Fasted cardio involves performing cardiovascular exercise after an extended period of not eating, typically lasting between 8 and 12 hours. This training method is based on the physiological principle that when the body is in a fasted state, its stored carbohydrates (glycogen) and insulin levels are low. Because the body’s preferred fuel source is less available, proponents suggest that the body is encouraged to use stored fat as the primary energy source during the workout. The goal is to maximize the rate at which the body burns fat during the exercise session itself.
Pre-Workout Preparation and Timing
Establishing a clear fasting window is the first step toward safe and effective fasted cardio. A period of 10 to 14 hours without consuming calories is generally sufficient to achieve a truly fasted state. This duration allows the body to fully digest the previous meal and lower blood glucose and insulin levels. Prioritizing hydration before and during any fasted session is paramount, as the body can become easily dehydrated.
Consuming water is the simplest component of preparation, and it does not break the fast. Experts suggest drinking roughly 16 to 20 ounces of water in the hours leading up to the workout to counteract the overnight fluid loss. For sessions lasting longer than 30 minutes, or if you are a heavy sweater, adding zero-calorie electrolytes is helpful to replenish minerals like sodium and potassium lost through sweat.
Some people choose to include fast-friendly supplements to support performance and muscle integrity. Black coffee is a popular option because it is virtually calorie-free and the caffeine content can increase focus and promote the mobilization of fat stores for energy. Branched-Chain Amino Acids (BCAAs) are sometimes used to help minimize muscle protein breakdown during prolonged exercise. However, the leucine found in BCAAs may trigger a small insulin response, which could negate the fully fasted state, making zero-calorie options a less controversial choice.
Fasted cardio is not suitable for everyone. Individuals with pre-existing conditions like diabetes or blood pressure issues should avoid this practice, as it can lead to dangerously low blood sugar levels. Pregnant women and those with a history of eating disorders should also seek alternative methods to ensure their health and safety. Anyone new to exercise or training for high-intensity performance events may also find that pre-workout fuel is necessary for optimal results and energy levels.
Structuring the Fasted Workout
The success of fasted cardio is highly dependent on controlling the intensity of the exercise. Low-Intensity Steady State (LISS) cardio is the preferred method because it primarily utilizes fat for fuel, whereas higher intensities rely heavily on glycogen stores. To keep the body in this fat-burning zone, the heart rate should remain at approximately 50 to 70% of your maximum heart rate.
An easy way to monitor this is by using the Rate of Perceived Exertion (RPE), aiming for a level of 5 or 6 on a scale of 10. At this effort level, you should be able to hold a conversation, though it may be slightly broken, which is often referred to as the “talk test”. Examples of appropriate LISS activities include brisk walking, light jogging, elliptical training, or steady-paced cycling.
Duration must also be carefully managed to prevent muscle breakdown, known as catabolism. Fasted sessions should be kept relatively short, typically lasting between 30 and 60 minutes. Extending the duration too far increases the risk that the body will start converting muscle protein into glucose for fuel, which is counterproductive to maintaining lean mass.
Therefore, High-Intensity Interval Training (HIIT) and heavy resistance training should be avoided entirely in a fasted state. These activities demand quick, explosive energy that can only be supplied efficiently by carbohydrates, and attempting them while glycogen-depleted can significantly impair performance. The higher stress they place on the body also potentially elevates cortisol levels, further increasing the risk of muscle tissue breakdown.
Post-Cardio Refueling Strategy
After the fasted cardio session, the focus must immediately shift to recovery. Consuming the “break-fast” meal within 30 to 60 minutes after exercise is recommended to optimize the body’s recovery processes. This window is when muscles are primed to absorb nutrients, which is crucial for repair and replenishing energy stores.
The post-workout meal should emphasize a combination of high-quality protein and carbohydrates. Protein is necessary to stimulate muscle protein synthesis, effectively stopping the muscle-wasting process that can occur after a fasted workout. Carbohydrates are equally important to replenish the glycogen stores in the muscles and liver that were utilized during the session.
For individuals whose goal is fat loss, a carbohydrate-to-protein ratio of 1:1 or 2:1 is often advised to support recovery without over-consuming calories. High-quality meal examples include eggs and whole-grain toast, Greek yogurt with berries, or a protein smoothie blended with fruit. These options provide the necessary mix of protein for repair and complex carbohydrates for sustained energy restoration, setting the body up for the rest of the day.