This portable device will tell you exactly how many calories you are burning in real-time.
The principle is simple: all energy is ultimately obtained from breaking down glucose (and related molecules through various biochemical pathways), and that results in CO2, which is exhaled. In laboratory conditions,
you could gather all exhaled CO2, and calculate how many calories a has burned. This is impractical for a portable device.
All we really need to know is the concentration of CO2 exhaled, and a good estimate of how much is being exhaled.
The Calorie Burn-O-Meter is comprised of two tiny intranasal CO2 sensors (optionally available magnetically attached nose-ring style, or as a transparent strip that goes above the lips), a mouth CO2 sensor (available as a tiny dental glue-on or a lip ring), and two chest attachments that monitor that estimate tidal volume (i.e., how much air is being inhaled/exhaled). The tidal volume sensors are placed in the front and back of the chest and are calibrated to estimate current lung volume by measuring the distance between each other (using short RF pulses and timing them or measuring change in EM permeability, etc).
The central unit of the Calorie Burn-O-Meter communicates with all sensors via a small wireless network, and calculates calories burned by estimating the absolute amount of CO2 exhaled (ambient pressure, temperature, etc. may have to be taken into account).
Clearly, the system must initially be calibrated to the user (esp. the tidal volume sensors).
This system may be marketed initially to weight-loss clinics and health-clubs who rent it out to clients starting a weight-loss or excercise program. The sensors would be semi-disposable (i.e., cheap enough to replace).