[Looking for studies]Regularly practicing fasting and speed at which body switches energy sources

So I know a lot of you have lots of info around on fasting. In particular what I’m looking for relates to respiratory quotient (RQ) and fasting.

So I think most of us are generally aware of the \~3 day process when initiating a fast of how your body will deplete glycogen in the order of 1. blood glycogen 2. blood glycogen and 3. liver glycogen.

One way I’ve seen this expressed is in the paper from Ray Cronise on Oxidative Priority, Meal Frequency and Energy Economy of Food and Activity

Excerpt:

> In the fasted state, metabolic energy is completely sourced from body stores. If the fast continues, glycogen stores will eventually be depleted and a shift to fat oxidation will ensue.33 Fatty acid-derived ketones can not only contribute to metabolic activity but also provide up to 60%–70% of brain energy needs during long periods of glycogen depletion; blood glucose concentrations are maintained by gluconeogenesis and glucogenic amino acids sourced from protein stores.

This is also tied it seems to RQ which is a shorthand for measuring what energy source the body is utilizing. With typical resting state being 0.85 which translates to 49.3% fat oxidation. Ray mentions in the paper that during times of sustained calories restriction resting RQ can drop to around 0.78 which translates to 74% fat oxidation.

To me this is great scientific explanation of muscle sparing and fat utilization that happens as you go ketogenic.

My question thus is are there any studies on people who fast more regularly switching more quickly to fat burn? I’d imagine this could likely be measured either by RQ or by has fast one enters ketosis. (I imagine you could test with those ketone strips)