I’ve got a needle in my arm, and I’m craving chocolate cake. The cake in question is my mom’s, flourless and rich, on a plate she’s just handed me at brunch. The needle—more like a filament, actually—is attached to a sensor called a continuous glucose monitor, or CGM, which measures my blood-glucose level in real time. I know two things: One, this cake is delicious. Two, it will probably jack my glucose reading sky-high, which has all sorts of possible unwanted consequences, ranging from me falling asleep on the couch in an hour to, over the long term, premature cardiovascular disease. But I’ve already had pancakes with syrup