Wednesday, February 09, 2011

Intensity and Blood Sugar

When I was diagnosed with type 1 diabetes, one of the guidelines I was given was that exercise would make my blood sugar drop.  Like most guidelines I received, this was an oversimplification.  During my years of daily running, I would go out the door without eating or taking any insulin, and often return with a slightly higher BG level.  I remember thinking, "'Guess I'm just weird."  And during track workouts, the response was even more perplexing.  After a few intervals at high intensity, my blood sugar would really start to climb.  I grew accustomed to this response and began taking some correction boluses or temporary basal rates to help temper the steep rise.  What I have learned since then, and which has hopefully become more common knowledge, is that high intensity physical efforts can increase the adrenalin and stimulate the liver to release more glucose into the bloodstream.  (Here is an article that explains the phenomenon and suggestions for how to manage it.  Also, fellow type 1 and exercise guru, Dr. Sheri Colberg-Ochs, has an excellent book devoted to the nuances of diabetes control and exercise.)

I think there is another reason that exercise does not always cause my blood sugar to drop, even when I am doing a more moderate-intensity workout: I have basically adjusted my basal rates to account for morning exercise, and I am consistent in exercising almost daily.  So, instead of constantly reducing my basal rate, I try to remember to increase it on my off days.  Of course, with anything diabetes-related, things don't always work out as planned--there are still days when I have to eat 3 gels to get through an hour of exercise!

When I first start a new exercise type, or increase volume and/or intensity of my current regimen, I will typically notice an increase in insulin sensitivity, as manifested by more frequent low blood sugar (unless I catch on early and adjust my basal rates).  This can be significant, especially initially; over a few weeks, the effect seems to be reduced, although I will still be more insulin sensitive than prior to the change in exercise (up to a point).  I remember, when I first started doing triathlon, I had a couple hours in the afternoon where I had to shut my pump off.  My body adapted to the changes and the effect did not last for more than a week or so; but still, I had an overall, significant decrease in my daily insulin dose that persisted.  I had been running 6 days/week for years so initially, at least, it was primarily the change in exercise type that was responsible for the effect.

When this effect diminishes and my insulin needs increase a little, I see it as a sign that my body is becoming more efficient at the exercise and that my fitness is improving.  Or maybe it's just time to take it up a notch again.

bike class at the beginning
I regularly attend a bike class that has efforts mainly in my upper power zones, equivalent in intensity to a track workout for running.  It takes a lot of focus to keep my intensity high because, basically, it hurts.  I have been watching my CGM during class to see what types of intervals cause my glucose levels to rise or fall.  Today's workout called for a five-minute warmup followed by isolated leg drills. We then moved into two five-minute high cadence intervals, following by two sets of six minutes at low cadence followed by a minute rest and then six minutes at high cadence.  These were all hard efforts and I tried to push it as hard as I could.  I came into the class right around 100 mg/dL and held off on eating anything. For the first half-hour, things remained steady.  I expected to see a rise after the two high cadence intervals, but it remained steady.  However, about 10 minutes later, there was a slight increase, which is the first bump you can see on the CGM display.  This came back down somewhat and I remained in a good zone throughout the class.  The high cadence intervals feel a lot harder to me; it takes much more determination to stay at 115 RPM for six minutes for me to do the equivalent Watts at 65 RPM.  For me to have the same heart rate at 65 RPM, I can increase my Watts by about 30.  This may explain my general observation that the high cadence intervals stimulate a greater blood sugar increase than the low cadence intervals.  Perhaps, today, the alternating high then low cadence intervals kept things more steady.  I need more data and translating this to the road may not be totally straightforward.  But I will add that the only bike race where I have had low blood sugar was the hill climb at Kern County Women's Stage Race.  (Of course, the hill climb being the second race of the day muddies the waters a little.)

Oh and now you can see, perhaps, why I want to have the power meter (and/or other fitness devices) and CGM all integrated.  I won't go off on another rant right now since I already took care of that here!

And one more thing: I think this stuff also applies to athletes without diabetes.  I am surprised that people haven't started using these tools to refine athletic performance.  Seriously, people pay big bucks to save 10 grams on their wheel, which is absolutely meaningless if you don't fuel properly and get sub-optimal blood sugar during a race.  It is really hard to nail down blood sugar levels unless you actually measure them.  (For example, I have bonked with normal blood sugar.)  And as luck would have it, there are plenty of tools to do so.  I would love to see some top athletes show their data during races, for example. Bueller? Bueller? Anyone?

Addendum: I'd like to add a reminder that exercise can of course cause a drop in blood glucose, which can become life-threatening if treatment (i.e., a carbohydrate source or glucagon) is not available.  I almost always have at least 50-75 g of fast-acting carbs with me, and more if I am exercising >1 hour or somewhere without quick access to a store etc. And I always bring along the CGM and/or BG meter.  (I rely on the CGM alone for workouts <2-3 hours, but will bring the meter as well if I'm going longer.)  Don't make fun of the bulging pockets on my jersey!


Gary said...

FANTASTIC post Anne.

Becca said...

Anne, this is cool. I would be interested in seeing what my bgs do throughout the day, especially when I'm exercising. I'm not a pro cyclist, but still, it would be interesting.

Joe B. said...

This mirrors my experiences to the letter. Nicely written, Anne. Bottom line: exercise does not necessarily lower BG despite repeated claims by some health professionals and publications!

Keith said...

I appreciate greatly this post. I wish you continued success, and I like the eduication you provide in this world of blood monitoring... Keep blogging! Thank you.

Mark said...

Great post, very inspirational.
Keep up the good work.