How does exercise impact the progression of a fast?Asked By: Elliot K.
The more you exercise, the more fuel you burn, and different types of exercise prioritize different fuels. So the volume and intensity of your training will definitely impact the speed at which you move through your Fasting Zones.
The progression of fast is largely dictated by liver glycogen (the storage form of glucose). To make the metabolic transition into the Fat Burning Zone and subsequently, Ketosis, one must first deplete liver glycogen and lower glucose availability. Depleting glycogen is the critical turning point during a fast where the body says, “Alright, glucose stores are low, and we gotta do something about this.” As a result, it turns to fat and ketones for energy which defines the fasted state.
Since exercise facilitates glycogen depletion, it can expedite the progression of a fast – meaning it can put you in the ketone-producing fasted state sooner than if you were to lay in bed all day. But not all forms of exercise are alike. The rate at which glycogen levels fall depends on the intensity of exercise since exercise intensity dictates which energy system the body relies upon most heavily (i.e., fat or carbohydrates).
Essentially, the higher the intensity of your workout, the more carbohydrate-dependent it is, and thus the more glycogen you burn. High-intensity exercise demands energy fast, and frankly, carbohydrates are the body’s quick fuel, compared to fat. This type of exercise facilitates both muscle and liver glycogen depletion, the latter of which is what sets the wheels in motion towards ketosis. Moderate intensity exercise (~50-70% of max effort) also depletes glycogen, just at a slower rate. It takes around 90 minutes of moderate to high or up to 120 minutes of continuous moderate exercise to significantly deplete glycogen stores. Subjectively, if your muscles feel fatigued, and you’re running out of steam, this is a sign that your glycogen levels are low.
Altogether, you can use exercise to burn through glycogen quicker and kick off the signaling cascade to get into the fasted state as quickly as possible, and in theory, achieve the benefits of a fast sooner.
Gonzalez JT. et al. 2016. Liver glycogen metabolism during and after prolonged endurance-type exercise. American Journal of Physiology-Endocrinology and Metabolism, 311(3):E543-E553.
Murray B, Rosenbloom C. 2018. Fundamentals of glycogen metabolism for coaches and athletes.Nutr Rev, 76(4):243-259.
About the Author: Kristi Storoschuk
Kristi Storoschuk is a science communicator who focuses her research on ketogenic diets, metabolic therapies, and fasting for health optimization. She currently works alongside the world’s leading ketogenic researchers providing scientific education for the mainstream audience. She completed her Bachelor of Science (Honours) at the University of Guelph in Ontario, Canada. Outside of her research, she spends her time doing CrossFit and optimizing her health with an ancestral approach.