What is Bile?
Bile is defined as a greenish-yellow fluid that breaks down fat into fatty acids for absorption by the digestive tract. The word bile comes from the Latin word bilis meaning "fluid secreted by the liver." Bile is made in the liver but stored by the gallbladder. It gets its greenish-yellow color from the pigments biliverdin (green) and bilirubin (brownish-yellow). Both pigments are molecular byproducts of hemoglobin degradation in red blood cells.
Bile was first discovered by a German chemist named Heinrich Otto Wieland. In 1927, he was awarded a Nobel Prize in Chemistry for investigating the role of bile salts in the human body.
Bile salts, also called bile acids, are a major constituent of bile. They are made from cholesterol in the liver. Conjugated bile salts are those that form amide bonds with amino acids. Since bile's discovery, it was believed that conjugated bile salts only formed bonds with the amino acids taurine and glycine. However, it was recently discovered that gut microbes have the ability to conjugate bile salts with many other amino acids. This new discovery fundamentally changes the scientific community's understanding of mammalian bile. At the time of writing, additional study is taking place to understand bile's additional roles and functions in the body.
As mentioned, bile salts are just one component of bile. As a percentage of total solute, bile contains:
- 61% bile salts
- 12% fatty acids
- 9% cholesterol
- 7% protein
- 5% other (copper, other metals, and waste products)
- 3% bilirubin
- 3% phospholipids
Overall, the total solute represents approximately 5% of bile with water constituting the rest.
Bile is secreted by both the liver and gallbladder; however, the gallbladder is bile's only storage unit. There are two stages of bile secretion:
- Hepatocytes (liver cells) secrete bile into small channels or ducts called canaliculi. Here, the composition of bile is rich in bile acids, cholesterol, bile pigments (bilirubin and biliverdin), and other organic material. The canaliculi converge into one relatively large duct called the common hepatic duct where sodium and bicarbonate are added. A portion of bile moves on to the small intestine, but the rest is moved to the gallbladder for storage.
- When bile reaches the gallbladder, sodium, bicarbonate, water, and chloride are absorbed to increase bile's concentration and pH. The gallbladder stores and concentrates bile during times of fasting. After a meal, the bladder releases alkaline bile through the common bile duct to emulsify (disperse and suspend in solution) fat in the small intestine.
An adult human liver produces approximately 400 to 800 milliliters of bile per day.
There are many factors that regulate the release of bile. Secretin is a digestive hormone released by the duodenum, the first portion of the small intestine. It is released in response to acidic foods in the gastrointestinal tract. Secretin initiates a series of pathways, including bile production by the liver, with the ultimate goal of neutralizing acidic foods after consumption.
Cholecystokinin (CCK) is an enzyme made in the pancreas and released by the small intestine when high fat and high protein food is consumed. CCK initiates contractions in the gallbladder to release bile. Somatostatin is a hormone produced by the nervous and digestive systems that inhibit bile release by blocking the release of cholecystokinin. Stimulation from the vagus nerve is also required for bile secretion from both the liver and the gallbladder. Gastrin is another hormone that increases both bile release as well as digestive proteins from the pancreas. Digestive enzymes, like lipase, are important because they break down fatty acids in bile so they can be absorbed through the digestive tract.
Functions of Bile
There are two major purposes of bile:
- To aid in digestion by breaking down fat into fatty acids
- To drain waste products from the liver to the digestive tract where they will eventually be expelled as a component of feces
Additional functions include:
- Absorbing fat, iron, calcium, and vitamins through bile salts
- Breaking down fat-soluble vitamins like vitamins A, D, E, and K
- Stimulating bowel movements
- Neutralizing stomach acids
- Lubricating the digestive tract
- Helping to regulate the metabolism of nutrients in the liver
Digestion & Absorption
Bile salts are responsible for taking fat consumed as food and breaking them up into small fat droplets of fatty acids that are easily absorbed. This is called emulsification.
Bile salts are amphipathic in nature, meaning they have a fat-soluble portion and a water-soluble portion. As bile salts surround fat and fat-soluble vitamins, their fat-soluble portion faces inward while the water-soluble portion faces outward. Multiple bile salts come together to form a micelle, a small sphere-like structure that is hydrophilic (water-loving) on the outside and hydrophobic (fat-loving) on the inside. This allows the bile salts to cover, conceal, and transport fat within the micelle while traveling among the hydrophilic environment of the gastrointestinal tract.
There are two forms of waste excreted by the body: urine and feces. The liver plays a significant role in detoxifying the body. Many of those toxins end up in bile so they can either be excreted through the gastrointestinal tract as a bowel movement or reabsorbed in the blood and excreted as urine.
Waste products of bile include:
- Metals like copper, zinc, and mercury
- Steroid hormones
- Excess calcium
- Drug metabolites (a substance formed in relation to the metabolism of pharmaceutical drugs)
Recently, bile salts have also been found to act like hormones in the body. Bile salts can bind to nuclear receptors (receptors in the nucleus that affect gene transcription) in two ways:
- Bile salts are made from cholesterol in the liver with the help of a series of enzymes. When the body has produced a sufficient number of bile salts, the salts bind to a region of DNA that stops the production of a key enzyme involved in its own synthesis. Essentially, bile salts have the ability to decrease their own production. This is called negative feedback.
- When bile salts enter the small intestine, they are absorbed by proteins on the surface of cells in the intestinal tract. Bile salts can bind to the DNA of intestinal cells to increase the production of these absorptive proteins. This is called positive feedback because they increase the rate of their own absorption.
Ultimately, both hormonal roles serve to reduce the number of active bile salts in the body once sufficient numbers have been reached.
The role of bile's additional functions are as follows:
- To act as a laxative by softening the stool and increasing the rate of expulsion
- To use its alkaline makeup to serve as a natural buffer for acidic compounds
- To serve as a lubricant by containing mucin. Mucin is a slimy material that helps coat the walls of the gastrointestinal tract so things move smoothly.
- To form micelles with the help of cholesterol's and lecithin's amphipathic nature
- To help with bile regurgitation (when bile enters the stomach and mixes with stomach acids). Bile salts can disrupt bile circulation and are sometimes prescribed as a medication for bile reflux.
Bile is an alkaline substance that breaks down fat into fatty acids and assists with the digestion of fat in the small intestine. It is produced by the liver and stored in the gallbladder. Cholecystokinin (CCK) is an enzyme made in the pancreas and released by the small intestine to promote the release of bile from the gallbladder. Bile gets its greenish-yellow color from the pigments biliverdin and bilirubin, which are produced from the breakdown of hemoglobin in red blood cells. Bile salts, also called bile acids, are a major component of bile. They help disperse fat by surrounding fat droplets and forming sphere-like carrier configurations called micelles. The dispersion and suspension of fat in a solution is called emulsification. Bile breaks down fat into fatty acids while digestive enzymes like lipase help the digestive tract absorb them.
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How does bile help digest fat?
Bile digests fat by breaking it down into fatty acids and dispersing it throughout the digestive tract for digestive enzymes to process.
How is bile created?
Bile is created by the liver and consists of many components. These components come from cholesterol, dead or damaged red blood cells, and accumulated waste products.
What are the three main functions of bile?
Bile has many functions, but the three main ones are to aid in the digestion of fat, rid the body of waste, and act as a buffer to neutralize acidic compounds.
What is bile made of?
Bile is made up of approximately 95% water. The remaining 5% percent consists of bile salts, fatty acids, cholesterol, protein, bilirubin, phospholipids, and other compounds.
What does having a bile mean?
"Having a bile" refers to the act of releasing bile from the liver and gallbladder after food is consumed. Bile enters the small intestine to break down fat and fat-soluble vitamins.
Is bile good or bad?
Bile is good. It assists with properly digesting fat through emulsification and assisting with absorption. It also helps with ridding waste out of the body.
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