Continuous glucose monitoring (CGM) devices are quite the rage in the endurance sport world at the moment. But in order to use them effectively, it’s essential to understand the typical blood glucose response to exercise.
Under normal resting conditions, our bodies regulate blood glucose via homeostatic mechanisms to ensure that blood glucose remains between ~70-100 mg/dl. When blood glucose is too low, glucagon targets the liver and kidneys to release glucose. When blood glucose is high (e.g. after a meal), insulin released from the pancreas facilitates uptake of glucose from the blood into liver, muscle, and fat cells.
During exercise, glucose is taken up from the blood into the exercising muscle independent of insulin. However, at the onset of exercise, there may be a slight increase in blood glucose compared to resting levels. Through a feed-forward response, the liver releases glucose into the circulation in response to the muscular contractions. The subsequent blood glucose response depends on exercise intensity (with higher intensity exercise, blood glucose may drift upward) and whether carbohydrate is consumed during exercise. If exogenous carbohydrate is not taken in as exercise continues, physiological feedback mechanisms regulate blood glucose release from the liver to accommodate the increased glucose needs of contracting muscle.
Carbohydrate is always used, to some degree, as an energy source during exercise; and, the rate of oxidation increases with exercise intensity. In fact, carbohydrate is designed to yield ATP at a quick rate and is, thus, the preferred energy source for supporting endurance performance. The availability of glucose during endurance exercise is dependent on liver and muscle glycogen stores before exercise and exogenous carbohydrate ingestion (e.g. sports drinks, gels etc.) during exercise. When the demand for glucose outpaces the supply of glycogen and blood glucose (e.g. prolonged endurance exercise and/or higher intensity), the body is forced to produce glucose from non carbohydrate sources (e.g. amino acids) through gluconeogenesis.
The body’s glycogen stores are limited, with ~ 100 g stored in the liver and ~ 300-500 g in skeletal muscle. Assuming full glycogen stores at the start of exercise, liver glycogen can only provide ~ 400 kcal of energy via blood glucose circulation. Muscle glycogen can contribute 1200-2000 kcal of energy, but muscle glycogenolysis is a localized process and does not contribute to circulating blood glucose (that is the role of the liver). This reinforces the importance of supplementing with exogenous carbohydrates to maintain blood glucose levels during longer training days, even when intensity is low.
If exogenous CHO is not taken in during prolonged endurance or during high intensity exercise, the body will maintain blood glucose through breakdown of liver glycogen and gluconeogenic mechanisms
Blood glucose response is individual but also related to exercise intensity and pre-exercise feeding
Blood glucose fluctuations do not necessarily result in hyper or hypoglycemia during exercise due to the body’s mechanisms to defend homeostasis
PMID: 10198303; 32747792, 12527976; 23112916