on-going mostly unedited stream of thoughts

my first week of wearing a continuous glucose monitor

I’ve mentioned in a recent post that I’d be inclined to try out a cgm (continuous glucose monitor) as I’ve been pricking my fingers more than 5 times a day in an attempt to understand my metabolism better.

I’d managed to pluck up the courage to finally try one last week. I got my partner to apply it for me because I couldn’t bring myself to press something that would puncture a hole in my arm. It was not entirely painless, my arm seemed to ache for at least half a day, but a week on I think it is worth that minor discomfort for the fidelity of the data I am getting.

I thought wearing a cgm would just be a more convenient version of pricking my fingers. But having an instant read of my glucose levels anytime I wanted taught me more than I expected. It also stores glucose values every 15 minutes in the background as long as you scan it every 8 hours, which you can then access as a downloaded spreadsheet online.

Things I learned

the first 12 hours are trash

I read on various online forums that the first 12 hours of the Freestyle Libre is inaccurate because the area around the CGM needs time to heal after the puncture (ouch). They recommend performing the first scan after 12 hours because the sensor counts its validity 14 days after the first scan. I did not heed this advice because I was too curious, so indeed my first day was full of low glucose readings.

how long does it take for glucose to spike after a meal

While using a standard glucose meter I tend to prick my fingers at the 1 hour mark post-meal, and sometimes 2 if the 1 hour reading was high. I had the assumption that the peak reading would be at 1 hour. But the cgm showed me that most of the time my peak reading would actually be at 30 minutes, and sometimes it actually peaks at 15 minutes, especially if I eat fruit. So if I was interested in how high my blood sugar would peak I should be pricking my fingers at 30 minutes instead. But 1 hour would be a good insight into how quickly my blood sugar can go back to its baseline.

This chart shows how I would have missed the 15-minute peak if I pricked my finger at the 1 hour mark as usual, but the CGM captured it (big blue dot).
This chart shows how I would have missed the 15-minute peak if I pricked my finger at the 1 hour mark as usual, but the CGM captured it (big blue dot).

how damaging are temporary spikes

For a very long time I was afraid of eating fruit because it would always seem to spike my blood glucose pretty badly. But with the cgm I learnt that while it does provoke a sharp spike, it does recover quickly. The medical literature is still unclear how high a temporary spike has to be to be damaging to our blood vessels. But it does show that high post-meal glucose is associated with an increased risk of cardiovascular disease, even when compared to a high baseline glucose levels:

Epidemiological studies have also suggested that postprandial spikes of high glucose levels may be a more robust determinant of CVD risk than average glucose levels. These episodes of high glucose levels increase oxidative stress, which in turn has several detrimental downstream effects, activating immune cells, and keeping the vasculature in a persistent state of elevated risk of cardiovascular events. To further support this hypothesis, postprandial blood glucose (PBG) levels are more predictive for CVD than HbA1c levels. Even in people without diabetes, PBG levels independently predict CVD in the non-diabetic glucose range.


But our glucose does spike pretty high whenever we perform intense activities, so the body must be able to tolerate a certain threshold of spikes. It is just that so far I haven’t come across any research that specifies the threshold. Personally I would not eat a lot of fruit though, because only the liver can metabolise fructose and modern day fruit is now sweet beyond what nature intended.

how close are the readings between the cgm and finger pricks

I have read horror stories of how cgms are a lot more inaccurate than finger pricking, but surprisingly for me the readings are quite close, especially if I account for the 15 minute lag (the cgm lags 15 mins behind because it measures the interstitial fluid instead of the blood). I was expecting to see readings to be off by 1 mmol/L consistently, but it seems like their difference is typically less than 0.5mmol/L. There are however some extreme spikes when the reading was off: the cgm measured 6.8mmol/L while the finger prick measured 7.8mmol/L:

the values of the CGM were quite close to finger pricking except for some extreme spikes
the values of the CGM were quite close to finger pricking except for some extreme spikes

stress causes glucose spikes

I always knew this intellectually, but it was still provoking to see it in action. There were a few events last week that caused some stress, and my glucose went from a 5+ baseline to spike over 6+. I can imagine how chronic stress can cause cardiovascular disease: repeated spikes cause arteries to be damaged and then hardened. Those stressful events were not even that stressful, I cannot imagine the sort of spikes I had when I was chronically stressed by work and people.

Since illness is also a form of stress, my baseline glucose seems to run higher during the menstrual cycle and its related migraines.

the first sourdough experiment

Sourdough is supposed to have a low glycemic load because of its fermentation. I miss bread a lot, so one morning I ate one small slice of sourdough with some tuna, ham and cheese as an experiment. Surprisingly my glucose didn’t spike much, but it also took much longer than usual to recover to baseline. This coincided with a day with some stressful events, so I’ll need to try it again.

general patterns

Apart from whatever I wrote above, it was still very interesting to observe my glucose patterns in general. I thought apart from meals my glucose would be stable around its baseline, but it actually fluctuates up and down around +/-0.5mmol/L. It seems that the liver releases some sugar from glycogen every time the circulating glucose runs low.

It is also nice to see that apart from stressful days when I may be ill, my glucose levels at night are pretty low and stable – not doing anything crazy. There are a couple of spikes here and there as the body is probably repairing itself.


This is just my first week so I am just documenting my initial observations. I’ll probably continue to wear a few more to decide whether it is something I would wear permanently. They are not cheap, but I was also spending quite a bit on glucose strips.

I am not sure what is the end game – if enough experimentation would form a subconscious database in my head and I wouldn’t need to do so much close monitoring to maintain my metabolic health. But I find the extra information useful for non-metabolic reasons as well, like if my glucose is higher than usual I should have an easier day because it indicates stress.

Also, it would be useful to monitor longer-term trends. For example, if I build muscle, would my glucose response improve with the same foods? Will there be a co-relation between my glucose control and my migraines? I’ve been allowing myself more carbs as an experiment, therefore producing higher post-meal spikes than usual. This cycle (right now in fact) I am experiencing a mild migraine. Is it due to my increased carb intake?

I’ll try to see if I can get a hba1c test (avoiding clinics because covid) to see if the reported a1c value by the cgm is accurate. I would also be interested to test more foods that I usually avoid.

related reading:

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