> Sure it's an extreme case, meant only as a thought experiment, but it's obvious that 0 calories in and non-zero calories out absolutely MUST mean weight loss. What are the alternatives?
Conversion of body weight to a form that has less stored energy per unit of weight mass (e.g., fat to muscle). That's unlikely with zero calorie intake (since activity, which is difficult to maintain with a too-extreme deficit, and protein are both important to it), but it balances the calorie equation just as well as weight loss does.
More generally on the point under discussion, adjusting caloric intake alone without changing calorie output is expected to produce weight loss if a bunch of other factors are held constant, but the way humans are constructed, its actually not at all simple to adjust calorie intake and not alter calorie output or any of the other relevant factors -- in fact, many of them the body will itself change as a result of a change in calorie input.
Conversion of body weight to a form that has less stored energy per unit of weight mass (e.g., fat to muscle). That's unlikely with zero calorie intake (since activity, which is difficult to maintain with a too-extreme deficit, and protein are both important to it), but it balances the calorie equation just as well as weight loss does.
More generally on the point under discussion, adjusting caloric intake alone without changing calorie output is expected to produce weight loss if a bunch of other factors are held constant, but the way humans are constructed, its actually not at all simple to adjust calorie intake and not alter calorie output or any of the other relevant factors -- in fact, many of them the body will itself change as a result of a change in calorie input.