Steve;
Thanks for writing in with your question. I doubt there’s much scientific literature about this. The reason for that might be that it comes down to the usually answer to rather complex questions: “It depends”.
There are many factors that impact both the rate at which fat adaptation occurs and declines.
If you were to cease all aerobic training and sit on the couch eating cookies I’m sure you could reveres the effects of several months of fat adaptation in a few days to a week. After all we are talking about an enzymatic change in the cells which occur on the time frame of hours to days.
If, on the other hand, you stay very actively doing aerobic activity you can even eat more carbs than your normal diet and maintain fat adaptation indefinitely. I coach a number of very high level athletes training in the 18-25 hours/week. They can eat a high carb diet and be very highly fat adapted. On the other end of the spectrum we have quite a few coached athletes who come to us with poor fat utilization and who are only able to train a few hours/week. For them some form of carb restriction is needed to help jump start and speed the fat adaptation process.
For a more in depth dive into mitochondrial function and fat adaptation I highly recommend listening to this podcast
I hope this helps,
Scott
