The term blood sugar is often used when discussing continuous glucose monitors (CGMs). Similarly you will sometimes hear it called blood glucose. We are here to break this down and explain these well-intentioned misnomers.
Sugar vs Glucose
All glucose is a sugar, not all sugars are glucose.
‘Sugars’ come in a range of shapes, sizes etc. The table sugar, most think of when using the word ‘sugar’, is more accurately called ‘sucrose’. Sucrose is one of a group of sugars called ‘disaccharides’ - the “di” prefix denoting two sugar molecules bonded together, in this case one molecule of glucose and one of fructose. Other familiar disaccharides include lactose (the sugar in milk) and maltose.
Single sugar molecules, known as ‘monosaccharides’ include the aforementioned glucose and fructose.
Sugars in Blood
It should be noted that there are more sugars in the blood than just glucose. That said, for all intents and purposes, the term ‘blood sugar’ refers to ‘blood glucose’.
A far bigger oversight than conflating ‘sugar’ and ‘glucose’ is the thought that glucose levels are consistent throughout all blood in the circulatory system. There is a narrow range of glucose in the blood (though this is being challenged with the increasing use of CGM in people without diabetes), but the exact amounts in different parts of the circulatory system varies. Classically, the total amount of glucose in the blood is quoted as 4g - speaking to the phenomenal system that is the body. This is because the circulatory system functions as a conveyor belt of sorts, dropping off nutrients along the way (and reciprocally picking up waste products), and so as the liver dumps glucose into the circulatory system or as it is absorbed from the digestive tract, it is progressively used. As such, sampling arteries (vessels coming from the heart), veins (vessels traveling back to the heart) or capillaries (small vessels supplying areas such as the skin) will yield slightly different glucose levels.
Compounding this at times for people using glucometers (aka fingerstick glucose monitors) where capillary blood is sampled following a small prick to the finger is sometimes samples are hard to produce. In these cases, the finger is often squeezed, yielding trauma to vessels, red blood cells etc which could cloud the measurement. This may not have a meaningful impact in measurements given it is probably smaller than the acceptable error in glucometers (as covered here).
Interstitial Glucose NOT Blood Glucose
As covered in this article CGMs actually measure glucose in the interstitial fluid, not in the blood. Of interest, is the fact that they do this via an enzyme (glucose oxidase), which reacts with glucose, causing an electrical current. Which is then algorithmically converted into a corresponding glucose value. It is worth noting that as many people with diabetes know, this enzyme can react with other substances, including things like vitamin C.
So whilst historically we have used capillary glucose as a gold standard to understand glucose metabolism as it pertained to those managing diabetes, we have seen both the technology and understanding change significantly. We are now at a point where we have continuous interstitial glucose measures and in people who do not have diabetes - which is changing the game in terms of our understanding. So much so, that there are also suggestions that interstitial glucose may actually be more relevant.
So now that you know - help us spread the word: we are on a mission to untangle this mess!
- Wasserman DH. Four grams of glucose. Am J Physiol Endocrinol Metab. 2009 Jan;296(1):E11-21. doi: 10.1152/ajpendo.90563.2008. Epub 2008 Oct 7. PMID: 18840763; PMCID: PMC2636990.
- Kropff J, van Steen SC, deGraaff P, Chan MW, van Amstel RBE, DeVries JH. Venous, Arterialized-Venous, or Capillary Glucose Reference Measurements for the Accuracy Assessment of a Continuous Glucose Monitoring System. Diabetes Technol Ther. 2017 Nov;19(11):609-617. doi: 10.1089/dia.2017.0189. Epub 2017 Aug 22. PMID: 28829160.
- Bowler, A. M., Whitfield, J., Marshall, L., Coffey, V. G., Burke, L. M., & Cox, G. R. (2023). The Use of Continuous Glucose Monitors in Sport: Possible Applications and Considerations. International Journal of Sport Nutrition and Exercise Metabolism, 33(2), 121-132. https://doi.org/10.1123/ijsnem.2022-0139
- Heinemann L. Interferences With CGM Systems: Practical Relevance? Journal of Diabetes Science and Technology. 2022;16(2):271-274. doi:10.1177/19322968211065065
- Siegmund T, Heinemann L, Kolassa R, Thomas A. Discrepancies Between Blood Glucose and Interstitial Glucose-Technological Artifacts or Physiology: Implications for Selection of the Appropriate Therapeutic Target. J Diabetes Sci Technol. 2017 Jul;11(4):766-772. doi: 10.1177/1932296817699637. Epub 2017 Mar 21. PMID: 28322063; PMCID: PMC5588840.