Bile works like washing-up liquid, breaking dinner's fat into tiny droplets
Fat and water do not mix, which poses a problem for a gut that digests everything in a watery soup. The liver's answer is bile, a yellow-green fluid whose salts behave like detergent. They coat globules of fat and break them into droplets small enough for enzymes to attack. Without it, most dietary fat would pass straight through.
An adult liver makes somewhere between 400 and 800 millilitres of bile a day, and it makes it constantly. Between meals the fluid is stored and concentrated in the gallbladder, then squirted into the duodenum, the first stretch of the small intestine, after eating. It is almost entirely water, about 97 to 98 percent, with small amounts of bile salts, cholesterol, fatty acids, lecithin and pigments.
The clever part is the shape of a bile salt. One face attracts water and the other repels it, so the salts cluster around fat with their oily sides inward, forming bubbles called micelles. Their outer faces carry a negative charge, which keeps the coated droplets from merging back together. That vastly increases the surface area available to pancreatic lipase, the enzyme that splits each triglyceride into two fatty acids and a monoglyceride small enough for the intestinal lining to absorb. The same process lets the body take up the fat-soluble vitamins A, D, E and K.
Bile has other jobs too. Being mildly alkaline, it helps neutralise stomach acid as food leaves the stomach, and its salts kill many microbes. It also serves as a disposal route for bilirubin, the orange-yellow leftover from recycled red blood cells. Bilirubin and its green oxidised cousin biliverdin give bile its colour, and ultimately give faeces their brown shade.
When bile is missing, fat goes undigested and emerges in pale, greasy stools, a condition called steatorrhea. Cholesterol in bile can also clump into gallstones inside the gallbladder. And the bitter green or dark yellow vomit that comes after repeated retching on an empty stomach may contain bile forced back up past the stomach's lower valve.
Source: Bile
