The larger a dose of insulin is, the faster it does it's work. Since I take fairly small doses of insulin, boluses are usually around 2-3 units or less, I hardly ever see my sensor with arrows showing that I'm trending down or up. In general, it's good to have changes in glucose happen more gradually, but when you are really out of range, you want faster change.
The one thing that has allowed me to experience rapid changes in blood glucose lately is exercise. I know I've already talked about it, but having a sensor to look at during every experiment has taught me a lot.
If I get on the elliptical for even 10 minutes I will soon see an arrow showing a rapid change in my blood sugar. Often, it will be a double arrow, indicating that I'm dropping faster than 5 mg/dl per 5 min.
I know that exercise makes you more sensitive to insulin (needing less), but this is clearly also an accelerated absorption rate. If I have a 2 units of insulin in my system and the rate of change is so slow that my sensor graph is barely showing it... 10 minutes of low-level activity will turn the insulin receptors on and have me in a fast correction. I'm finding that it allows me to spend less time out of range, when I make a mistake.
Here's an example from yesterday:
I was going into lunch feeling hypo and my sensor was reading 80. I decided to eat the baked potato that came with my ribs, as a precaution. Since I'm very sensitive to fast-acting carbs (plus, I generally underestimate them), I ended up at 275 mg/dl later in the day. I took a correction bolus, but didn't see much change. About an hour before dinner I got on the elliptical for 15 minutes. My blood sugar began dropping (double-down arrows) and dropped all the way through dinner. By 7pm I was using glucose tabs to keep myself in range. Overall, I didn't take that much insulin, but with exercise, I could have probably taken half as much for correction (and with dinner), and still gotten it fixed in 2 hours.
In the future, I'll have to be more conservative with correction doses if I am going to use exercise alongside. And, because of the immediacy... I definitely will.
When I first started really focusing on my glucose control in 2003, I spent most of my energy on managing my food intake and insulin. I felt that exercise was just a monkey wrench that would make things more unpredictable. Now that I am tinkering with this, I feel like I just discovered the other 30% of my toolbox. Ironically, I'm spending less time exercising than ever, a little bit seems to be more effective than an hour, so it's much easier to find time for it.