Blood Glucose

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Blood glucose can be tested quite easily and cheaply, and may provide some useful information for endurance runners.

1 Why test blood glucose

For runners who race marathon or ultramarathon distances it is important to optimize fueling. The human body stores carbohydrate as Glycogen in the muscles and liver, and the depletion of that glycogen can cause fatigue and impaired performance. A common pattern of blood glucose while exercising is for the level to rise during the first part of training as the body Warms up, then gradually declines until exhaustion. (The exhaustion can be caused by many different factors besides low blood sugar.) The level at which blood glucose starts to impact performance appears to be somewhat different for different individuals. For me, my performance suffers when my blood glucose drops below 100 mg/dl, and is significantly impaired if it goes below 80 mg/dl. If you know how your blood glucose impacts your performance you can then evaluate how effective your fueling is.

1.1 What's involved in testing?

The biggest downside to blood glucose testing is the need to prick your finger to draw blood. If you're doing the testing to gather data on your running, you're probably not going to be doing the test often enough for the discomfort to be a problem. The drop of blood goes onto a test strip that's inserted into a meter, which measures the chemical reaction between the glucose in the blood and the test strip, giving you a reading in a few seconds. The test strips are not reusable and the cost of the test strips works out far more than the cost of the meter.

1.2 Accuracy

Generally blood glucose meters are accurate to +/- 20%. Normally glucose meters define their accuracy in terms of the percentage of samples that have a given error. For instance the TRUE2go meter has 55% of samples within 5%, 82% within 10%, 96% within 15% and 99.7% within 20%. What's that mean in practice? A single reading is good enough for clinical use, but for more rigorous use it's worth taking several reading and averaging them. For my TRUE2go, averaging multiple samples improves the accuracy like this:

Samples Percent within +/-5% Percent within +/-10% Percent within +/-15% Percent within +/-20%
1 54.9% 86.8% 97.7% 99.7%
2 71.3% 96.7% 99.9% 100%
3 80.7% 99.1% 100% 100%
4 86.9% 99.8% 100% 100%
5 91% 99.9% 100% 100%

For instance, I recently took readings of 82, 88, 95 and 93. That first reading of 82 indicates the actual blood glucose is between 66 and 98, which is rather a wide range. The average of the four readings is 89.5, which is probably a little closer to reality. Taking just the first reading of 82 would have been quite misleading.

1.3 Which glucose meter

I have three glucose meters, each with their own pros and cons.

  • TRUE2go is a tiny meter that actually clips onto the top of the pot that holds the test strips. The test strips only require a tiny drop of blood, much smaller than many other meters. The TRUEtest test strips are also cheaper than some others. The TRUE2go has a memory of readings, but it does not display the time with the result. This can make it tricky if you record a number of samples during a long run, then want to record them later. (
  • TRUEresult is a slightly larger meter than the TRUE2go, and I got it because it has a better display which includes the time of each sample recording. (
  • Precision XTRA is unusual in that it will measure blood ketones as well as glucose. I only use mine for ketone measurement because the XTRA glucose strips are more than twice the price of the TRUEtest strips and require a 50% larger blood sample. The XTRA requires you to insert a calibration strip each time you open a new box of test strips, something that is not required with the TRUE system. However, if you want to only have one meter that does both blood and ketone testing, this is it. (