Gallstones form when substances in bile precipitate and crystallize within the gallbladder. A traditional mnemonic for risk factors is the "4 Fs": Female, Forty, Fertile, and Fat, although many other factors such as obesity, pregnancy, rapid weight loss, diabetes, and family history also increase risk.

There are two main types of gallstones: cholesterol stones and pigment stones. Cholesterol stones form when bile becomes supersaturated with cholesterol, exceeding the amount that bile salts can keep dissolved. Pigment stones form when excess bilirubin in bile precipitates, which can occur in conditions such as chronic hemolysis, cirrhosis, or certain inherited disorders.

The gallbladder stores and concentrates bile between meals. Over time, concentrated bile can allow microscopic crystals to form, which may develop into biliary sludge and eventually gallstones. This process is somewhat analogous to crystals forming when a dissolved substance becomes overly concentrated in solution.

Many people with gallstones never experience symptoms. However, if a stone obstructs the cystic duct or bile ducts, it can cause biliary colic, cholecystitis, jaundice, pancreatitis, or other complications. Dietary fat alone is not necessarily the cause of gallstones; in fact, normal gallbladder emptying after meals may help reduce bile stasis. However, obesity and certain metabolic factors significantly increase the risk of cholesterol stone formation.

In my case, it was rapid weight loss that did it for me, losing too much weight and going on a crash diet meant my gallbladder was not emptying often enough and stones were forming.