If I had a unlimited budget I'd do comparison studies between BMR and fasting, fat fasting, ketogenic metabolism, and omnivore Mediterranean - a cross over design with a 6 week wash in period, and a 6 week adaption window to each intervention.
The FGF-21 signal appears to be very strong in mitochondrial decoupling, but I'm not sure if there is any benefit to fat fasting vs just water fasting.