This is the version of the explanation I wish somebody had given me, written down before I forget what confused me. It is about protein intake, and it is deliberately narrow — everything I am not confident about is marked as such.
What is actually established
The RDA is the wrong reference and it is worth understanding why. 0.8 g/kg is the intake at which nitrogen balance is not negative in healthy weight-stable adults — a floor for deficiency prevention, not an optimum, and derived in a population that is not in a deficit. In a substantial energy deficit, protein requirement rises because amino acids are being oxidised for energy and because muscle protein synthesis is blunted. The literature on preserving lean mass during weight loss lands around 1.4 to 2.0 g/kg of reference body weight, which is roughly two to two and a half times the RDA.
The condition it depends on
The caveat that reference weight and current weight are different numbers. Using current weight at the start of a large loss produces a target that is both unachievable and unnecessary.
The practical version
The arithmetic that makes it tractable: reference weight in kg times 1.6 gives the target; divide by four for a per-sitting figure; then find four foods that hit that figure in a volume you can actually finish. Most people need two of the four to be liquid or semi-liquid early on.
What I am not sure about
The bit I cannot resolve on my own is whether the RDA is the wrong reference entirely during rapid weight loss, and what the actual target should be. Practical detail welcome, however dull — the duller the better.
— labquiet_amy · corrections welcome and will be edited into this post with credit