observed in women without exposure to these factors. In this case, stress is the cause.

In general, there is a type of people who are extremely prone to stress. They tend to kick ass, don't like change (like that client from my younger days), and their already elevated cortisol production is exacerbated by a combination of too much exercise and too few calories. A proposal to increase calories or reduce activity is inevitably met with resistance (as was the case with that client). What such people really need is to relax and slow down a little. Or not even slightly.

And the approach that they are intuitively inclined towards is not suitable for them: for such comrades, moderate deficits and moderate activity always work better. But getting them to do this is what is really hard.

By the way, I suspect that the classic hardgainer is a classic representative of this type, but this is a topic for another time.
(I recalled the advice that the aftar gave to one guy from the forum, whose weight did not go away, and he wrote a detailed post about his problem, nutrition and training schedule, asking for advice. And the aftar told him that looking at his post, one gets the impression, that this person is just one of those who are prone to excessive production of cortisol, and it would be best for him to lie in the bath, drink a couple of glasses of wine, and then he, the author, is sure that the situation will change for the better).

Summary
Well, this was a look at one of the oddities of the weight loss process, where the combination of extreme calorie deprivation and excessive exercise seems to do more harm than good for burning fat, and my speculations (with just enough research references to make it seem I understand what I'm talking about) about what might actually be happening.

Although from a practical point of view, most of this is irrelevant. There is simply a fact that is true for most people: you can either cut calories OR train hard and hard. You cannot do both. Okay, actually you can, but in most cases you just shouldn't do it.

Author's thoughts on what happens when people talk about dead metabolism and that they are gaining 1000 kcal, from a forum post.
Definitely there is evidence of a metabolic disorder where the % of fat is reduced very much, and these problems are further aggravated by the influence of overtraining, in which most losers are at this point. But I don't see any physiological rationale for a person to actually gain FAT on such a low calorie intake. I believe that fluid retention is to blame (and the neurosis that makes people equate this water retention in the body with weight gain and fat).