Magnetic resonance imaging of the behaviour of oil-in-water emulsions in the gastric lumen of man

Biophysical insights into modulating lipid digestion in food emulsions

During the last decade, major scientific advances on understanding the mechanisms of lipid digestion and metabolism have been made, with a view to addressing health issues (such as obesity) associated with overconsumption of lipid-rich and sucrose-rich foods. As lipids in common foods exist in the form of emulsions, the structuring of emulsions has been one the main strategies for controlling the rate of lipid digestion and absorption, at least from a colloid science viewpoint. Modulating the kinetics of lipid digestion and absorption offers interesting possibilities for developing foods that can provide control of postprandial lipaemia and control the release of lipophilic compounds. Food emulsions can be designed to achieve considerable differences in the kinetics of lipid digestion but most research has been applied to relatively simple model systems and in in vitro digestion models. Further research to translate this knowledge into more complex food systems and to validate the results in human studies is required. One promising approach to delay/control lipid digestion is to alter the stomach emptying rate of lipids, which is largely affected by interactions of emulsion droplets with the food matrices. Food matrices with different responses to the gastric environment and with different interactions between oil droplets and the food matrix can be designed to influence lipid digestion. This review focuses on key scientific advances made during the last decade on understanding the physicochemical and structural modifications of emulsified lipids, mainly from a biophysical science perspective. The review specifically explores different approaches by which the structure and stability of emulsions may be altered to achieve specific lipid digestion kinetics.

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.

Palmitic acid-rich oils with and without interesterification lower postprandial lipemia and increase atherogenic lipoproteins compared with a MUFA-rich oil: A randomized controlled trial

BACKGROUND

Interesterified (IE) fats are widely used in place of trans fats; however, little is known about their metabolism.

OBJECTIVES

To test the impact of a commonly consumed IE compared with a non-IE equivalent fat on in vivo postprandial and in vitro lipid metabolism, compared with a reference oil [rapeseed oil (RO)].

METHODS

A double-blinded, 3-phase crossover, randomized controlled trial was performed in healthy adults (n = 20) aged 45-75 y. Postprandial plasma triacylglycerol and lipoprotein responses (including stable isotope tracing) to a test meal (50 g fat) were evaluated over 8 h. The test fats were IE 80:20 palm stearin/palm kernel fat, an identical non-IE fat, and RO (control). In vitro, mechanisms of digestion were explored using a dynamic gastric model (DGM).

RESULTS

Plasma triacylglycerol 8-h incremental area under the curves were lower following non-IE compared with RO [-1.7 mmol/L⋅h (95% CI: -3.3, -0.0)], but there were no differences between IE and RO or IE and non-IE. LDL particles were smaller following IE and non-IE compared with RO (P = 0.005). Extra extra large, extra large, and large VLDL particle concentrations were higher following IE and non-IE compared with RO at 6-8 h (P < 0.05). No differences in the appearance of [13C]palmitic acid in plasma triacylglycerol were observed between IE and non-IE fats. DGM revealed differences in phase separation of the IE and non-IE meals and delayed release of SFAs compared with RO.

CONCLUSIONS

Interesterification did not modify fat digestion, postprandial lipemia, or lipid metabolism measured by stable isotope and DGM analysis. Despite the lower lipemia following the SFA-rich fats, increased proatherogenic large triacylglycerol-rich lipoprotein remnant and small LDL particles following the SFA-rich fats relative to RO adds a new postprandial dimension to the mechanistic evidence linking SFAs to cardiovascular disease risk.

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.

Structural and physicochemical changes in almond milk during in vitro gastric digestion: impact on the delivery of protein and lipids

Almond milk (about 3% protein and 7% lipids) was prepared using wet disintegration of raw almonds and then subjected to in vitro gastric digestion using an advanced dynamic digestion model (i.e., a human gastric simulator). Microstructural changes, physicochemical behavior, and protein digestion were examined; the release of lipids and protein during digestion was quantified. Under acidic gastric conditions, almond oil bodies flocculated. Proteolysis by pepsin led to destabilization and coalescence of the oil bodies, resulting in creaming and phase separation. This phase separation significantly delayed the delivery of lipids to the small intestine. After 225 min of digestion, ∼42% of the lipids remained in the stomach. In contrast, protein release was not significantly affected by the gastric behavior of the almond oil bodies. This study provides a better understanding of how the digestive system manages plant lipids, and may be useful in the microstructural design of foods to achieve a controlled physiological response during digestion.

Lipid droplet size and emulsification on postprandial glycemia, insulinemia and lipidemia

Previous studies have suggested that a smaller lipid droplet size results in a greater rate of lipolysis. However, acute health impacts of emulsification and small lipid droplet size are not well understood. We aimed to investigate the effect of emulsification and lipid droplet size on postprandial lipidemia, glycemia and insulinemia. Fifteen healthy Chinese males (mean ± SD, age of 26 ± 6 years and BMI of 22.2 ± 1.2 kg m^-2) participated on 3 separate occasions in a randomized order. Participants received an olive oil-water beverage and white bread as test meals. The three test beverages were as follows: (1) an olive oil-water mixture (non-emulsified, control), (2) fine olive oil-water emulsion (small lipid droplet size) and (3) coarse olive oil-water emulsion (large lipid droplet size). Glucose, insulin, triglyceride, non-esterified fatty acid (NEFA), gastric antral distention and appetite measurements were recorded for 4 hours. Glucose and insulin concentrations increased rapidly after administration of non-emulsified beverages as compared to fine and coarse emulsions with a significant difference at 30 min (95% confidence interval, P < 0.05). Fine emulsion led to a significant increase in triglyceride responses, a smaller suppression of NEFA responses and slowed gastric emptying compared to the non-emulsified beverage and coarse emulsion (iAUC, 95% confidence interval, P < 0.05). Emulsification and alteration of lipid droplet size have acute effects on glucose, insulin, triglyceride and fatty acid responses.

Gastric Mixing During Food Digestion: Mechanisms and Applications

Gastric mixing is a complex process that is governed by meal properties, such as food buffering capacity, physical properties, and the rate of breakdown as well as physiological factors, such as the rate of gastric secretions, gastric emptying, and gastric motility. Gastric mixing processes have been studied through the use of experimental and computational methods. Gastric mixing impacts the intragastric pH distribution and residence time in the stomach for ingested materials. Development of a fundamental understanding of the advective and diffusion processes and their roles in gastric mixing will be important in furthering our understanding of food breakdown, microbial survival, and drug dissolution during gastric digestion.

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

BACKGROUND

Oil-in-water emulsions have recently become of interest to nutritional sciences because of their ability to influence gastrointestinal digestive processes and ultimately benefit human health. MRI offers the potential to noninvasively characterize the interaction between emulsified lipids and gastric secretion within the stomach.

OBJECTIVES

We determined noninvasively how emulsion stability modulates volumes of fat and secretion, layering of fat, and the mixing of emulsified fat with secretion within the stomach. This required the development of MRI technology for quantifying fat and secretion concentrations inside the stomach.

METHODS

Twenty-one healthy adults [13 men, mean ± SD age: 22.5 ± 2.5 y, mean ± SD body mass index (in kg/m²): 22.7 ± 1.8] were analyzed in a single-blind, randomized, parallel design. MRI was used to acquire the distributions of fat and secretion in the stomach after ingestion of 2 emulsions: a stable emulsion (E1) or an unstable emulsion (E4) with 20% fat fraction and ∼0.3 mm droplet sizes. Layer, volume, and mixing variables were fitted to the data and compared between the 2 emulsions.

RESULTS

The intragastric mixing between fat and secretion was better with the E4 than the E1 [increase in content heterogeneity of 17.1% (95% CI: 12.3%, 21.9%)]. The E4 demonstrated a linear relation [slope 1.57 (95% CI: 0.86, 2.29)] between the degree of layering and mixing. In contrast, no such relation was detected for the E1. Accumulated secretion volume in the stomach was lower with the E4 [decrease in volume variable k_s of 2.3 (95% CI: -3.9, -0.7)] and correlated with the degree of layering (r = 0.62, P < 0.001).

CONCLUSIONS

In healthy adults, intragastric fat layering was influenced mainly by the degree of intragastric mixing, rather than the overall dominance of secretion. The E1 triggered a higher accumulation of gastric secretion, which in turn facilitated homogenization of intragastric content in comparison with its unstable counterpart. This trial was registered at clinicaltrials.gov as NCT02602158.

Functional food microstructures for macronutrient release and delivery

There is a need to understand the role of fat, protein and carbohydrate in human health, and also how foods containing and/or structured using these macronutrients can be designed so that they can have a positive impact on health. This may include a reduction in fat, salt or sugar, the protection and targeted release of micronutrients or active ingredients from/to particular parts of the digestive system, improvement of gastrointestinal health or satiety enhancing properties. Such foods can be designed with various macro- and microstructures that will impact on macronutrient release and delivery. These include simple and double emulsions, the use of Pickering particles and shells, nanoparticles, liposomes, gelled networks, fluid gels and gel particles, foams, self-assembled structures, and encapsulated systems. In order to design foods that deliver these benefits understanding of how these structures behave in the gastrointestinal tract is also required, which should involve utilising both in vitro and in vivo studies. This review aims to draw together research in these areas, by focusing on the current state of the art, but also exciting possibilities for future research and food development.

The visualisation and quantification of human gastrointestinal fat distribution with MRI: a randomised study in healthy subjects

We aimed to study the fate of fat during digestion. For this purpose, we validated and investigated the non-invasive quantification of gastric and duodenal fat emptying and emulsion processing (creaming and phase separation) using the MRI method iterative decomposition with echo asymmetry and least squares estimation (IDEAL). In total, twelve healthy subjects were studied on two separate visits in a single-blind, randomised, cross-over design study. IDEAL was utilised to repeatedly acquire quantitative fat fraction maps of the gastrointestinal tract after infusion of one of two fat emulsions: E1 (acid stable, droplet size 0·33 mm) and E4 (acid unstable, 0·38 mm). In vitro and in vivo validation was carried out using diluted emulsion and gastric content samples, respectively, and resulted in Lin’s concordance correlation coefficients of 1·00 (95 % CI 0·98, 1·00) and 0·91 (95 % CI 0·87, 0·94), respectively. Fat fraction maps and intragastric emulsion profiles enabled the identification of features of intraluminal phase separation and creaming that were not visible in conventional MRI. Gastric fat emptying was faster for E4 compared with E1 with a difference of 2·5 (95 % CI 1·9, 3·1) ml/h. Duodenal content volumes were larger for E1 than for E4 with a difference of 4·9 (95 % CI 3·9, 8·5) ml. This study demonstrated that with IDEAL it was possible (1) to visualise the intragastric and duodenal fat distribution and (2) to quantify the differences in emptying, phase separation and creaming of an acid-stable and an acid-unstable emulsion. This method has potential to bridge the gap between current in vitro digestive models and in vivo behaviour and to be applied in the development of effective functional foods.

Impact of gastric pH profiles on the proteolytic digestion of mixed βlg-Xanthan biopolymer gels

Gastric pH profile duringin vitrogastric digestion is critical for proper assessment of mixed biopolymer gel proteolysis.

Effects of inhomogeneity on triglyceride digestion of emulsions using an in vitro digestion model (Tiny TIM)

The concentration of fatty acids in the small intestine duringin vitrodigestion of emulsions is reported.

In vitro digestibility of highly concentrated methylcellulose O/W emulsions: rheological and structural changes

The changes in structure during the digestion of highly concentrated methyl cellulose (MC) O/W emulsions and of hydrated MC were investigated.

Cross-linking of sodium caseinate-structured emulsion with transglutaminase alters postprandial metabolic and appetite responses in healthy young individuals

The physico-chemical and interfacial properties of fat emulsions influence lipid digestion and may affect postprandial responses. The aim of the present study was to determine the effects of the modification of the interfacial layer of a fat emulsion by cross-linking on postprandial metabolic and appetite responses. A total of fifteen healthy individuals (26·5 (sem6·9) years and BMI 21·9 (sem2·0) kg/m2) participated in a cross-over design experiment in which they consumed two isoenergetic (1924 kJ (460 kcal)) and isovolumic (250 g) emulsions stabilised with either sodium caseinate (Cas) or transglutaminase-cross-linked sodium caseinate (Cas-TG) in a randomised order. Blood samples were collected from the individuals at baseline and for 6 h postprandially for the determination of serum TAG and plasma NEFA, cholecystokinin (CCK), glucagon-like peptide 1 (GLP-1), glucose and insulin responses. Appetite was assessed using visual analogue scales. Postprandial TAG and NEFA responses and gastric emptying (GE) rates were comparable between the emulsions. CCK increased more after the ingestion of Cas-TG than after the ingestion of Cas (P< 0·05), while GLP-1 responses did not differ between the two test emulsions. Glucose and insulin profiles were lower after consuming Cas-TG than after consuming Cas (P< 0·05). The overall insulin, glucose and CCK responses, expressed as areas above/under the curve, did not differ significantly between the Cas and Cas-TG meal conditions. Satiety ratings were reduced and hunger, desire to eat and thirst ratings increased more after the ingestion of Cas-TG than after the ingestion of Cas (P< 0·05). The present results suggest that even a subtle structural modification of the interfacial layer of a fat emulsion can alter the early postprandial profiles of glucose, insulin, CCK, appetite and satiety through decreased protein digestion without affecting significantly on GE or overall lipid digestion.

Reduced-fat foods: the complex science of developing diet-based strategies for tackling overweight and obesity

Fat plays multiple roles in determining the desirable physicochemical properties, sensory attributes, nutritional profile, and biologic response of food products. Overconsumption of fats is linked to chronic diseases, such as obesity, coronary heart disease, diabetes, and cancer. There is therefore a need to develop reduced-fat products with physicochemical properties and sensory profiles that match those of their full-fat counterparts. In addition, foods may be redesigned to increase the feelings of satiety and satiation, and thereby reduce overall food intake. The successful design of these types of functional foods requires a good understanding of the numerous roles that fat plays in determining food attributes and the development of effective strategies to replace these attributes. This article provides an overview of the current understanding of the influence of fat on the physicochemical and physiologic attributes of emulsion-based food products and highlights approaches to create high-quality foods with reduced-fat contents.

Imaging gastric structuring of lipid emulsions and its effect on gastrointestinal function: a randomized trial in healthy subjects

BACKGROUND

Efficient fat digestion requires fat processing within the stomach and fat sensing in the intestine. Both processes also control gastric emptying and gastrointestinal secretions.

OBJECTIVE

We aimed to visualize the influence of the intragastric stability of fat emulsions on their dynamics of gastric processing and structuring and to assess the effect this has on gastrointestinal motor and secretory functions.

DESIGN

Eighteen healthy subjects with normal body mass index (BMI) were studied on 4 separate occasions in a double-blind, randomized, crossover design. Magnetic resonance imaging (MRI) data of the gastrointestinal tract and blood triglycerides were recorded before and for 240 min after the consumption of the following 4 different fat emulsions: lipid emulsion 1 (LE1; acid stable, 0.33 μm), lipid emulsion 2 (LE2; acid stable, 52 μm), lipid emulsion 3 (LE3; acid unstable, solid fat, 0.32 μm), and lipid emulsion 4 (LE4; acid unstable, liquid fat, 0.38 μm).

RESULTS

Intragastric emulsion instability was associated with a change in gastric emptying. Acid-unstable emulsions exhibited biphasic and faster emptying profiles than did the 2 acid-stable emulsions (P ≤ 0.0001). When combined with solid fat (LE3), different dynamics of postprandial gallbladder volume were induced (P ≤ 0.001). For acid-stable emulsions, a reduction of droplet size by 2 orders of magnitude [LE1 (0.33 μm) compared with LE2 (52 μm)] delayed gastric emptying by 38 min. Although acid-stable (LE1 and LE2) and redispersible (LE4) emulsions caused a constant increase in blood triglycerides, no increase was detectable for LE3 (P < 0.0001). For LE3, MRI confirmed the generation of large fat particles during gastric processing, which emptied into and progressed through the small intestine.

CONCLUSIONS

MRI allows the detailed characterization of the in vivo fate of lipid emulsions. The acute effects of lipid emulsions on gastric emptying, gallbladder volume, and triglyceride absorption are dependent on microstructural changes undergone during consumption. Gastric peristalsis and secretion were effective at redispersing pools of liquid fat in the stomach. This trial was registered at clinicaltrials.gov as NCT01253005.

Additive effects of gastric volumes and macronutrient composition on the sensation of postprandial fullness in humans

Background/Objectives:

Intake of food or fluid distends the stomach and triggers mechanoreceptors and vagal afferents. Wall stretch and tension produces a feeling of fullness. Duodenal infusion studies assessing gastric sensitivity by barostat have shown that the products of fat digestion have a greater effect on the sensation of fullness and also dyspeptic symptoms than carbohydrates. We tested here the hypothesis that fat and carbohydrate have different effects on gastric sensation under physiological conditions using non-invasive magnetic resonance imaging (MRI) to measure gastric volumes.

Subjects/Methods:

Thirteen healthy subjects received a rice pudding test meal with added fat or added carbohydrate on two separate occasions and underwent serial postprandial MRI scans for 4.5 h. Fullness was assessed on a 100-mm visual analogue scale.

Results:

Gastric half emptying time was significantly slower for the high-carbohydrate meal than for the high-fat meal, P=0.0327. Fullness significantly correlated with gastric volumes for both meals; however, the change from baseline in fullness scores was higher for the high-fat meal for any given change in stomach volume (P=0.0147), despite the lower energy content and faster gastric emptying of the high-fat meal.

Conclusions:

Total gastric volume correlates positively and linearly with postprandial fullness and ingestion of a high-fat meal increases this sensation compared with high-carbohydrate meal. These findings can be of clinical interest in patients presenting with postprandial dyspepsia whereby manipulating gastric sensitivity by dietary intervention may help to control digestive sensations.

Behaviour of whey protein emulsion gel during oral and gastric digestion: effect of droplet size

A set of whey protein stabilized-emulsion gels with different droplet size distributions (D4,3 = ∼1, 6 and 12 μm) was produced, and the mechanical properties of the gels in the linear viscoelastic region and at large deformation were measured, along with the physicochemical and structural changes of the gels during oral mastication and gastric digestion. The gels containing 1 μm oil droplets had an aggregated particle structure with proteins coating at oil droplets whereas the gels containing 12 μm oil droplets had a particle-filled structure with spatially continuous matrix. During oral processing, the release of oil droplets from the gels increased as the droplet size increased, with coalescence being seen in gels containing oil droplets of 6 and 12 μm diameter. Under gastric digestion, high degrees of coalescence and phase separation of oil droplets occurred in the gels containing 6 and 12 μm oil droplets because of oil droplet release from the gel matrix; this led to slow gastric emptying. The gels were finally broken down into peptide aggregates and oil droplets (or free oil). The gels, containing 1 μm oil droplets disintegrated into various particles of several to several tens of microns with a low degree of oil droplet release and coalescence. Protein breakdown was slower in these gels, suggesting that the protein structures of the gel matrices were affected by the sizes of the incorporated oil droplets.

A standardised static in vitro digestion method suitable for food - an international consensus

Simulated gastro-intestinal digestion is widely employed in many fields of food and nutritional sciences, as conducting human trials are often costly, resource intensive, and ethically disputable. As a consequence, in vitro alternatives that determine endpoints such as the bioaccessibility of nutrients and non-nutrients or the digestibility of macronutrients (e.g. lipids, proteins and carbohydrates) are used for screening and building new hypotheses. Various digestion models have been proposed, often impeding the possibility to compare results across research teams. For example, a large variety of enzymes from different sources such as of porcine, rabbit or human origin have been used, differing in their activity and characterization. Differences in pH, mineral type, ionic strength and digestion time, which alter enzyme activity and other phenomena, may also considerably alter results. Other parameters such as the presence of phospholipids, individual enzymes such as gastric lipase and digestive emulsifiers vs. their mixtures (e.g. pancreatin and bile salts), and the ratio of food bolus to digestive fluids, have also been discussed at length. In the present consensus paper, within the COST Infogest network, we propose a general standardised and practical static digestion method based on physiologically relevant conditions that can be applied for various endpoints, which may be amended to accommodate further specific requirements. A frameset of parameters including the oral, gastric and small intestinal digestion are outlined and their relevance discussed in relation to available in vivo data and enzymes. This consensus paper will give a detailed protocol and a line-by-line, guidance, recommendations and justifications but also limitation of the proposed model. This harmonised static, in vitro digestion method for food should aid the production of more comparable data in the future.

Enzymatically structured emulsions in simulated gastrointestinal environment: impact on interfacial proteolysis and diffusion in intestinal mucus

Fundamental knowledge of physicochemical interactions in the gastrointestinal environment is required in order to support rational designing of protein-stabilized colloidal food and pharmaceutical delivery systems with controlled behavior. In this paper, we report on the colloidal behavior of emulsions stabilized with the milk protein sodium caseinate (Na-Cas), and exposed to conditions simulating the human upper gastrointestinal tract. In particular, we looked at how the kinetics of proteolysis was affected by adsorption to an oil-water interface in emulsion and whether the proteolysis and the emulsion stability could be manipulated by enzymatic structuring of the interface. After cross-linking with the enzyme transglutaminase, the protein was digested with use of an in vitro model of gastro-duodenal proteolysis in the presence or absence of physiologically relevant surfactants (phosphatidylcholine, PC; bile salts, BS). Significant differences were found between the rates of digestion of Na-Cas cross-linked in emulsion (adsorbed protein) and in solution. In emulsion, the digestion of a population of polypeptides of M(r) ca. 50-100 kDa was significantly retarded through the gastric digestion. The persistent interfacial polypeptides maintained the original emulsion droplet size and prevented the system from phase separating. Rapid pepsinolysis of adsorbed, non-cross-linked Na-Cas and its displacement by PC led to emulsion destabilization. These results suggest that structuring of emulsions by enzymatic cross-linking of the interfacial protein may affect the phase behavior of emulsion in the stomach and the gastric digestion rate in vivo. Measurements of ζ-potential revealed that BS displaced the remaining protein from the oil droplets during the simulated duodenal phase of digestion. Diffusion of the postdigestion emulsion droplets through ex vivo porcine intestinal mucus was only significant in the presence of BS due to the high negative charge these biosurfactants imparted to the droplets. This implies that the electrostatic repulsion produced can prevent the droplets from being trapped by the mucus matrix and facilitate their transport across the small intestine mucosal barrier.

Development of the Digestive System-Experimental Challenges and Approaches of Infant Lipid Digestion

At least during the first 6 months after birth, the nutrition of infants should ideally consist of human milk which provides 40–60 % of energy from lipids. Beyond energy, human milk also delivers lipids with a specific functionality, such as essential fatty acids (FA), phospholipids, and cholesterol. Healthy development, especially of the nervous and digestive systems, depends fundamentally on these. Epidemiological data suggest that human milk provides unique health benefits during early infancy that extend to long-lasting benefits. Preclinical findings show that qualitative changes in dietary lipids, i.e., lipid structure and FA composition, during early life may contribute to the reported long-term effects. Little is known in this respect about the development of digestive function and the digestion and absorption of lipids by the newborn. This review gives a detailed overview of the distinct functionalities that dietary lipids from human milk and infant formula provide and the profound differences in the physiology and biochemistry of lipid digestion between infants and adults. Fundamental mechanisms of infant lipid digestion can, however, almost exclusively be elucidated in vitro. Experimental approaches and their challenges are reviewed in depth.

Preventing gastric sieving by blending a solid/water meal enhances satiation in healthy humans

Separation of solids and liquids within the stomach allows faster gastric emptying of liquids compared with solids, a phenomenon known as sieving. We tested the hypothesis that blending a solid and water meal would abolish sieving, preventing the early rapid decrease in gastric volume and thereby enhancing satiety. We carried out 2 separate studies. Study 1 was a 2-way, crossover, satiety study of 22 healthy volunteers who consumed roasted chicken and vegetables with a glass of water (1008 kJ) or the same blended to a soup. They completed satiety visual analogue scales at intervals for 3 h. Study 2 was a 2-way, crossover, mechanistic study of 18 volunteers who consumed the same meals and underwent an MRI to assess gastric emptying, gallbladder contraction, and small bowel water content (SBWC) at intervals for 3 h. In Study 1, the soup meal was associated with reduced hunger (P = 0.02). In Study 2, the volume of the gastric contents after the soup meal decreased more slowly than after the solid/liquid meal (P = 0.0003). The soup meal caused greater gallbladder contraction (P < 0.04). SBWC showed a biphasic response with an initial "gastric" phase during which SBWC was greater when the solid/liquid meal was consumed (P < 0.001) and a later "small bowel" phase when SBWC was greater when the soup meal was consumed (P < 0.01). Blending the solid/liquid meal to a soup delayed gastric emptying and increased the hormonal response to feeding, which may contribute to enhanced postprandial satiety.

In vitro assessment of the bioaccessibility of fatty acids and tocopherol from soybean oil body emulsions stabilized with ι-carrageenan

The present investigation aimed to expand the knowledge of the in vitro bioaccessibility of fatty acids and tocopherol from natural soybean oil body emulsions stabilized with different concentrations of ι-carrageenan. Several physicochemical parameters including proteolysis of the interfacial layer, interfacial composition, and microstructure were evaluated with regard to their impact on the bioaccessibility of fatty acids and tocopherol. Results from simulated human digestion in vitro indicated that the bioaccessibility of total fatty acids and tocopherol decreased (62.7-8.3 and 59.7-19.4%, respectively) with the increasing concentration of ι-carrageenan. During the in vitro digestion procedure, ι-carrageenan affected physicochemical properties of the emulsions, thereby controlling the release of fatty acids and tocopherol. These results suggested that soybean oil body emulsions stabilized with ι-carrageenan could provide natural emulsions in foods that were digested at a relatively slow rate, the important physiological consequence of which might be increasing satiety.

In vitro assessment of the bioaccessibility of tocopherol and fatty acids from sunflower seed oil bodies

The in vitro digestibility (proteolytic and lipolytic) and bioaccessibility of nutritionally important compounds (alpha-tocopherol and fatty acids) have been studied for natural sunflower ( Helianthus annuus ) oil body suspensions in comparison to artificial emulsions emulsified with polyoxyethylene-20-sorbitan-monolaurate (Tween 20) or whey protein isolate. Proteolytic digestion of emulsions with pepsin (pH 2) promoted significant increases in mean particle size of the whey protein isolate stabilized emulsion (1.8-2.9 mum) and oil bodies (2.3-22.5 mum) but not the Tween 20 stabilized emulsions. SDS-PAGE of proteolytic digestion products suggested degradation of the stabilizing oleosin protein (ca. 18-21 kDa) in oil bodies. The rate of oil body hydrolysis with lipase was significantly slower than the lipase-catalyzed hydrolysis of the artificial emulsions and exhibited a prolonged lag phase. Results from simulated human digestion in vitro suggested that the mean bioaccessibility of alpha-tocopherol and total fatty acids from oil bodies (0.6 and 8.4%, respectively) was significantly lower than that from the Tween 20 stabilized emulsion (35 and 52%, respectively) and the whey protein isolate stabilized emulsion (17 and 33%, respectively). These in vitro results suggest that oil bodies could provide a natural emulsion in food that is digested at a relatively slow rate, the physiological consequence of which may be increased satiety.

Intragastric layering of lipids delays lipid absorption and increases plasma CCK but has minor effects on gastric emptying and appetite

Intestinal intubation studies have demonstrated that lipids induce satiety, but the contribution of lipid processing by the stomach on satiety remains poorly understood. In this explorative, randomized, placebo-controlled, crossover study we tested whether delayed lipid absorption, increased cholecystokinin (CCK), decelerated gastric emptying (GE), and increased satiety can be achieved by controlling lipid distribution in the stomach. Six healthy men were intubated nasogastrically. Two treatments were performed and repeated in duplicate. In the oil-on-top treatment (OT), subjects received a fat-free liquid meal (LM, 325 ml, 145 kcal) followed by intragastric infusion of 4 g of high-oleic-acid rapeseed oil (4.6 ml, 36 kcal) labeled with 77 mg glyceryl-[(13)C]trioleate. In the emulsion treatment (EM, control), 4 g of labeled rapeseed oil was incorporated into the LM (325 ml, 181 kcal); 4.6 ml of saline was infused as a control. In OT and EM a second LM was consumed at time t = 270 min. Plasma (13)C-C18:1, CCK and satiety were measured over 480 min. GE was determined by the paracetamol absorption test. OT delayed oleic acid absorption shown by an increased lag time of absorption (EM: 37 +/- 7 min; OT: 75 +/- 10 min; P < 0.01) and time at maximum concentration (EM: 162 +/- 18 min; OT: 280 +/- 33 min; P = 0.01). OT released more CCK than EM (P = 0.03), including increased CCK after the second meal. OT accelerated initial GE until 30 min postprandial. OT showed a tendency (P = 0.06) to suppress hunger and increase satiety and fullness 120-270 min postprandially. The results demonstrate that low amounts of lipids, when separated from the aqueous phase of a meal, delay lipid absorption and increase CCK. An escalating-dose study should determine whether this could have implications for the development of weight-control foods.

MRI in ocular drug delivery

Conventional pharmacokinetic methods for studying ocular drug delivery are invasive and cannot be conveniently applied to humans. The advancement of MRI technology has provided new opportunities in ocular drug-delivery research. MRI provides a means to non-invasively and continuously monitor ocular drug-delivery systems with a contrast agent or compound labeled with a contrast agent. It is a useful technique in pharmacokinetic studies, evaluation of drug-delivery methods, and drug-delivery device testing. Although the current status of the technology presents some major challenges to pharmaceutical research using MRI, it has a lot of potential. In the past decade, MRI has been used to examine ocular drug delivery via the subconjunctival route, intravitreal injection, intrascleral injection to the suprachoroidal space, episcleral and intravitreal implants, periocular injections, and ocular iontophoresis. In this review, the advantages and limitations of MRI in the study of ocular drug delivery are discussed. Different MR contrast agents and MRI techniques for ocular drug-delivery research are compared. Ocular drug-delivery studies using MRI are reviewed.

Effect of intragastric acid stability of fat emulsions on gastric emptying, plasma lipid profile and postprandial satiety

Fat is often included in common foods as an emulsion of dispersed oil droplets to enhance the organoleptic quality and stability. The intragastric acid stability of emulsified fat may impact on gastric emptying, satiety and plasma lipid absorption. The aim of the present study was to investigate whether, compared with an acid-unstable emulsion, an acid-stable fat emulsion would empty from the stomach more slowly, cause more rapid plasma lipid absorption and cause greater satiety. Eleven healthy male volunteers received on two separate occasions 500 ml of 15 % (w/w) [13C]palmitate-enriched olive oil-in-water emulsion meals which were either stable or unstable in the acid gastric environment. MRI was used to measure gastric emptying and the intragastric oil fraction of the meals. Blood sampling was used to measure plasma lipids and visual analogue scales were used to assess satiety. The acid-unstable fat emulsion broke and rapidly layered in the stomach. Gastric emptying of meal volume was slower for the acid-stable fat emulsion (P < 0·0001; two-way ANOVA). The rate of energy delivery of fat from the stomach to the duodenum was not different up to t = 110 min. The acid-stable emulsion induced increased fullness (P < 0·05), decreased hunger (P < 0·0002), decreased appetite (P < 0·0001) and increased the concentration of palmitic acid tracer in the chylomicron fraction (P < 0·04). This shows that it is possible to delay gastric emptying and increase satiety by stabilising the intragastric distribution of fat emulsions against the gastric acid environment. This could have implications for the design of novel foods.

Adsorption of protein-coated lipid droplets to mixed biopolymer hydrogel surfaces: role of biopolymer diffusion

The adsorption of charged particles to hydrogel surfaces is important in a number of natural and industrial processes. In this study, the adsorption of cationic lipid droplets to the surfaces of anionic hydrogels was examined. An oil-in-water emulsion containing cationic beta-lactoglobulin-coated lipid droplets was prepared (d32=0.24 microm, zeta=+74 mV, pH 3.0). An anionic hydrogel containing 0.1 wt % beet pectin and 1.5 wt % agar (pH 3.0) was prepared. Emulsions containing different lipid droplet concentrations (0.3-5 wt %) were brought into contact with the hydrogel surfaces for different times (0-24 h). The adsorption of lipid droplets to the hydrogel surfaces could not be explained by a typical adsorption isotherm. We found that the electrical charge on the nonadsorbed lipid droplets became less positive or even became negative in the presence of the hydrogel and that extensive droplet aggregation occurred, which was attributed to the ability of pectin molecules to diffuse through the hydrogels and interact with the lipid droplets. These results may have important consequences for understanding certain industrial and biological processes, as well as for the design of controlled or triggered release systems.

Enhancement of intragastric acid stability of a fat emulsion meal delays gastric emptying and increases cholecystokinin release and gallbladder contraction

Preprocessed fatty foods often contain calories added as a fat emulsion stabilized by emulsifiers. Emulsion stability in the acidic gastric environment can readily be manipulated by altering emulsifier chemistry. We tested the hypothesis that it would be possible to control gastric emptying, CCK release, and satiety by varying intragastric fat emulsion stability. Nine healthy volunteers received a test meal on two occasions, comprising a 500-ml 15% oil emulsion with 2.5% of one of two emulsifiers that produced emulsions that were either stable (meal A) or unstable (meal B) in the acid gastric environment. Gastric emptying and gallbladder volume changes were assessed by MRI. CCK plasma levels were measured and satiety scores were recorded. Meal B layered rapidly owing to fat emulsion breakdown. The gastric half-emptying time of the aqueous phase was faster for meal B (72 +/- 13 min) than for meal A (171 +/- 35 min, P < 0.008). Meal A released more CCK than meal B (integrated areas, respectively 1,095 +/- 244 and 531 +/- 111 pmol.min.l(-1), P < 0.02), induced a greater gallbladder contraction (P < 0.02), and decreased postprandial appetite (P < 0.05), although no significant differences were observed in fullness and hunger. We conclude that acid-stable emulsions delayed gastric emptying and increased postprandial CCK levels and gallbladder contraction, whereas acid-instability led to rapid layering of fat in the gastric lumen with accelerated gastric emptying, lower CCK levels, and reduced gallbladder contraction. Manipulation of the acid stability of fat emulsion added to preprocessed foods could maximize satiety signaling and, in turn, help to reduce overconsumption of calories.

The effect of macronutrients on gastric volume responses and gastric emptying in humans: A magnetic resonance imaging study

The effects of macronutrients on gastric volume changes, emptying, and gastrointestinal symptoms are incompletely understood. Three liquid meals of 500 ml (fat emulsion, 375 kcal; protein solution, 375 kcal; glucose solution, 400 kcal) were infused into the stomach of 12 healthy volunteers on three occasions. Studies were performed in seated body position using an open-configuration magnetic resonance imaging (MRI) system. MRI imaging sequences, assessing stomach and meal volumes, were performed prior to and at times t = 0, 3, 6, 9, 12, 15, 25, 35, 45, 60, 75, and 90 min after meal administration. Areas under the curve for the early emptying phase (0-15 and 0-45 min) were calculated, and characteristics of the volume curves were analyzed by a gastric emptying model. Gastrointestinal symptoms were assessed by a self-report scale. Initial (t = 0 min) and early postprandial gastric volumes were highest for glucose because of lower initial emptying. However, in the early emptying phase the characteristics of the volume curves for stomach and meal were uniform for all macronutrients. Perceptions of fullness and satiety were linearly associated with postprandial gastric volumes, but not with macronutrient composition. Isovolumic macronutrient meals modulate gastric volume response by initial meal emptying patterns. Macronutrient specific accommodation responses, as shown in barostat studies, are not reflected as gastric volume responses under noninvasive conditions.