Bile Acids
Definition
The bile acids are a collection of detergent-like compounds (predominantly cholic acid and chenodeoxycholic acid in mammals). The primary bile acids are synthesized from cholesterol in the liver and are secreted in bile to aid in digestion and absorption of fat and fat-soluble vitamins.
Physiology
• Bile acid synthesis in mammals and birds occurs in the liver and is the primary pathway in the metabolism of cholesterol, which is the precursor molecule for bile acid synthesis.
• Bile acids are conjugated in the liver, to glycine or taurine, to inhibit intestinal bile acid resorption and promote lipid metabolism. Bacteria in the intestine further modify bile acids.
• Bile acids are secreted via the bile duct into the small intestine and emulsify ingested fats to be solubilized for digestion and absorption.
• In birds (with or without a gallbladder), secretion of bile acids is continuous.
Typical Normal Range
Although reference ranges for bile acid concentrations have not been established for all avian species, bile acid concentrations are considered to be elevated in pigeons and psittacine species if they are greater than 70 mmol/L in fasted (12-hour) samples. Postprandial bile acid concentrations vary between bird species and are challenging to interpret.
• As an indicator of the severity of a liver disease, avian values for enzymatic (spectrophotometer) methods are as follows (radioimmunoassay shows lower values):
| Liver Disease Severity | Bile Acid Values, µm/l |
| Minimal | 50-150 µM/L |
| Mild | 150-250 µM/L |
| Moderate | 250-500 µM/L |
| Severe | 500-700 µM/L |
Bile Acid Values of Selected Avian Species as Measured by Radioimmunoassay (RIA) and the Enzymatic Method (Spectrophotometer) in µMol/L
| RIA | Colorimetric | |
| Birds | ||
| psittacines | ||
| African grey parrot | 18-71 | 12-96 |
| Amazon parrots | 19-144 | 33-154 |
| Budgerigar parakeet | 20-65 | 32-117 |
| Caique | 12-112 | |
| Cockatiel | 25-85 | 15-139 |
| Cockatoos | 34-112 | 8-11 |
| Conure | 32-105 | 8-15 |
| Eclectus parrot | 30-110 | 8.8-9.8 |
| Grey-cheek parakeet | 15-96 | |
| Lory | 20-97 | |
| Lovebird | 25-95 | 12-90 |
| Macaw | 7-100 | |
| Pionus parrot | 15-92 | |
| Quaker parrot | 21-90 | |
| Senegal parrot | 0-85 | 20-94 |
| passeriformes and columbiformes | ||
| Canary | 23-90 | |
| Pigeon | 22-60 | |
| piciformes | ||
| Toucan | 20-40 | |
| Emu | 6-45 | |
| Ostrich | 4-40 | |
| raptors | ||
| Peregrine falcon | 20-118 | |
| Mammals | ||
| Guinea pig | 57 µg/mL | |
| Rat | 56.0 ± 6.6 µmol/L (male) 27.7-41.1 µmol/L (female) | |
| Ferret | 0.0-8.0 µmol/L1-28 µmol/L (mean, 9.1 µmol/L) for males | |
| 2-14 µmol/L (mean, 4.4 µmol/L) for females |
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