Adipose tissue retains an epigenetic memory of obesity after weight loss

As someone who's struggled with weight loss, and have known others to struggle with it well, I think we colloquially called this "slow metabolism".

It always did feel like it was easier to gain weight than lose it, especially fat weight and not muscle weight for me.

I was recently sent a video about fat adaptation (basically teaching your body to be better at burning fat) by a very fit friend, but I wonder how much of that is bro science and how much of it is grounded in reality. Maybe worth looking into more deeply if it can counteract or balance out this.

A lot of people blame failure to lose fat on a "slow metabolism" but this is usually incorrect. Have you had an actual resting metabolic rate (RMR) test to quantity your baseline total daily energy expenditure?

Fat adaptation is a real thing. Endurance athletes focused on longer events will target some of their training around that energy system. This is more complex than can really be explained in an online comment but basically you want to do long training sessions below your lactate threshold in a glycogen depleted state.

There is also a conflation of a slowing metabolism and low energy availability, which can reduce the amount of energy expended during the day (because you feel tired and do less). It can be quite subtle but when I've done some extended periods in a calorie deficit I start to notice subtle things, like a propensity to sit a bit longer, or to reduce my overall body movements. My resting metabolism is the same (I've had it measured a few times) but my body looks for ways to expend less energy.