Just white bread with a slice of cheese and spiced ketchup.

A slice of bread with whatever is available to me.

I see, looks like the book is available in English.

Conquering Fat Logic

How to Overcome What We Tell Ourselves about Diets, Weight, and Metabolism

Nadja Hermann

There is a great book in German called " Fett Logik überwinden" ( Overcome fat logic) that scientifically clears up a lot of the myths around gaining and losing weight. What you write about are the classics mentioned in this book.

You need the protein and minerals as building blocks for the muscles. That is why you need to take special care to ingest enough of them with that low calories.

More muscles burn more energy even when idle, that helps losing weight. Looks like you did that right.

As far as I know: forget this thing about the lowered metabolism. Your body needs the energy it needs for basic functionality.

You may feel less active, lowering the energy used above the basics, but still your heart, lungs, brain, temperature management and all the other stuff need roughly the same energy. If your body does not get it from food then it will use up the fat.

But eating this low level of calories you must make sure that you consume all needed vitamins, minerals and enough protein.

And being less active may end up in a decline of muscle mass. In the end that may lead to lower basal metabolsk hastighet, but not your metabolism shutting down.