Saturday Morning Research Review – February 3, 2018

Gran Fondos and delayed hypoglycemia
by Adam Burrack, PhD
I had the opportunity last summer to race in two duathlons and two longer-distance cycling events. In this post I͛ll share
how my diabetes management worked – or didn͛t work as the case may be – for these events. I raced the regional ͚
championship͛ race for the Duathlon on Memorial Day weekend, as well as another duathlon race in late July. This
summer I added a couple of ~65 mile cycling events in the rolling hills of southwest Wisconsin via the Wisconsin Gran
Fondo series. All four of these events provided unique challenges,
described below.
As I͛ve written about previously in our series, I enjoy the duathlon (run-bike-run) race format and manage my diabetes for these 80-100 minute events with
a pretty straight-forward method: I eat a normal breakfast the day of the event (50-80 g CHO) 2-4 hours before race
start, perform the normal bolus for current BG level and CHO in the meal, and suspend my insulin pump for the duration
of the event. Given there at least 2 hours between breakfast and the exercise, and that I suspend the pump for the
event, I͛ve never had problems with low BG levels during these events. Upon completion of the event – when I am
uniformly in the physiologic normal BG range – I add-back 50% of the basal insulin I lost as soon as possible (as a quick
bolus). This approach works well for me for races of 1-2 hours duration. This includes my sweet spot of 10 mile to half-
marathon running races, most duathlons, cycling time trials, and shorter road cycling races. This approach was
successful at the USAT regional duathlon race Memorial Day weekend where I took 2nd in my age group
and the Minnesota Duathlon the final Sunday in July, where
I took 7th overall.
The challenge with these events, for my diabetes management, occurs 2-6 hours after finishing the event: there have
been several occasions when I͛ve gotten low blood sugars 4-ish hours after finishing the event, presumably due to
enhanced insulin reception expression – and therefore enhanced insulin sensitivity – following these events. Enhanced
insulin sensitivity following these events is expected, and is something I should be able to trouble-shoot. From my
experience, this becomes a particular challenge for me as events increase in duration beyond 90 minutes. Also from my
experience, as liver glycogen is replenished the following overnight, I seem to be at higher risk of low blood sugar levels
for several nights afterward.
A particular challenge I͛ve run into (technically biked into, pardon the pun) this summer is bumping the duration of a
hard aerobic effort up to 4 hours for the ~65 mile cycling event with rolling hills. Happily, I was well-trained for these
events – the Tour de Coulee and the Tour de
Circus in July – and experienced no problems during
the events or in the 4-6 hour window following completion of these events. Briefly, my in-race strategy for diabetes
management for these races was the same as I͛ve done for longer-duration hikes in the mountains: 50% temporary
basal rate for the duration of activity, along with 30-50 g CHO (via solid food) per hour of exercise (with 25-50% of the
normal bolus for the CHO in the food). I finished both of these events in 4 hours +/- 5 minutes and finished strong.
However, I had severe overnight low blood sugars following both events. These low events precipitated insulin-resistant
rebound high BG events the following days. This can be quite frustrating in the days following a successful long-distance
endurance event; it can feel as if the purpose of the exercise – improving diabetes management – has been defeated. In
addition, in the 4-7 days following these events, I felt as if I was getting low BG levels on my (8-mile) bike commute to
work, which again would be defeating the purpose of the exercise to a degree.
In light of these challenges, I under-took a literature search. I was looking for relationships between improved aerobic
fitness with T1D and the potential for increased frequency of severe hypoglycemia (ie, really bad low blood sugar level
events). Interestingly, I found an article from the journal Diabetes Medicine, published in 2016, which concluded a
similar point: that better VO2max (proxy for aerobic fitness) correlates with more frequent low blood sugar level events
during exercise for people with T1D. One limitation of applying this
study to my own situation is the study was not following competitive athletes with relatively high aerobic capacities, this study was designed to find relationships in fit, healthy, non-competitive individuals. In addition, this was a static study; these authors did not test whether this tendency becomes more pronounced throughout a training season, which was
specifically what I thought I might be experiencing. I know from previous Dexcom use that I do not dip into low BG levels during exercise in general, including mile-specific running training and last year͛s duathlon season.
To test this, I have resumed my Dexcom device. Initial results confirm my suspicion that I am not getting low more often and missing the signs and symptoms. In contrast, it appears I am ͞floating͟ around 150-200 mg/dl throughout the day.
Therefore, if anything I need to increase my basal rate, decrease my CHO ratio, and inject more insulin overall. This is
more in line with my expectations than the paper cited above. In previous cross country/half marathon training
experience, as I͛ve worked my way into better aerobic shape I͛ve had to do inject more insulin. My experience following the 4-hour cycling events with challenging hills is more in line with an hypothesis that I was entirely glycogen-depleted
following these efforts, and that it took several days to replenish, and that during those 4-7 day windows following the events I probably was getting low BG levels more often. I conclude that I should have been more mindful to fully recover from these events, rather than plowing forward with my training plans. This is another example of why hypotheses are great to test; you can find out what͛s really happening through a series of logical tests, and act to improve the outcome in the future 

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