Hi @Belindab and @Bleubell
I have had another look through the scientific literature to see what I could find out about gallstone disease (GSD). Not much about fasting still, but some interesting work on the higher risk of GSD with high fasting blood glucose and non-alcoholic fatty liver disease (NFLD), which indirectly point to insulin resistance as a risk factor. http://www.ncbi.nlm.nih.gov/pubmed/24775330
As NFLD has been suggested to be associated with a high fructose intake, this may also be a risk factor. I also found a study showing that GSD risk was reduced with higher alcohol intake (but not excessive alcohol consumption obviously).
The type of gallstones is also relevant, with mixed cholesterol GS being common in those with Moor ethnicity and/or a body mass index above 25, while black pigment GS were significantly common among patients with Type 2 diabetes: http://www.ncbi.nlm.nih.gov/pubmed/24884475
There is an association between thyroid disorders and the presence of bileduct stones: http://www.ncbi.nlm.nih.gov/pubmed/24829684
and longer fasting may lower thyroid function. Although studies of fasting and thyroid function suggest any decrease in thyroid function happens after 36 hours, biological variation means that for some it happens earlier.
Curiously, an antibiotic, ceftriaxone, has a tendency to cause temporary gallstones and fasting can worsen this.
I am sure you are right that it is the lack of contraction of the gallbladder during the fasting period that encourages sludge formation, as this is what seems to happen on a low fat high carb diet also. I was wondering whether the bulletproof coffee that the low carbers are so keen on might be of value. By having some fat it keeps insulin down and the body in fasting mode but might the gallbladder a work out? I found this article on gallstones and low carb high fat diet: http://www.dietdoctor.com/gallstones-and-low-carb http://www.dietdoctor.com/gallstones-and-low-carb
. The advice there is to eat a high fat diet but to increase the fat levels slowly if you already have gallstones.
What the risk is of developing gallstones with routine 24 hour fasts is hard to work out as the other factors will be important too (the three Fs - or perhaps four if we add fructose - speed of weightloss, ethnic heritage and background diet etc.).
I do hope you are able to find your way through this minefield and end up with a healthy gallbladder and also able to find a way to continue your way of fasting and eating.