I’ve noticed that if my blood sugar is running too high (I’m a type 2 diabetic), a spate of exercise will almost immediately drop it by anywhere from 20 to (on one notable occasion) 100 mg/dL. It seems like the more vigorous the exercise is, the more dramatic the effect, regardless of duration. In other words, 5 minutes of pedalling on the exercise bike just as fast as I possibly can and turning myself into a limp dishrag has more effect than pedalling at a quick-but-not-crazy pace for 20 minutes.
I realize that a lot of this probably varies by individual, but can someone explain the physiological mechanism at work here? I assume that the exercise is lowering insulin resistance somehow, but how?
This is a well-known but marginally understood phenomenon. Basically, muscle contraction causes muscle cells to rapidly burn up their internal energy sources, and they rapidly become dependent on the delivery of fuel via the bloodstream. At different levels of exercise and at different durations of exercise, the muscle cell uses different mixes of glucose and free fatty acids for fuel. In general, muscle cells take up a lot more glucose during exercise than they do at rest: for one, they are getting a lot more blood flow, so the delivery is increased; for another, they put more glucose-transporting molecules on their cell membranes, so they take up more; and for a third, they burn the glucose more quickly, so they are always very glucose-hungry – the delivery and transport lags behind demand.
Despite extensive research, the exact mechanisms by which this occurs are not yet fully hammered out. Some of it appears to be insulin-independent, but it is potentiated by insulin. The complex environment of active muscle (low pH, low O2, elevated epinepherine, changes in calcium concentration, etc) make things hard to figure out. It may be more accurate to say that there are a number of possible mechanisms, but it’s not clear which are significant/dominant in the real world. Experimentally, it does seem that the increase in glucose uptake is more dependent on exercise intensity than duration, as you noted. Also, there are differences in the response of ‘trained’ vs. ‘naive’ muscle tissue, and some of the effects of exercise on glucose metabolism last for many hours after the exercise is over.
There is a nice, but somewhat technical, review of this here
One would assume that this effect is at least partially due to the difference in aerobic vs. anaerobic energy systems. The glycogen-lactic acid system obviously burns glycogen and is used primarily at higher intensities.
Here’s the dope on the different muscle energy systems:
There is also a lot of information available about the effect of exercise on insulin levels and insulin efficiency. However, diabetics react differently and so you need to be careful about the conclusions you draw. Here’s a start for learning about the differences: