This is more or less considered a given by most people, that severe dieting or starvation will slow your metabolism. However you would figure that if the human body (or any organic body) was capable of making due on far less calories then it would do so.
There are several ways the metabolism would slow down that i find believable.
- The body would cannibalize muscle tissue, which would provide calories, lower metabolic rate (more muscle = more calories burned) and induce lethargy.
- Lethargy would set in so people are incapable of physical activity, saving caloric needs
- The thyroid will not convert as much T4 to T3, slowing metabolism
Other than that i do not see how it is possible, none of these things really imply that on the microscale that the body is working more efficiently (except for maybe number 3) I once read that people in concentration camps who worked all day in manual labor were fed around 1600 calories a day and weighed 80ish pounds. If you consider 10 lbs of that being fat that is around 23 calories per pound of lean body mass, which doesn’t sound like something a person with a super efficient metabolism would need. So for people in concentration camps (for example) did their bodies become more efficient at using calories or did they lose their muscle tissue and experience lethargy to cut down caloric use?
Essentially, does starvation cause your body to burn less calories (by destroying muscle and inducing lethargy) but at the cellular level still use the same amount of calories in the same way or is there some way it can become more efficient, shutting down less efficient usages of calories and getting more energy from the calories it gets.