Let us repair to the workshop and try to turn this back of the envelope sketch into hardware. Rummaging through the parts bin, we quickly find all the components at the top of the diagram. Integrators, comparators, and indicators are available for pennies apiece.
We come up short, however, looking for those confounded meters that read calories in and out. It's possible to measure these quantities, at least in principle. If you were crazy enough to do so, you could calculate calories in by looking up everything you ate, as you ate it, in a calorie counting book and punching in the numbers on a keyboard. You could track calories burned by measuring body temperature, blood sugar, heart and respiration rates, etc., especially if you were willing to calibrate the readings for your own body over a month. Biosensors that measure these quantities have been used on astronauts, athletes, and intensive care patients for years.
But the whole idea of the eat watch is to be unobtrusive and not disrupt your life! Being slim and trim and the envy of your peers loses a lot of its gloss if it means spending the rest of your life wired up like a labrat. ``Bend over, please, this won't take but a moment.''
No.
But it could be done. This is encouraging.