Gallstones are made from solidified bile, a yellow/green digestive juice produced by the liver. It is produced continuously and stored in the gallbladder, where water is removed, concentrating it for use in emulsifying consumed fats.
When fat is eaten, the gallbladder is sent a message to contract. This sends the concentrated bile into the duodenum where it breaks apart and coats consumed fatty acids with a water soluble lipoprotein shell to allow them to enter the bloodstream.
When the gallbladder is not called on to contract over a long period of time, the stored bile becomes overly concentrated and begins to solidify, first forming a sludge and then stones in the gallbladder. Gallstones take years to form, they do not happen overnight.
After the stones have formed, when fat is eaten, pain is caused by the contractions of the gallbladder trying to push the sludge and stones down the rather tiny bile duct. (Biliary colic).
When a switch is made to a low carb way of eating, if you have gallstones the increase in fat intake will cause pain. This is why a very low fat diet is prescribed to control the condition. It removes the cause of the pain, but actually makes the issue worse over the long term.
There are several medical interventions that can be made. Using lithotripsy to break down the stones into sludge or administering ursodeoxycholic acid. Consuming olive oil and garlic has also been shown to aid the body break down gallbladder sludge.
The body can survive without the gallbladder if removal (cholecystectomy) is required as a cure. If a removal is required, remember that fresh bile is not concentrated at optimal levels for fat digestion. Increasing consumed fat must be done slowly, to allow the body to adjust. Diarrhoea can occur if too much fat is eaten too soon.
Gallstones require a high level of inconvenient diet management to avoid attacks of biliary colic. Most people find that if removing a stone filled gall bladder is required, this is a very positive thing.
Image used:GallbladderAnatomy By GallbladderAnatomy.png by LukesAnatomy (talk); conversion to SVG by Angelito7 (talk) – GallbladderAnatomy.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=29818837