Gallstones

I'm studying for a test I have on Monday and I find it interesting that two risk factors for gallstones are prolonged fasting and rapid weight loss.  I work with someone who suffers pretty frequently, at least a few times a year, from gallstones.  This same person practices intermittent fasting, which is the practice of fasting for 16 hours and consuming all your calories in an 8 hour window span. Additionally, this same person has reported a 20-30 lb weight loss in a year, which he attributes to intermittent fasting.  (I must note that he looks fabulous.)  While I wouldn't necessarily call a loss of 20 lbs in a year "rapid," I wonder if loosing a lot of weight, when someone is within normal weight to begin with, contributes to gallstones.  I would think that the intermittent fasting also contributes to gallstones, but I'm not sure if either statement is correct.  Something to consider...