Carb Loading before a 14 mile run

In preparation for a 14 mile run on a Saturday morning a few weeks ago, I broke from my normal low-carb diet the night before and decided to carb load. The week before, I had lost energy the last few miles of a 13+ mile run. I wanted to see whether a change in my diet could help.

I ended up getting fluctuations in my blood sugar in really unexpected ways. For the entire 2 hour run, I struggled to get my blood sugar up to normal levels despite eating massive amounts of carbs along the way.

Friday night dinner:

I cooked some vegetables along with a can of black beans and 1/2 a cup of rice. I chose brown Basmati rice, thinking it is more nutritious than normal white rice. The rice alone has 70 g of carbs, although I couldn’t finish all that I prepared (I probably ate about 50g worth). The black beans were another 20g of carbs. The broccoli was probably about 10g of carbs. I also added 2 eggs (a minimal amount of carbs).

My blood sugars were spot on at my 8:00pm dinner time. I took 8.0 u of Humalog to cover the 80g oc carbs meal (these days my insulin-to-carb ratio seems to be about 1:10). The first surprise was that my blood sugar went low with a 58 mg/dL reading at 10:55pm. I had a small glass of orange juice and finished off the left-over rice (I probably over did it with ~35g of carbs. I went to bed shortly after, but was woken up by a CGM high blood sugar alarm at 5:00 am. I took 1.5 u of insulin, assuming this would put me in good shape at breakfast time.

Saturday morning:

But when I woke up at 8:00 am, my blood sugar was still high according to my CGM. When I tested with my OneTouch meter, my blood sugar was significantly higher (266!). For any of you that have exercised with blood sugar that high, it is not fun. I always feel sluggish for at least the first hour. Not an ideal way to start off 2 hours of exercise.

I took 6.0 u of insulin to get the blood sugar down and cover my breakfast. I normally take 3.2 u of insulin for breakfast when I am not about the exercise. If I am about to run, I dial that down to about 2.0 u. But with my blood sugar that high, I would probably need a little more insulin for the food anyway. At the same time, I dialed down my basal rate to .05 u an hour (from a normal rate of .65 u/hr) for two hours.

The run:

When I tested @ 9:45 am before starting to run, I was already down to 86 mg/dL! What a steep drop! Also not ideal before exercise. So I bought an orange juice with 45g of carbs and then started running.

At 10:38 (around mile 5), my blood sugar was even lower (55 mg/dL)! I drank another 35g worth of orange juice and kept running. At 11:01 (around mile 7.5) , it was 59 mg/dL. I ate an exercise bar at 11:10 (mile 9), which kicked in another 25 g of carbs. At 11:39 (around mile 12), my blood sugar 54 mg/dL. When we finally stopped at 12:08, it was at 81 mg/dL.

95 g of carbs throughout the run! And my blood sugar finished at about the same level as when I started. I know this is a lot of exercise. But the shear quantity of carbs that I ate, I can’t believe that my blood sugar didn’t swing up more. I’ve quite simply never had a run like that where I couldn’t get a bounce back up in blood sugar.


Looking back on it, I suspect that the 266 mg/dL reading I got when I woke up was probably a little off. My Dexcom had been giving me predictable readings around that time and was showing (INSERT READING). Yes, normal blood glucose monitors are supposed to be more accurate than CGMs. But those readings can also be off. If in reality my blood sugar was in the low 200s, 6.0 u of insulin was way too much.

It’s a bit frustrating to have to deal with this during a long run. Diabetic runners definitely have another challenge beyond just getting your muscles in shape. In the future, I will do a second blood glucose meter check should I get such a big discrepancy before a run. If nothing else, I’d like to do the run again just to be able to focus more on enjoying the exercise and not worrying about whether things were going to bounce back or not.


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