Always when you read about diets, you also read a warning saying that a too small calorie intake will slow down your metabolism. However, I can't find any sources explaining what exaclty slows down and how much engery is conserved through this slow metabolism (1%, 5%, 50% or maybe 99%?).

I know from experience that I can't reach the max. performance in sport during a diet as glycogen reserves are depleted faster and ketogenic processes take a while to kick in, but these kinds of reduced enegery consumption are in general additonal accitivies that are not required to keep up life.

When I sit on a chair and just fart around, is my energy "consumption" reduced through the following things, as compared to a normal food routine?

* Does the heartrate sink?
* Is blood renewed slower?
* Does my brain work less?
* Do hair and nails stop growing?
* Does my immune system take a holdiay?
* Do my bowls drain the food more completely?
* Do I produce less semen?
* Do injuries heal slower?
* Do my cells get older (recycled later), thus maybe even increasing the cancer risk?
* Is my temp lower?

I presume most of these things can only be reduced slightly in order to survive, so where does the body optimize energy "consumption", and how much savings are we talking about? 

Even when eating massivly less than required (e.g. 5000KJ under maintainance requirements over more that 2 weeks) I do not notice massive drawbacks in bodily functioning, therefore I am questioning these warnings. Furthermore you often read about a "starvation mode" and even a "ruined metabolism" - is there any scientific indication that such things exist and are inreversible (slowly reversable)? If so, why does the body in gerneral not work in this more effiecent state if there are no drawbacks?

I'm not questioning that lifting heavy weights while eating less will conserve more muscle mass. Also please don't give answers along the lines of changing your food consumption in general to lose weight (e.g. what the word diet really means). I'm using the word "diet" in the wrong modern way, as a limited time change of food intake.