There is a (relatively small) subset of Muslims that fast every Monday and Thursday, in addition to fasting the month of Ramadan. Perhaps that’s a better population sample to study?

And how much fasting?

Is intermittent enough? 12 hours? 18 hours?

Or do we need frequent (e.g. weekly or 2-3 times weekly) 24-hour fasting?

14 hours every day is considered great (don't eat late, skip breakfast), a solid 48 hour fast every month or so is also great. Small human studies, for example, have measured 5x to 10x increases in HGH after 48 to 120 hr fasts respectively. Free steroids, basically; the body repairing itself while not being drowned in food all day every day. Telomeres too!

Fasting during Ramadan is only during the daylight hours, so hardly even enough to be considered intermittent fasting.