.

KEY RESULTS

Gastric function was measured in a population of 73 HVs stratified for age and sex. NTM induced moderate satiety and fullness. Labeled fluid was observed in the small bowel in all subjects after meal ingestion ("early-phase" GE). Secretion was rapid such that postprandial gastric content volume was often greater than meal volume (GCV0 > 400 mL), and there was increasing dilution of the meal during the study (P < 0.001). Gastric half-time was median 66-minutes (95% reference interval 35 to 161-minutes ["late-phase" GE]). The number of intact agar beads in the stomach was 7/12 (58%) at 60-minutes and 1/12 (8%) at 120-minutes. Age, bodyweight and sex had measurable effects on gastric function; however, these were small compared to inter-individual variation for most metrics.

CONCLUSIONS AND INFERENCES

Reference intervals are presented for MRI measurements of gastric function assessed for the mixed liquid/solid NTM. Studies in patients will determine which metrics are of clinical value and also whether the reference intervals presented here offer optimal diagnostic sensitivity and specificity.

Tailoring Emulsions for Controlled Lipid Release: Establishing in vitro-in Vivo Correlation for Digestion of Lipids

The use of oil-in-water emulsions for controlled lipid release is of interest to the pharmaceutical industry in the development of poorly water soluble drugs and also has gained major interest in the treatment of obesity. In this study, we focus on the relevant in vitro parameters reflecting gastric and intestinal digestion steps to reach a reliable in vitro-in vivo correlation for lipid delivery systems. We found that (i) gastric lipolysis determines early lipid release and sensing. This was mainly influenced by the emulsion stabilization mechanism. (ii) Gastric mucin influences the structure of charge-stabilized emulsion systems in the stomach, leading to destabilization or gel formation, which is supported by in vivo magnetic resonance imaging in healthy volunteers. (iii) The precursor structures of these emulsions modulate intestinal lipolysis kinetics in vitro, which is reflected in plasma triglyceride and cholecystokinin concentrations in vivo.

Clinical assessment of gastric emptying and sensory function utilizing gamma scintigraphy: Establishment of reference intervals for the liquid and solid components of the Nottingham test meal in healthy subjects

BACKGROUND

Current investigations of stomach function are based on small test meals that do not reliably induce symptoms and analysis techniques that rarely detect clinically relevant dysfunction. This study presents the reference intervals of the modular "Nottingham test meal" (NTM) for assessment of gastric function by gamma scintigraphy (GSc) in a representative population of healthy volunteers (HVs) stratified for age and sex.

METHODS

The NTM comprises 400 mL liquid nutrient (0.75 kcal/mL) and an optional solid component (12 solid agar-beads (0 kcal). Filling and dyspeptic sensations were documented by 100 mm visual analogue scale (VAS). Gamma scintigraphy parameters that describe early and late phase Gastric emptying (GE) were calculated from validated models.

KEY RESULTS

Gastric emptying (GE) of the liquid component was measured in 73 HVs (male 34; aged 45±20). The NTM produced normal postprandial fullness (VAS ≥30 in 41/74 subjects). Dyspeptic symptoms were rare (VAS ≥30 in 2/74 subjects). Gastric emptying half-time with the Liquid- and Solid-component -NTM was median 44 (95% reference interval 28-78) minutes and 162 (144-193) minutes, respectively. Gastric accommodation was assessed by the ratio of the liquid-NTM retained in the proximal:total stomach and by Early phase emptying assessed by gastric volume after completing the meal (GCV0). No consistent effect of anthropometric measures on GE parameters was present.

CONCLUSIONS AND INFERENCES

Reference intervals are presented for GSc measurements of gastric motor and sensory function assessed by the NTM. Studies involving patients are required to determine whether the reference interval range offers optimal diagnostic sensitivity and specificity.