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Continuous Glucose Monitor (CGM)

A continuous glucose monitor is a small device that attached to your arm and then gives you an approximation of your blood glucose level. Rather than taking glucose readings from your blood, CGM sensor readings are taken from interstitial fluid, a thin layer of fluid that surrounds the tissue cells below your skin. These readings are not quite as accurate as a pin prick blood sample and there is a short lag (about 10 minutes), however the device records a reading every 5 minutes which can then be read using a meter or simple scanned into your phone. This almost real time information then allows you to see how different foods impact on your blood glucose levels.


CGMs are not just for diabetics. Originally they were created to allow type 1 diabetics to help them manage their own insulin injections by giving them trends in glucose changes and the latest devices can give a warning if blood sugar is out of the target range. Everyone who is interested in their blood glucose levels can now have access to this data by wearing a monitor. I would not recommend using one all of the time as they are expensive and, although not obtrusive, they are still something that is attached to your body. I would, however, use them for short periods of time to establish what happens when: you eat different types of food, how long it takes to get back to base levels of blood glucose and what happens during periods of fasting.


My main reason for using a CGM was to track how my blood glucose changed as I ate different foods. I had an A1C which indicated pre diabetes and so I was keen to avoid sugars that kept my glucose levels high. Within a week I was able to see that any form of wheat caused a large spike and potatoes and rice were also a problem. Over the next few weeks, I was able to see how to keep my level quite stable and then introduce a high carb food or an alcoholic drink and to see the change in level and well as the time to return to normal. I was also able to see a prominent ‘dawn phenomenon’ where my blood glucose levels would be elevated in the morning even without eating. With a stable low carb diet I was able to reduce this effect but the biggest impact was when I started to increase the length of fasts beyond 24 hours.


The second use of the CGM was in monitoring during and after a fast. I was able to see my glucose levels drop, and from day 2 the early morning rises were insignificant. This led me to speculate that by depleting my stored glucose, my body was not able to produce the morning glucose boost and so my weak insulin was able to handle any glucose that was in the system.


I used the FreeStyle Libre 2 CGM. There are positive and negative reviews on this system but it was the one I was able to get in China so I don’t have a comparison. The first time I used it there seemed to be a significant error. It was erratic during the first 48 hours but even after it settled down it did not correlate with pin prick blood samples. The error was, however, relatively constant and so although not giving me absolute values I could trust the trends were enough to allow me to make good use of the data. Each sensor lasts for 14 days. I learnt a little more, watched some more YouTube clips and I did not have this problem with any subsequent sensor.


These are my key points when using the Libre 2:

  • Make sure that the area of the arm is cleaned and disinfected with an alcohol wipe. I also shave the area to make sure that I get a good contact and that the adhesive sticks well.

  • Apply the sensor to your arm 12 – 24 hours before you first scan it to activate it. That way the readings are stable as soon as you start taking them.

  • Don’t worry about trying to get exactly the same readings on the CGM and from the glucose pin prick test. I now have 3 different blood monitors and a CGM to compare. The blood glucose monitors differ by a significant amount and the difference between the CGM and the direct blood readings is often less than the difference between the different blood glucose monitors.

  • When using a pin prick blood test to check your readings on the CGM are reliable, try to use an individually wrapped testing strip. I found that some packets of testing strips that were within their use by date were very inaccurate.

These are the YouTube clips that I found helpful when first using a CGM:

  • Low blood sugar level key to losing weight, VOA News https://www.youtube.com/watch?v=01l-9tMpQa8 Explains that different people have different glucose rises by eating the same food. It was very important for me to understand this as it changed how I thought about the food I was eating and how often I ate.

  • To lose weight you must control insulin, Jason Fung https://www.youtube.com/watch?v=3d7KkyXnyB4 Links Glucose, Insulin and fasting. Explains the need for low insulin levels to lose weight.

  • The perfect treatment for diabetes and wight loss, Diet Doctor (Jason Fung) https://www.youtube.com/watch?v=1a2Fsfa8e4I The link between what we eat, fasting and weight loss. The way this can be used to reverse diabetes.

  • Using CGM to better understand metabolic health – Diet Doctor Podcast with Dr. Casey Means https://www.youtube.com/watch?v=336fjAmY8JQ&t=790s Why use a CGM and how to understand the link between blood glucose levels and food choices.


Demonstrations of how to use the Libre CGM

Further tips and explanations when using the Libre CGM

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