Bile acids are a component of bile and are a large family of molecules that have a steroidal structure. Bile acids are made in the liver and stored in the gall bladder, which help with the digestion of foods, particularly fat. Whenever food is eaten, the body sends a signal to the gall bladder to contract and push bile acids into the small intestine.
It is synthesized from cholesterol in the liver and is actively secreted along with cholesterol and phospholipids into the bile. Bile acids play a key role in human metabolism. It facilitates the digestion and absorption of lipids, maintains the dissolved state of cholesterol in bile, and inhibits its precipitation.
Bile acids also act as signalling molecules to regulate self-metabolism, lipid metabolism, energy balance, and glucose metabolism by binding to the “membrane bile acid receptor.”
Significance of bile acid
Bile that is flowing from the liver is concentrated in the gallbladder. It is released into the upper intestine each time in response to a meal. Once it reaches the intestines, bile acids act as detergents helping to emulsify fats, and aiding in their digestion and absorption.
After digestion in the small bowel, bile acids are completely (almost 95%) reabsorbed in the distal ileum and later retaken up from portal blood by the liver (enterohepatic circulation).
The primary bile acids that are synthesized in the liver are cholic and chenodeoxycholic acid and are typically conjugated to glycine or taurine before secretion. In the intestine, colonic bacteria often convert the primary bile acids to secondary bile acids, predominantly deoxycholic acid and lithocholic acid. The reabsorbed bile acids are then transported to the liver in portal blood.
Conjugated bile acids are retaken by hepatocytes via the sodium taurocholate cotransporter (NTCT). The unconjugated bile acids are taken by organic anion transporters that also take up bilirubin and other anions.
The total bile acid pool in humans is controlled by coordinated regulation of the expression of genes involved with the synthesis, secretion, reabsorption, and reuptake of bile acids by the liver.
The major components of the bile acid pool
- cholic and chenodeoxycholic acid
- lesser amounts of deoxycholic acid
- lithocholic acid
- minor amounts of ursodeoxycholic acid.
Functions of Bile acids
- Act as signaling molecules
- Are important in the regulation of their own synthesis, uptake, and secretion
- Important in the control of cholesterol synthesis
- Regulates lipid and glucose metabolism
- Can be used as therapeutic agents in patients with cholestatic liver diseases
- Are also useful for the medical treatment (dissolution) of gallstones by increasing bile acid and decreasing cholesterol concentrations in bile (causing less saturated bile)
- Useful as replacement therapy in patients with bile acid synthetic defects
- Other metabolic effects of bile acids can be useful in treating metabolic diseases including non-alcoholic steatohepatitis.
Bile acid levels are increased in the serum and liver in patients with obstructive jaundice or cholestasis and, perhaps because of their inherent detergent activities, can cause hepatocyte injury. Thus, increased bile acid levels in hepatocytes may account for some of the liver damage in cholestatic liver diseases.
Purpose of the bile acid test
This test evaluates the normal functioning of the liver. The bile acids – serum test is done to diagnose hepatobiliary diseases such as sclerosis of the liver, acute hepatitis, chronic hepatitis, cancer of the liver, and intrahepatic cholestasis of pregnancy.
The bile acid test is useful in determining if the liver is working properly. Specifically, the test lets in determining:
- If the liver has enough healthy cells to do its job
- To know if the liver has a good blood supply
- Understand whether bile moves freely through and out of the liver
Intended use of the test
Tests for bile acids are can be helpful in diagnosing a number of conditions, including cholestasis, portosystemic shunt, and hepatic microvascular dysplasia. Excess concentrations of bile acids in the colon are usually associated with chronic diarrhoea. Studies have shown that bile acids are carcinogens and tumour promoters in experimental models and their role in carcinogenesis are best documented in Barrett's oesophagus and adenocarcinoma at the gastroesophageal junctions.
When would the bile acid test be recommended?
Whenever there are signs that suggest the liver is damaged or defective. This would include when:
- There are abnormally high liver enzyme values in the blood causing damage to the liver and reducing the healthy cells.
- There is a low albumin level in the blood signalling that the liver may not have enough healthy cells to do its job.
- There may be a congenital defect in the blood supply that is preventing the liver from doing its job.
- There are seizure disorders because of poor blood supply or because there are not enough healthy liver cells to do the job.
Interpretation of bile acid test
If the liver is healthy and functioning, the bile acids levels will be low in the resting blood sample and only slightly higher levels of bile acids will be there in the postprandial sample. This indicates that bile acids released from the gall bladder during the test meal were adequately recaptured by the liver during the 2-hour period following the meal.
Hence the conclusion from the test will be that the liver has enough cells to do its job, there is a blood supply, and bile is flowing properly.
But a normal result needn’t always mean the liver is completely healthy. Because, sometimes the bile acid test results will show normal, even when there is a problem in the liver. This happens when the problem is mild or affects only a small portion of the liver.
In such situations, there is often no reduction in the overall ability of the liver to do its job. In other situations, the gall bladder may not contract as expected, and bile acids levels will be low, even though a liver problem may exist.
Conclusion
Bile acid tests are highly helpful in understanding if the bile and the liver are functioning well. If the bile acid test is abnormal, the result indicates that there is a problem in the liver. Depending on how sick the person is and how abnormal the test results are, the doctor may recommend monitoring the situation, or may suggest moving on to additional diagnostic tests including an ultrasound of the liver, and possibly a liver biopsy.
