Original episode & show notes | Raw transcript
This section delves into the fundamental science behind Continuous Glucose Monitors (CGMs) and the physiological context of glucose regulation during endurance exercise. Understanding these basics is crucial to appreciating the nuances of their application in sport.
A common misconception is that CGMs provide a real-time measurement of blood glucose, the standard metric used in medical settings. However, this is not the case.
Interstitial Fluid vs. Blood: CGMs measure glucose concentration in the interstitial fluid, the fluid that surrounds your body’s cells. They use a tiny filament inserted just under the skin to do this.
The Physiological Time Lag: Glucose moves from the bloodstream into the interstitial fluid to be taken up by cells. This process is not instantaneous. As a result, there is an inherent time lag of 5-15 minutes between a change in blood glucose and the corresponding change in interstitial fluid glucose. During intense exercise, when glucose levels can fluctuate rapidly, this delay is significant.
Accuracy: While related, interstitial glucose is not identical to blood glucose. The podcast notes that measurements can differ by as much as 10%, even without considering the time delay.
A CGM reading represents the net balance of a highly complex and dynamic system. Thinking of the body’s available glucose as a “bucket,” a CGM only measures the level of the bucket at any given moment. It does not provide information about the rate at which glucose is entering or leaving.
Key factors influencing the “glucose bucket” include:
Glucose Inflow (Sources):
Dietary Intake: The carbohydrates you consume. The rate of appearance in the blood is affected by gastric emptying (how quickly food leaves the stomach) and absorption in the intestine. These rates are influenced by fuel type (liquids vs. solids), hydration status, heat stress, and gut trainability.
Liver Glycogenolysis: The liver stores glucose as glycogen and can release it into the bloodstream to maintain levels, a process critical during exercise.
Glucose Outflow (Sinks):
Working Muscles: During exercise, active muscles are the primary consumer of glucose. The rate of uptake is mediated by transporters like GLUT4. Highly trained athletes have a greater capacity for glucose uptake.
Other Tissues: The brain and other organs require a constant supply of glucose.
The podcast’s experts argue that focusing solely on the CGM reading oversimplifies this intricate process. A low reading could be due to numerous factors, and by the time it’s visible on the device (considering the time lag), an athlete’s performance may already be compromised.