Next time you're back from a run, questioning whether it's been worth the agony effort, you might actually be able to find out for sure. Well, perhaps not the next time, but soon, thanks to a prototype breathalyzer developed by NTT DoCoMo that promises to instantly let you know whether your body is currently burning fat. We've seen the device before, but a recently published journal on its latest test results brings it one step closer to reality. The handheld unit uses gas and pressure sensors to monitor levels of acetone -- a substance created during fat burning, partly expelled via the lungs -- in the breath, and can display fat burning potential on a phone via bluetooth (or cable). The creators tested the prototype on a small group, and found that those who didn't diet or exercise, or did light exercise only, didn't show signs of fat burning, but those that watched what they ate, and upped activity did -- boosting hopes of its real-world use. No word on commercial availability just yet, but we're already wondering if it might up the ante on our potential second income.