I developed type 1 diabetes in March 1991, when I was 21 years old (which is pretty late—most people with type 1 get it as children or teenagers). My pancreas makes no insulin at all, so I've had to inject it and monitor my blood sugar by taking tiny blood samples from my fingers several times a day ever since.
While I have my blood sugars under pretty good control, I do occasionally have a low-blood-glucose incident (also known as hypoglycemia, or in severe cases, insulin shock), when I haven't eaten enough food to compensate for the insulin I've taken, or have taken too much insulin for the amount of exercise I've done.
People who know about that sometimes wonder what it feels like. Every insulin-dependent diabetic gets hypoglycemia from time to time, but we all feel it differently. Some people don't notice at all until they're in a stumbling stupor, but my body has (fortunately) always alerted me as soon as I approach the 4.0 mmol/L threshold of low blood sugar.
The best way to describe the sensation is to think of the jittery feeling you get when you almost drop a glass but catch it in time, or think you've lost your wallet and keys but remember they're in another pocket, or narrowly miss a car crash. Your body pumps out adrenaline, and you feel all wired and shaky. (It therefore also feels a bit like having drunk way too much coffee.) Yet when I have hypoglycemia, that feeling doesn't abate, but keeps going until I consume some sugar to boost my blood glucose levels back to normal. It's a disturbing, annoying, and frightening feeling, which is one reason insulin isn't a drug anyone is likely to abuse.
I also have more direct physical symptoms. Blotchy, pulsating light patterns appear in my field of vision, like very pale versions of what you see when someone takes a flash photo. My nostrils flare involuntarity. My hands shake, and I sweat. I also feel ravenously hungry, which makes sense, since for normal people low blood sugar is a sign of not having eaten enough. I'm also lucky that, in the rare occasions hypoglycemia hits me at night, I wake up and can go deal with it.
Those who haven't wrestled directly with the physiology of diabetes frequently find it puzzling that I generally can't eat refined sugars, but when I have hypoglycemia, I must consume them to stave it off. What it comes down to is that, while most people's bodies regulate their blood sugars automatically—responding to food intake, exercise, and other factors—I have to do that manually. My general goal is to keep my blood glucose from getting too high (i.e. much above 8–10 mmol/L), so I must take insulin and regulate my intake of sugars and other carbohydrates.
Every once in a while I don't get it precisely right, and my blood glucose goes too low, so I have to dump some quick carbs (sugar, such as a can of Coke or even a couple of packets of table sugar) into my system to keep from passing out. In my 12 years as a diabetic, I have only blacked out once, and that was right after I was diagnosed, when I still didn't have a handle on how my body reacted to all this stuff. But it happened in the middle of a sidewalk near Granville Street and Broadway in Vancouver, a very public place, and I woke up being wheeled from an ambulance into Vancouver Hospital's Emergency ward, with elbows sore where they hit the pavement when I fell over. I've worked hard to avoid that happening again ever since.
So I carry a can of Coke wherever I go: when I go out for a walk, when I ride my bike, when I take my kids to school, in a gym bag by the side of the pool when I swim. When I feel the jitters, I drink it and maybe eat a bit of bread or yogurt, and I soon feel better.