Fasted Versus Fed Exercise — Effects on Blood Sugar Levels

Dominic D’Agostino, PhD, is an associate professor at the University of South Florida where he develops tests on metabolic-based therapies, including the ketogenic diet. Based in Tampa, Dominic D’Agostino, PhD, has done research on ketone supplementation and ketone technologies and published articles on ketonutrition.org.

KetoNutrition recently published an article highlighting the effects of exercising in a fasted state (no carbohydrates) versus exercising after eating carbohydrates. An interesting revelation from the article was that fasted exercise didn’t significantly change blood sugar levels.

When a person exercises after eating carbohydrates, their baseline glucose levels rise and then fall during moderately intense exercise. Similarly, a person who exercises without eating carbs beforehand will also experience a slight increase in blood sugar. This is likely due to glycogen (glucose) breakdown in the liver.

Glucose enters the body’s muscles by two ways: ingestion and muscle contraction (exercise). When a person eats carbohydrates, they increase their insulin levels. Similarly, when they exercise, they trigger insulin-independent glucose intake pathways. In one study, researchers uncovered that eating before exercise did improve postprandial glucose disposal. However, this was offset by new glucose entering the blood, hence the limited effect on blood sugar levels.

Following exercise, however, in certain circumstances, people who eat a meal after fasted exercise have higher glucose levels than those who eat before and after exercise.