How does starving or dieting slow your metabolism

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.

  1. The body would cannibalize muscle tissue, which would provide calories, lower metabolic rate (more muscle = more calories burned) and induce lethargy.
  2. Lethargy would set in so people are incapable of physical activity, saving caloric needs
  3. 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.

There’s a baseline to maintaining critical body functions, called the BMR (basal metabolic rate). Any calories you consume beyond that and do not burn are stored in various forms (like fat). If you don’t consume enough calories to provide energy for basic body functions, you start losing those functions. When you are starving, you are barely taking in sufficient calories to maintain basic body functions, which does not allow for much activity beyond sitting around–even digestion requires energy, and cannot be done properly on starving metabolism, so I suppose it depends on your definition of “making do”.

The body does cannibalize itself for a source of energy to maintain its essential functions in the absence of food intake, but it will harvest glycogen and fat stores far more readily than muscle tissue, which is pretty much a last resort. Low levels of T4 and T3 result from inadequate food intake (partially because iodine, which we get from food, is essential for their production), resulting in slower metabolism (though I don’t know the exact cellular mechanism).

Neither. More efficient use of calories would imply that fewer calories were needed to do the same amount of work, but instead they just took in fewer calories and were capable of less work. Therefore, their bodies burned calories more slowly, but not more efficiently.