This would be considered a fast and metabolism can actually increase during a fast, but will not likely decrease.
Chronic low calorie is the problem because at that point the body begins to predict that calories will continue to be limited and will down-regulate the metabolism to be conserving through what it thinks is an impending famine.
Yeah, this is part of my problem too and why I track everything in MFP so I can at least review the information to detect patterns and associate them with how I feel.
Using MFP, I noticed that my IF was actually being combined with calorie restriction and although keto does seem to naturally reduce calories, I was just too low considering how I felt.
I find it best for me to restrict carbs to under 20 grams, protein between 70 and 100 grams and eat fat until I feel full and some days that means eating very little and some days I eat a lot (and I really mean a lot) and I’ve been hovering around the same bodyweight for a couple of years. I have problems when I try to increase calories using carbs or protein.
I would recommend against eating just to get your macros unless that is based on empirical evidence that eating too little of something consistently results in feeling cold or hungry, etc… This is taken from personal experience because I noticed that feeling cold correlated with eating too little in general and specifically too little fat as well as eating too little protein was associated with more soreness and slower recovery from activity and exercise.
In other words, spend some time eating what you want and evaluate the results. In your case and mine this can be scary knowing that we have a history of eating for emotional reasons so it’s constantly second-guessing whether or not the hunger is real.
I’ve used the information I log to find out what works for me and review it weekly to see if I’m drifting too high or too low and I will occasionally correct with an extra fast or skipping a fast and eating more in the next couple of meals.
That’s probably not very helpful, although you’re not alone when it comes to really understanding hunger and eating to satiety, and I will say that I have gotten much, much better at understanding real hunger and eating appropriately because there are now many more times when I’ve eaten to satiety and spontaneously fast which is something that never happened before keto.