The ATkins diet discussed teh “tarvation” mode. It is a well-known side effect of calorie restriction and many several professionally-planned diets take it into account.
IIRC what the process was described as is this - after a few days (bout 3 or so) of severe calorie restriction, the body will notice “Holy cow! I’m starving” and your metabolism will adjust to reduce its calorie consumption. As noted above, you become sluggish, less energetic (duh!). How much calorie restriction? I don’t know, leave that to the experts.
Another interesting side effect is that your body starts to consume muscle. One purpose of getting fit as a means to reduce weight is that muscle mass burns more calories, even at rest. If you actually starve, your body sheds that excess weight. I know one person who went from 300 to 100 lbs (and slowly put it all back on). She now has a heart problem at a young age, because the effect also consumes heart muscle. Remember perhaps that in the 80’s the "liquid protein"diet (no, the other one!) was popular in Hollywood. people would drink nothing but this pink goop - until some of them started keeling over with heart attacks - same effect.
The Atkins and some other diets try to exploit the fact that what triggers the starvation mode is lack of overall calories - fat and protein too. Whereas, what triggers consumption of the fat stored in the body is lack of carbohydrates - or so they claimed. ANother fad says that the “low fat” diet we try to follow nowadays is actually what helps cause obesity, because we eat too much carbs to compensate and the body thinks it need to store fat due to the shortage.
You can monkey with the proportions and quantity of the 3 basic food types - fat, carbs, and protein - but in the end, a moderate amount of food and healthy exercise - to both burn calories and build muscle mass - is what’s needed.
Whether there’s a rebound effect, whether it’s phyisological or psychological - who knows?
As for toxic materials - there is a residual level of toxic materials in the environment, and they tend to accumulate in fat cells. PCB’s, for example, are found there. The higher up the food chain the more toxins; the toxins accumulate for example in plants, which are eaten by herbivores, which in turn are eaten by carnivores. Marine animals are the worst affected because the toxins are pervasive and because fish and sea mamals acumulate a lot of oil/fat. The eskimos have alarmingly high toxin levels because of the amount of fish and seals they consume.
Normally, we consume and the levels accumulate over our lives with no noticeable effects…
In the Biodome experiment (not the Pauley Shore one) thet eco-nuts refused to listen to the scientists, so as a result the dome ecology crashed. One effect was that in an attempt to survive the inhabitants were on a starvation diet. They lost about one quarter of their body weight, and regular monitor tests showed a large increase in body toxins such as PCB’s. This set off a panic search for the source and any toxic leaks until the doctors realized it was just coming out of their fat cells.
So - one more reason why a starvation diet is not good; but we’re still talking about toxin levels that are below limits, and that toxic surge gets flushed (since it’s not being stored any more); so all in all, it’s not an “OMG gonna die” situation, as long as you get some food later.