Bile is the digestive secretion produced by the liver and used to emulsify dietary fats in the small intestine. The horse is unusual among domestic mammals in one important respect: it has no gallbladder. In most species the gallbladder stores bile between meals and releases a concentrated bolus when fat enters the duodenum. Because horses evolved as near-continuous grazers, they instead secrete bile continuously, directly from the liver into the common bile duct and then into the duodenum, at a steady low rate that matches their trickle-feeding pattern rather than the intermittent meal-feeding pattern of omnivores and carnivores.
This anatomical fact carries practical consequences. The horse’s digestive tract is calibrated for small, frequent forage intake; large, infrequent high-fat meals can overwhelm the continuous-drip delivery and impair fat absorption. It also means there is no gallbladder to form or obstruct with stones, so the equine-specific differential list for colic and liver disease differs from that of cattle or dogs — cholelithiasis (bile duct stones) is rare but not impossible, and when it occurs it typically lodges in the bile duct rather than a storage organ.
Bile itself is an alkaline, bicarbonate-rich fluid containing bile acids synthesized from cholesterol, bilirubin (a breakdown product of hemoglobin from spent red blood cells), phospholipids, and electrolytes. Its two functions in digestion are emulsification of dietary lipids — breaking fat globules into smaller droplets that lipase can reach — and carrying waste products, principally bilirubin, out of the body via the feces. Bilirubin gives equine manure its characteristic color and, when liver function is compromised, accumulates in the blood to produce the yellow discoloration of the mucous membranes (icterus) that veterinarians check on the sclera and gums during a colic or liver exam.
Elevated serum bile acid concentration is a primary diagnostic marker for hepatic disease in the horse. Because the liver recycles bile acids through enterohepatic circulation — absorbing them from the small intestine and returning them to the liver via the portal blood — a failing liver allows bile acid levels to rise in systemic circulation. A bile acid assay is accordingly part of the standard bloodwork panel when clinicians suspect hepatic insufficiency, pyrrolizidine alkaloid toxicity (from plants such as ragwort or Senecio spp.), Theilers disease, or chronic liver failure from any cause.