I think have in a sense but I kind of powered through it. On the spreadsheet image I posted, in the notes, there's a reference to 'crashed' metabolism.
As I understand it, and I don't understand it very well: If the brain thinks it might be in a potential starvation situation it reacts by doing a range of things to reduce your energy expenditure. For example it slows digestion, trying to extract as much energy from your food as possible, and it slows your metabolism. I've seen this referred to as a 'crashed' metabolism or a 'damaged' metabolism, or 'starvation mode', mostly on fitness websites. I believe trying to lose weight too fast causes it but perhaps it's at least partially about what you're eating, not necessarily the size of the calorie deficit. One of the signs that this is happening is a reduced Resting Heart Rate (RHR). The heart beats slower when you're sitting still or lying down, to conserve energy.
I bought my Fitbit (Charge 6) thinking it would help with tracking calorie burn. I discovered that it is not very good at this at all, for me at least, but one of the things the newer Fitbits with optical heartrate sensors do quite well is measure resting heart rate. It's considered a useful indicator of overall cardiovascular fitness. It goes up and down a little depending on things like stress, how much sleep you've had, and alcohol intake, but big changes lasting days at a time are a sign that something unusual is afoot, such as illness. I've attached two screenshots from the Fitbit app which appear to show my metabolism crashing at one point to a new, lower level, and where it is today after resuming my diet fully on January 1st. It was almost flat at around 66 Beats Per Minute (BPM) during the first two weeks in December.
At the time I didn't even notice it was happening. I had just started using the Fitbit so I didn't have baseline RHR data to compare it to. I was in the midst of changing my diet and increasing my exercise little by little, focused on my rate of weight loss to tell me when enough was enough. I think the reason I didn't plateau was because I was increasing my calorie deficit at the time - I powered through it. The only thing I noticed at the time was that I became quite lethargic, drained of energy, shortly after sitting down after I exercised and lasting an hour or two if I wasn't being active. It's possible that part of the reason I've lost so much weight this past week is that I last made changes to my diet 'plan' while I was in this 'starvation mode', and my metabolism hasn't had time to adjust yet since Christmas and crash again.
The danger of being in this state is that the body will consume muscle mass for fuel. I've read that resistance training, lifting weights, can help to avoid this, as will eating lots of protein, but I have no idea if that's true or not. I did buy some small weights over Christmas and intend to use them - not with the intention of building muscle just yet but to try to retain what I have. Also to try to improve my insulin resistance levels by burning off the ectopic fat stored in all my muscles regularly. My thinking is, and this is pure amateur speculation, the body will not consume muscle mass that is in frequent, regular use. It wouldn't make much evolutionary sense to burn up muscle mass that is needed for the basics of survival - hunting and gathering or whatever - while it makes complete sense to reduce the size of muscles that aren't in regular use, in a potential starvation situation. If my metabolism crashes again, and I think it's about to as my RHR is starting to fall, I plan to work all my muscles a bit every few days, eat lots of tuna and salmon, and hope for the best. If I do end up losing some muscle mass I intend to just build it back up again after I reach my target weight. I'm not not exactly 'ripped' so building back the little muscle I have should be doable. I'm not so advanced in years that building muscle is impossible - difficult perhaps, but not impossible.
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