Thanks. BG measurements are useful because glucose concentration in the blood directly determines a number of health parameters. Insulin (and glucagon!) measurements would also be very useful, but as far as I know there are no home devices available for it.
Ketone measurements, on the other hand, are a proxy for fat metabolism and even then only measure β-hydroxybutyrate concentration, the somewhat more stable storage form of acetoacetate, which is the actual ketone energy packet. β-hydroxybutyrate concentration does not tell us how much fat and/or ketones are actually being utilized.
Breath acetone (BrAce) measurements reflect the concentration of acetoacetate. This because BrAce along with CO2 are the resulting products of acetoacetate breakdown and utilization. BrAce is a better indicator of actual fat burn of acetoacetate, but the concentration in the breath is so dilute that it is very difficult to measure accurately. Also, the amount of acetone produced varies markedly and rapidly.
Yes, all of the above change constantly because our metabolism is a very dynamic system. It responds to inputs/outputs continuously, minute to minute.
A non-invasive device that accurately measures RER determines the ratio of carb and fat burn. No need to estimate fat burn based on the measurements of moving proxy targets! The possibilities and potential for this are very great indeed. For example: do you really need to eat sub-20 grams of carbs to stay in ketosis? What if you eat 50 grams and still have an RER of .7 or .71? That would be good to know, donât you think? Or the opposite. Someone thinks âI can eat 50 grams of carbs and Iâm still in ketosisâ. Then discovers his/her RER is .8.