Hey Jacqueline. The increase in autophagy that occurs with extended fasting is due primarily to nutrient deprivation, almost entirely the absence of protein and carbohydrates. Fat has vastly less effect on mTOR and AMPK, as well as on insulin and glucagon - these are the primary things that speed autophagy up or slow it down, when we’re talking general autophagy all over the body.
Protein is a potent inhibitor of autophagy, followed by carbohydrates. Fat may make some difference via its presence in the stomach and small intestine, due to hormone secretion in response…? I’m not sure if this has been studied.
Fasting itself, where the body is using its own stored fat, avoids that, but there are genetic controls/limits on autophagy, and it’s also a self-limiting process to an extent, as it breaks down some protein structures and cellular components into the constituent amino acids, including leucine, which is the most potent inhibitor of autophagy of all. Thus, even with no protein consumed from external sources, the body is still ‘making’ some leucine, etc.
There’s an argument that eating only fat won’t affect the increased autophagy that we gain from fasting, or won’t affect it much. As far as I know, this is far from settled. But eating protein or carbs every day (even once a day) pretty well cuts you off from increased autophagy.
Getting rid of loose skin operates differently for different people. Not everybody loses all of it or most of it through fasting; it really varies. Overall there is a big difference between extended fasters and those who don’t do it - the real consumption of loose skin tends to occur in the extended fasters, although it doesn’t happen with every individual.
Even at the increased rate of autophagy after a few days of fasting, the amount of protein that’s processed in a day is far less than the amount of fat. It’s mostly two independent things that are going on - the body using fat for fuel, and then also having an increased rate of “house cleaning” going on. As one becomes very lean, this changes somewhat - the portion of lean mass consumed, versus fat mass, increases while fasting.
For you personally, perhaps staying at your new lower weight would help with loose skin. The elasticity of one’s skin makes a difference of course, to start with. I don’t know about taking collagen or any other treatments, but my gut feeling is that almost everybody would see some improvement, even if over time. Just a guess on my part, there - if the body does not need as much skin, perhaps it won’t replace all the cells as they die off?