The century old scheme for estimating how much energy in food is used by our metabolism is only a crude approximation, but many of us use it as if it was fact. It can be a useful guide - particularly when you are trying to lose or maintain weight and journal what you eat, but sorting out how your particular body behaves is very complex and an individual task. A nice post by Rob Dunn appears in SciAm
Fortunatly people have bodies that are good at self regulation. Unfortunately many of us have managed to break that system by being overweight for a systained period.
This sort of thing will greatly expand and become much less expensive in the next few years.... I suspect water quality monitoring will be important at the neighborhood level. Ultimately it may drive court cases, so calibration and reference will be an issue.
I wouldn't be surprised to see prices drop dramatically over time.
I've been wondering about what supports the notion that the paleodiet makes sense - after all, it comes from cherry picked "data" from a period where few people made it to a 35 year oldage. The things that kill us now after the age of 60 were complete non-issues, but getting enough calories to make it through the week were, for many (not all) of these peoples, very pressing.