What is Fasted Cardio?
Fasted cardio is when you engage in cardiovascular exercise during a fasted state, meaning while your body is not digesting food. When food is being digested, glucose and insulin levels are elevated. You cannot burn fat when insulin levels are elevated since insulin signals the body to store fat. Only when insulin levels are low can you burn fat efficiently. Insulin levels are at their lowest during a fasted state.
Ideally, a fasted state would be about 8-12 hours after your last meal but shorter durations, as little as three hours, can also provide benefits. After a meal, your body is working to break down food, use it for energy, and store the rest as fat. If your body is busy allocating energy resources coming in, it can’t break down and burn fat at the same time. A longer fast allows for insulin to drop lower, which promotes more fat-burning potential. Performing exercise with no food forces your body to look for energy elsewhere and a great resource is in the fat cells. Theoretically, you can burn more fat in a fasted state compared to non-fasting, but there are limits and cautions to this type of exercise. So before you jump on that treadmill, let’s get a deeper understanding of all this.
Fat Storage & Fat Burning
Fat is stored energy. When we consume extra calories that are not needed the body will store excess glucose in fat cells. Insulin is a growth hormone that is released by the pancreas in response to elevated blood sugar. Food enters the stomach and is sent to the liver for processing. Glucose enters the bloodstream as food is broken down where insulin transports it to every cell in your body. When cells are full or there is insulin resistance built up, blood sugar will remain elevated. In efforts to reduce blood sugar, more and more insulin starts being made. Weight gain occurs when there are persistent levels of elevated insulin causing food to be stored in fat cells. Therefore, to burn fat, insulin levels must be low.
Insulin levels are lower in a fasted state compared to a non-fasted state, so you are more likely to burn fat when you are fasting.
The liver and skeletal muscles store their own supply of glucose. While you’re catching zzz’s at night, your blood sugar naturally drops since you aren’t eating. Insulin drops too. However, if blood sugar drops too low you become very sick and can be potentially fatal. To prevent this, during the night the liver releases stored glucose to keep blood sugar normal. The same is true for our skeletal muscles during a fasted workout. With their own supply of glucose on board and readily available, skeletal muscles are well equipped to handle energy needs even without food. So you don’t need to eat before a workout, but remember your storage supply is limited. Once your muscles run out of glucose for fuel they take it from fat cells and muscle cells. Fasted cardio should be a lower intensity shorter duration type of workout.
To burn fat, we must reduce the amount of glucose you put in the body and use up available stored energy.
Health & Safety Concerns
While this type of exercise seems great for fat-burning potential, it’s not without risks. Working out in a fasted state increases the potential for safety concerns like passing out. Blood pressure, heart rate, and blood sugar levels are important to consider.
Hypoglycemia- With a limited supply of glucose during a fast, you can experience symptoms of hypoglycemia (low blood sugar) very quickly. Symptoms include fatigue, dizziness, nausea, sweating, shaking, and can progress to confusion, syncope (pass out), and death in extreme cases.
If you are experiencing these symptoms, stop, sit, hydrate, and eat. The key to fasted cardio is to start slow and build your way up until you become more conditioned over time.
Muscle Loss- Muscle mass can also be used for fuel. The body can derive energy from breaking down skeletal muscles. To combat this, it’s important to include routine weight training and ensure you’re getting adequate nutrition with protein and carbohydrates. Limit the duration of exercise to less than 40 minutes.
Stimulates Hunger- Fasted cardio can stimulate hunger. As the metabolism revs up and you’re burning more fat, the body will naturally send out hunger signals to replenish glucose storage. This is where the post-workout meal is crucial to recovery. Meal planning is a good idea when doing any fast.
Just clarify, fasted cardio is not the only way to burn fat. HIIT (High-Intensity Interval Training) is another great way to burn fat and maintain/build muscle mass. Cardio spurt intervals are a great way to burn fat during a workout. Cardio alone is a good way to lean down, but it’s not going to build a lot of muscle unless you’re doing sprints. To optimize your fat-burning potential, focus on building muscle because more muscle burns more fat.
My Advice
Work your way up– Start out with only 10-20 minutes of low-moderate intensity workouts. Gradually build up to 30-40 minutes. You should eat before any workout over 40 minutes. Avoid high-intensity workouts. If you’re doing HIIT you should eat before.
Can also burn muscle– Fasted cardio burns stored glucose first and then taps into fat reserves for energy. Problem is that your body can also take energy from muscles. To prevent muscle loss, limit exercise to less than 40 mins, low-moderate intensity, also do weekly weight training and ensure you are getting enough protein and aren’t cutting calories.
Bring a snack- If you are experiencing dizziness it’s important to listen to your body and stop. Rest, hydrate and eat a snack
Avoid working out alone- Until your body is conditioned to this type of exercise you should ensure you are safe. Avoid going outside alone, especially in hot weather conditions for fasted cardio.
Post-workout meal- Eat a meal with protein and carbs to replenish glucose reveres and reduce muscle loss. Since you’re likely going to be more hungry throughout the day on these days, have a plan for your meals. Be sure to eat carbs like oats, brown rice, and sweet potato that are also full of fiber to reduce cravings.
Hydrate- Always stay hydrated
Results will not be overnight- Fat loss takes time. you won’t see results after a few workouts, so don’t expect it. Focus on how you feel not how you look. Be patient, positive, and consistent and the rest will come.
Unfortunately, we have no control over where we store and burn fat in our bodies. Genetics and other influences play a role in where and how much fat storage we accumulate. Diet, exercise, mental health, and sleep are all important areas to consider when starting a weight loss plan. Small changes in each of these areas over time are what build healthy habits for a healthy lifestyle.
**Disclaimer** The information in this article is designed for educational purposes only and is not intended to be a substitute for informed medical advice or care. This information should not be used to diagnose or treat any health conditions without consulting your healthcare provider. Information used is based on experience and opinion, not 100% evidence. Always consult with a health care practitioner before relying on any information in this article or on this website. Some links in this article may contain affiliate links.