Is there any actual science behind IF that isn't just basically consuming less calories overall? And what does it do to your metabolism ? Does there not come a point where your body becomes so used to running on so few calories that to reach a calorie deficit you have to reduce drastically
It's both - science and consuming fewer calories. If you eat/snack every couple of hours throughout the day, your insulin spikes like a yoyo. If your body doesn't actually need food, it will store it in the form of fat - and mostly around the liver, which is really bad.
If you only eat a couple of times a day, when you are actually hungry, these spikes dont happen, particularly if you eat mostly nutrient dense foods such as protein, vegetables and healthy fats, rather than refined carbs, sugar and - especially - UPFs. The latter is just industrially produced fodder, cleverly designed not to satiete, so one is still hungry and craves more.
There are many doctors and scientists who have written in depth about all this, such as Mark Hyman, Jason Fung, Tim Spector, Michael Mosely, Andrew Jenkinson and Becky Gillaspy. It can get a bit overwhelming, but I found The Sugar Solution, Why We Eat (Too) Much, Ultra Processed People, some of Jason Fung's talks and Becky Gillaspy's short videos on YouTube, as well as Michael Moseley's podcasts, very interesting.
NB: one thing to bear in mind is that after the menopause we may require fewer calories. I used to be one of those annoying people who could eat anything without gaining weight. Not anymore - I had to adjust my daily intake. I don't count calories as such, but if I've learnt the hard way how much I can eat without gaining weight. Not drinking alcohol has been a game changer - no empty calories and less inclined to eat crap...