The Curious Case Of Fasting Blood Glucose And Average Running Heart Rate

In the previous post I looked into variables that seem to be associated with my morning fasting blood glucose. However, I haven’t reported the strongest, yet very surprising correlation that I observed between my fasting blood glucose and the mean heart rate I record using my Polar chest strap during my runs.

A little bit of context. I tend to run 1-2 times a week during the warmer weeks of the year. I always run the exact same ~5km track. I run light, roughly at the same effort level, generally aiming for the Zone 2 intensity. Yet, subjectively, I always noticed that some days my heart rate during the run is considerable higher than on others, without any obvious connection to my effort level.

Interestingly, it turns out that there is a strong positive association (R=0.33; N=70; p<0.005) between the fasting glucose on the day of the run and the average heart rate the chest strap recorded during the run:

Alcohol vs. FSS

If I shift the data by one day, such that I correlate the blood glucose from the previous day, a smaller but still statistically highly significant association remains (R=0.27; p=0.02). On the other hand if I shift the data the other way around - i.e. testing the correlation between the running heart rate and blood glucose the next morning the correlation disappears. Altogether, I take this as a mild hint that there is a direct causal link from blood glucose to running heart rate, but of course I can’t be sure without a well controlled experiment.

What I also find intriguing is that I could not find any other link between other variables reported by Fitbit, such as resting heart rate, or mean heart rate at night and the blood glucose. It is only the specific strong association with the Polar recorded running heart rate that shows up.

Having found this strange relationship, I have searched the literature if similar effect has been previously found, but while I can find couple of studies looking into relationship between fasting blood glucose and HRV, I could not find any study specifically looking at the relationship betweeing fasting blood glucose and mean running heart rate. Neither could I find studies evaluating the link between general running and performance and fasting blood glucose before the run. If any reader knows of any relevant research on this topic please let me know below in the comments! It would be most appreciated.