Bile acids play their first critical role in lipid assimilation by promoting emulsification. As derivatives of cholesterol, bile acids have both hydrophilic and hydrophobic domains (i.e. they are amphipathic). On exposure to a large aggregate of triglyceride, the hydrophobic portions of bile acids intercalate into the lipid, with the hydrophilic domains remaining at the surface. Such coating with bile acids aids in breakdown of large aggregates or droplets into smaller and smaller droplets. As monoglycerides and fatty acids are liberated through the action of lipase, they retain their association with bile acids and complex with other lipids to form structures called micelles. Micelles are essentially small aggregates of mixed lipids and bile acids suspended within the ingesta.

Crude emulsions of lipids enter the duodenum as fine lipid droplets and then mix with bile and pancreatic juice to undergo marked changes in chemical and physical form. Emulsification continues in the duodenum along with hydrolysis and micellization in preparation for absorption across the intestinal wall. These products of lipolysis are removed from the water–oil interface when they are incorporated into the mixed micelles that form spontaneously when they interact with bile salts. Having both hydrophilic and hydrophobic components, bile salts are able to facilitate micelle formation; MAG and PL enhance their ability to form mixed micelles.

Bile salts are amphipathic molecules that orient at the lipid–water interface with their hydrophobic surface facing the lipid phase and their hydrophilic surface facing the aqueous phase, thereby emulsifying dietary fat into smaller droplets accessible to lipase. Mixed micelles formed in the intestinal lumen contain bile salts together with phospholipids and lipid digestion products such as monoacylglycerols and free fatty acids rather than pure lipid cores.

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.[6]

Dietary lipids (mostly triglycerides), upon their entry into the small intestine, are emulsified by bile salt (also called bile acid) released from the gall bladder. Bile salt functions as a detergent (due to their OH and COOH groups), and large lipid molecules form smaller lipid droplets surrounded by a layer of bile. The lipid digestion products are assembled into micelles. These are temporary combinations of bile salt, fatty acids, monoglycerides, and other fat-soluble substances such as vitamins and cholesterol. The micelles are water soluble and enable the lipid digestion products to be transported to the small intestinal surface for absorption.

These studies taken together suggest that reverse-phase HPLC mobility and equilibrium cholesterol-solubilizing capacities are inverse functions of each other and correlate closely with the hydrophilicity of bile salt molecules. In addition, the evidence here deduced further strengthens our recent deductions… that cholesterol may be solubilized in micellar bile salt solutions by both hydrophobic and hydrophilic association with the external ("hydrophilic") surface of bile salt micelles rather than with the hydrophobic surface of the micelle's interior.

Contrary to most common surfactants that have a polar head and hydrophobic tail, bile salts have two surfaces, one hydrophilic and the other hydrophobic. This property leads to the formation of very small disc-shaped micelles. Hydrophobic solutes interact efficiently with the nonpolar core of the micelles, and this step is essential for their emulsification and solubilization in aqueous media. Mixed bile salt/phospholipid micelles play an important role in the digestion of fats in the gastrointestinal tract.

Lipid droplets are composed of a hydrophobic core of triacylglycerol (TAG) and cholesteryl esters, "surrounded by a phospholipid monolayer."[3] The neutral lipid core occupies most of the volume, and the surrounding monolayer presents its hydrophilic head groups to the aqueous cytosol while the hydrophobic tails face inward toward the neutral lipids. This basic organization—lipid core inside, amphipathic molecules at the surface, water outside—is a general feature of lipid droplets and emulsified fat particles.

This work focuses on colloidal transformations during the digestion of dietary triglyceride-lipids under physiological conditions present in the intestine. In the intestine, the interface-active pancreatic lipase–colipase complex adsorbs to the interface of the emulsified lipids and quantitatively hydrolyses the tri- and diglyceride to 2-monoglycerides and fatty acids as the final digestion products. With time of lipase action, the interior of the emulsion particles was found to self-assemble to oil continuous structures with increasing hydrophilicity of the interface. A transition from oil emulsion to emulsified microemulsion, micellar cubic, inverse hexagonal, and bicontinuous cubic liquid-crystalline droplets was found.

The products of digestion are solubilized by bile acids and phospholipids in the intestinal lumen into self-assembled structures such as bile salt micelles and other colloidal structures. In lipid digestion in food emulsions, bile acids and phospholipids play an important role in interfacial processes and in the formation of mixed micelles that remove lipolytic products from the oil–water interface and transport them in the aqueous phase.

Bile salts molecular structure has an impact on their aggregation and solubilisation behaviours. Structural studies with liposomes show that the addition of bile salts induces their solubilisation into mixed micelles… This unusual planar polarity enables them to self-assemble in solution into micelles… Structural studies… have suggested that bile salts’ distinct functions during lipolysis arise from their architectural diversity, in particular the very different disposition of their hydrophobic and hydrophilic surfaces.

Bile extracts can adsorb to lipid droplets and form fine oil-in-water emulsions.[3] Bile extracts can replace sodium caseinate at the oil/water interface.[3] In vitro lipid digestion in emulsions was largely dependent on its presence in the unadsorbed phase rather than the adsorbed phase. Clearly, the bile salts adsorbed onto the interface had relatively less influence on lipolysis than the presence of bile salts in the aqueous phase.[3]

Micelles are aggregates of bile salt that forms a polar outer shell and a hydrophobic inner core.[5] Long chain fatty acids, cholesterol and other hydrophobic molecules dissolves in the core and are transported to the surface of the enterocytes for processing.[5]

Adipocytes synthesize fatty acids and triacylglycerol (TG), and "the TG accumulated in large amounts is stored in the cell as lipid droplets."[7] TG synthesis occurs in the endoplasmic reticulum membrane, and "the TG accumulated between the phospholipid bilayer membranes is eventually released into the cytoplasm as lipid droplets covered by a phospholipid monolayer." Thus, the droplet has a core of neutral lipids, enveloped by a single layer of phospholipids whose hydrophilic heads face the aqueous environment.

"Lipid droplets have a core portion made of lipid esters such as cholesterol esters (CE), triacylglycerols (TG), and diacylglycerols (DG), which occupies most of the volume, and the surface is covered by a phospholipid monolayer."[5] Perilipin, ADRP and other lipid droplet proteins are thought to be anchored in this phospholipid monolayer. This schematic assumes a central mass of neutral lipids inside, an amphipathic phospholipid layer at the surface, and the aqueous phase outside the monolayer.

Amphipathic bile breaks dietary fat into small lipid droplets allowing enzyme access for digestion.[8] Bile is an amphipathic fluid with bile salts that have a hydrophilic water-loving end and an hydrophobic water-fearing end.[8] Bile acids can break down a large lipid droplet into smaller lipid droplets by orienting the hydrophilic ends towards the outside and the hydrophobic ends towards the center lipid.[8]

The results demonstrate that the bile salts not only contribute to the emulsification of lipids but also possess “membrane curvature power” to manipulate the nanostructural organization of lipid systems.[9] Bile salts are described as facially amphiphilic molecules that can interact with lipid assemblies and alter their structure.[9]

Emulsification is the process of breaking down fat globules into smaller, more manageable droplets, allowing for better interaction with digestive enzymes. Bile salts, which are components of bile, act as emulsifying agents. Their role in lipid digestion is to disperse large fat globules into smaller droplets and to assist in the formation of micelles, which help transport lipid digestion products through the aqueous environment of the intestine.

The hydrophobic and hydrophilic properties of bile salts have the capability to form micelles at the lipid–water interface. As a result, there is a high amount of bile acids/salts in the small intestine which helps in solubilisation of lipid molecules. The micelles formed by the bile acids help the lipases (enzymes that break down fat) in the digestion of lipids and brings them close to the intestinal brush border that aids in fat absorption.

"Lipid droplets have a core (center) composed of triacylglycerols and cholesteryl esters, and a structure covered by a phospholipid monolayer, and are widely present not only in adipocytes but in cells throughout the body."[8] The neutral lipid core is hydrophobic, whereas the phospholipid monolayer has hydrophilic head groups exposed to the surrounding aqueous environment. This architecture parallels that of emulsified fat droplets in digestion, with lipids inside, amphipathic molecules at the surface, and water outside.

Bile salts emulsify lipids by breaking large droplets into smaller ones. This increases the surface area for lipase action.[4] Diagram illustrating how bile salts emulsify large fat globules into smaller droplets by surrounding them.[4] Micelles: Small transport structures composed of bile salts, fatty acids, and monoglycerides.[4]

A micelle is a spherical aggregate of amphiphilic molecules with a hydrophobic core and hydrophilic shell. In this arrangement, the hydrophobic tails cluster together in the core, shielded from the water, while the hydrophilic heads face outwards, interacting with the surrounding aqueous environment. In biology, a prime example is the role of bile salts, which form micelles in the digestive system to aid in the absorption of dietary fats and fat-soluble vitamins.

Bile is composed of amphipathic molecules such as phospholipids and bile salts.[2] When bile enters the small intestine, it will mix with the fat globules and will cause them to break down into smaller units called emulsion droplets. This process is called emulsification.[2] Since fatty acids are hydrophobic, the bile phospholipids or bile salts can surround the fatty acids and form a tiny spherical structures called micelles.[2]

It is found that the unusual arrangement of the hydrophobic and hydrophilic groups leads to unusual micellar shapes. This effect is particularly evident in bile salts where the hydrophobic and hydrophilic moieties are arranged on different sides of the rigid steroid nucleus, giving rise to facial amphiphilicity rather than the linear head–tail structure of conventional surfactants. Computer simulations show that bile salt aggregates expose both hydrophobic and hydrophilic regions to the surrounding solution rather than forming a simple core–shell micelle.

"Emulsification is the process of dividing total fat globules into small droplets, thereby providing a larger surface area on which enzyme action can occur."[2] In biological systems, emulsification is usually carried out by emulsifying agents that help stabilize fat in water; "the most common emulsifier in the human body is bile, whose components include bile salts and phospholipids, which cover fat droplets and prevent them from fusing." By surrounding the fat droplets, these amphipathic bile components allow the hydrophobic lipids to be dispersed in an aqueous environment.

Bile increases the surface area of the big fat globules by breaking them up into smaller fat droplets.[7] The increased surface area of the lipid droplets increases the rate of digestion of lipids by the lipase enzymes because more lipid is available for digestion at any given time.[7]

Bile extracts can adsorb to lipid droplets and form fine oil-in-water emulsions.[1] The present study showed that the impact of added bile extract on the kinetics of in vitro lipid digestion in emulsions was largely dependent on its presence in the unadsorbed phase rather than the adsorbed phase.[1] Clearly, the bile salts adsorbed onto the interface had relatively less influence on lipolysis than the presence of bile salts in the aqueous phase.[1]

"Emulsification means turning fat into small droplets and changing it into a form that mixes easily with water, and fat becomes easier to break down once emulsified."[9] The agent that emulsifies fat is bile acid salts; pancreatic lipase is the enzyme that then digests the emulsified fat. This description implies that after emulsification, microscopic fat droplets are dispersed in the surrounding aqueous intestinal contents.

The hydrophilic–hydrophobic balance of bile salt monomers can be readily quantified by their elution sequence during reverse-phase high-performance liquid chromatography. Together with other physical–chemical data, these studies support the concept that bile salt micelles are not simple hydrophobic-core structures but have significant hydrophilic character at their surfaces where cholesterol and other lipids can associate.

Bile salts, secreted from the gallbladder, emulsify large lipid droplets into smaller micelles, increasing surface area for pancreatic lipase activity. Bile salts come over and attach on to these little fat droplets and help to make the fat globules soluble in the intestinal fluid. They bind to small fat droplets and help make them even smaller, assisting colipase and pancreatic lipase to bind and break down triglycerides into monoglycerides and free fatty acids.

Micelles are formed when bile salts arrange themselves with their hydrophobic parts inwards and hydrophilic parts outwards, creating a structure that can carry lipids in an aqueous environment. Inside the micelle, only lipids and other hydrophobic molecules are present, while water surrounds the micelle externally.[5][2]