Discussion of metabolism and diet is squarely outside my areas of expertise. I have not studied these formally for decades, so the only information I have is the same information everyone else has readily available. But I do read and think about these all the time, and I do have a first-hand insight into how science and medical research actually work (and sometimes fail to work).
I would be very skeptical of any single-paper findings. Surprising or otherwise ostensibly controversial findings need to be replicated several times, by different researchers, in different locations, before taken as “scientific gospel”.
This pervasive problem with medical research is described very well in the book
“Rigor Mortis: How Sloppy Science Creates Worthless Cures, Crushes Hope, and Wastes Billions”, by Richard Harris, an NPR Health and Medicine reporter.
I suggest everyone read this book before combing through the scientific literature to find any stand-alone gems. This book gives a very clear and coherent description of the most important problem with and limitations of modern medical research. This is only part of the problem, but it is a very important part indeed.
I mention this because of your concern over the findings in one paper on animals that developed insulin resistance while fasting. Unless that paper can be repeated, I would only consider it an interesting and important finding that needs to be replicated before further action, not as a guideline for dietary planning.
Another problem is that the animal type in the study may have been entirely inappropriate for modeling human metabolism. For example, mice may be excellent models for some types of studies (such as fundamental molecular mechanisms), but can be very inaccurate and misleading for other studies that relate to human health. And in particular, rodents and humans have very different metabolic needs. And I do not know that any study of humans during fasting has shown similar results; quite the opposite so far as I know.
The main impression I get is that, for HUMANS, most forms of fasting are quite beneficial, appear to be safe, and no effects such as insulin resistance seem to have emerged as a repeatable outcome. If that were shown in humans, it would certainly make the headlines, and so far as I know, it has not.
IMHO, this is an example of the inappropriate application of animal models in medical research, resulting in misleading and inaccurate extrapolation to human health. But I could be wrong, it would not be the first time.
One more thing I would like to say about fasting, just my opinion, but it makes sense to me: I think human metabolism and the many cycles and states driven by the feeding cycle in humans should be viewed in the context of human evolution. I don’t think human metabolism is optimized to have a beneficial response only when a precise eating pattern is followed. Precise feeding patterns that you see in the scientific literature are a necessary expedient for research. But it is wrong to think that people evolved to eat precise amounts of macronutrients at precise intervals. I think that metabolic agility arises from variations in feeding/fasting intervals, and therefore I do not follow strict patterns.
I base this on my own self-experimentation. It is really just common sense and I think everyone observes this. I have observed it by monitoring my own metabolic shifting during IF using a Ketonix meter and strips (they correlate well for me).
Any given pattern of feeding/IF that induces a shift to ketones for me will get better and faster over time. This is a general metabolic agility that also works when I use very different patterns of feeding/IF: the shift to a keto metabolism gets more efficient with time and fasting cycles, even for very different feeding patterns. I view fasting as a form of exercise for my metabolism. So, I think it is valuable to vary the pattern of fasting, just as it is valuable to vary any other form of exercise. And this is precisely what I do, and it seems to work well for me.