Bile plays a significant role in the metabolism of cholesterol. The liver converts excess cholesterol into bile acids, which are then released as part of bile. This conversion is a primary method for the body to regulate and reduce high levels of cholesterol. Additionally, bile facilitates the digestion and absorption of dietary cholesterol. Once in the intestine, cholesterol is incorporated into micelles formed by bile salts, allowing it to be absorbed by the intestinal cells. However, not all bile acids and cholesterol are absorbed; a portion is excreted in the feces. This excretion of cholesterol with bile acids is an important mechanism for cholesterol homeostasis and reducing the risk of hypercholesterolemia and associated cardiovascular diseases. Thus, bile serves both as a pathway for the elimination of excess cholesterol and as a facilitator for the digestion and absorption of dietary cholesterol.

The regulation of bile release in response to dietary fats is primarily controlled by hormonal and neural signals. When fats enter the small intestine, they stimulate the intestinal cells to release a hormone called cholecystokinin (CCK). CCK travels through the bloodstream to the gallbladder, signaling it to contract and release stored bile into the small intestine. Additionally, the presence of fats triggers the vagus nerve, part of the parasympathetic nervous system, which also signals the gallbladder to release bile. This coordinated response ensures that bile is available in the intestine when it is needed for the emulsification and digestion of fats. The liver continuously produces bile, but the gallbladder's ability to store and concentrate bile allows for a more efficient and targeted release in response to dietary fat intake.

Bile acid malabsorption can have significant implications for digestive health. It occurs when the intestines fail to properly reabsorb bile acids, leading to excessive bile acids in the colon. This can cause a range of digestive symptoms, such as diarrhoea, abdominal pain, and bloating, as the presence of bile acids in the colon can increase water secretion and speed up intestinal transit time. Chronic diarrhoea due to bile acid malabsorption can lead to dehydration, electrolyte imbalances, and nutrient deficiencies. Furthermore, the reduced reabsorption of bile acids signals the liver to increase bile acid production, which may deplete the body's cholesterol and potentially affect the synthesis of fat-soluble vitamins. Treating bile acid malabsorption often involves dietary adjustments and medications such as bile acid sequestrants, which bind to bile acids in the intestines, reducing their diarrhoeal effect. This condition highlights the importance of balanced bile acid metabolism in maintaining overall digestive health.

The body has limited capacity to compensate for a lack of bile. If bile flow is reduced or blocked, the digestion of fats becomes less efficient, but the body attempts to adapt in several ways. The liver may increase the production of bile, but this is only effective if the bile ducts are unobstructed. The intestinal mucosa can increase its absorptive efficiency to try and maximise the uptake of the fats that are digested. However, these compensatory mechanisms are often insufficient, especially in cases of severe or chronic bile deficiency. Long-term bile deficiency can lead to fat malabsorption, resulting in nutrient deficiencies, particularly of fat-soluble vitamins (A, D, E, K), and essential fatty acids. This can have widespread effects on health, including poor growth, vision problems, weakened bones, and a compromised immune system. Dietary adjustments, such as consuming medium-chain triglycerides (MCTs) which require less bile for digestion, may help but do not completely resolve the issue.

Bile salts have a unique molecular structure that is amphipathic, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts. This dual nature is essential for the emulsification of fats. The hydrophobic side interacts with fat molecules, while the hydrophilic side interacts with water in the digestive tract. When bile salts surround a fat droplet, their hydrophobic sides face inward towards the fat, and their hydrophilic sides face outward, forming a micelle. This arrangement breaks down large fat globules into smaller droplets, significantly increasing the surface area available for enzyme action. The small size and water-soluble nature of these micelles make it easier for them to move through the aqueous environment of the intestines, facilitating the efficient digestion and absorption of fats. This structural functionality of bile salts is a key aspect of their role in fat digestion.