carb to fat for fuel, How long, at what point, how do you know
How long does it take for your body to swap sources of energy.
At what carb level and sustained for how long creates the switch over.
How do you know once its begun?
Though i have asked i haven't learned.
In what context?
Assuming a healthy metabolism your body can "swap" energy sources within a matter of minutes several times a day.
If you are talking "low carb flu" then depending on how damaged your metabolism is you can expect to feel better within a few weeks even though your body has "swapped" fuel sources as soon as blood glucose drops to a certain level after the first few days of restricted carbs.
You know that your body is using fat for energy when the egg, tbs. butter, and slice of salami at breakfast keep you full until dinner.