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Fasted Cardio: What Is It And Should You Be Doing It?

Patrick Scheel, RDN, LDN

Published in Fitness

6 min read

April 5, 2024
A woman with dark hair and in fitness gear, running outdoors
A woman with dark hair and in fitness gear, running outdoors

In the world of fitness and weight management, fasted cardio has surged in popularity as a strategy to maximize fat-burning potential. Simply put, fasted cardio involves engaging in cardiovascular or aerobic exercise on an empty stomach, typically in the early morning before breakfast.

But is this just a buzzy workout trend, or is fasted cardio truly effective for weight loss? Should it be a staple in your fitness routine?

In this article, we’ll explore the concept of fasted cardio, diving into its definition, potential benefits, and scientific research. Read on to see if incorporating fasted cardio into your routine aligns with your fitness goals.

What is Fasted Cardio?

A person wth dark hair n a ponytail, wearing boxing gloves and training indoors
Source: Pexels

Fasted cardio involves engaging in cardiovascular exercises like running, cycling, or brisk walking without having eaten any food for 10-12 hours beforehand. Proponents of this morning workout practice claim that it leverages the body's overnight fasting period, during which glycogen stores are depleted, and insulin levels are low, so that the body will rely more on stored fat as a primary fuel source during exercise, potentially leading to greater fat loss.

In contrast, traditional cardio performed after eating involves exercising in a fed state, where the body has recently received glucose and other nutrients from food. In this scenario, blood glucose and insulin levels are elevated due to recent meal consumption.

Consequently, the body's primary energy sources during exercise are derived from the carbs and fats consumed in the meal. While this form of cardio still contributes to calorie expenditure and overall fitness, some believe it does not prioritize fat oxidation to the same extent as fasted cardio due to the presence of readily available carbohydrates.

Potential Benefits of Fasted Cardio

Yellow background with beige text box, with black text with the nutrisense logo and bulleted list of the benefits of fasted cardio: May Increase Fat Oxidation, May Improve Insulin Sensitivity, May Improve Metabolic Flexibility

It's important to note that the research on the specific benefits of fasted cardio remains somewhat mixed, and, like all physical activity, its effectiveness can vary depending on individual factors such as metabolism, overall dietary habits, and other lifestyle habits.

However, it's widely recognized that engaging in any form of exercise, whether fasted or not, offers numerous health benefits. That being said, proponents of fasted cardio suggest several potential advantages associated with this practice. 

One of the primary reasons some people turn to fasted cardio is it has been shown to enhance the body's capacity to burn fat while exercising. As we mentioned earlier, some studies suggest that fasted exercise may indeed lead to greater fat utilization during the workout compared to exercising in a fed state.

Research also suggests that fasted cardio may positively impact insulin sensitivity, a crucial factor in overall metabolic health. It's worth noting, however, that non-fasting exercise also greatly benefits insulin regulation, which highlights the importance of individualized approaches to exercise.

Does It Support Weight Loss?

Research suggests that fasted cardio can indeed support weight loss efforts for some people. However, it's important to remember that weight loss outcomes and body composition changes can vary significantly and are influenced by other factors.

Weight loss occurs when you burn more calories than you consume, regardless of the timing of cardio exercise or whether it's performed in a fasted state. While fasted cardio may enhance fat burning during steady-state exercise and improve metabolic parameters, its effectiveness ultimately hinges on your overall energy intake, or caloric balance. 

Therefore, while fasted cardio may be a useful tool for weight loss, it’s important to consider the importance of maintaining a healthy diet and overall lifestyle habits to achieve sustainable results. 

Is Fasted Cardio Safe?

Two people cycling outdoors
Source: Unsplash

Fasted cardio is generally considered safe for most people, but it's important to be mindful of potential risks and limitations. Concerns about low blood sugar levels and decreased workout performance are valid, as fasted exercise may affect energy levels and endurance.

While performance may be impacted during fasted cardio, particularly for intense or prolonged workouts, many people find it manageable with proper hydration and pacing. However, engaging in high-intensity workouts such as HIIT workouts while fasted may require caution due to the potential for decreased energy levels and increased risk of fatigue or lightheadedness without readily available fuel sources from food intake.

For those considering fasted cardio, it's advisable to start gradually and listen to your body's signals. Individuals with medical conditions should consult with a personal trainer, a registered dietitian or nutritionist, or another healthcare professional before attempting fasted exercise, as it may pose additional risks or require adjustments to medication or dietary management.

How to Do Fasted Cardio

When incorporating fasted cardio into your fitness routine, it's essential to approach it with care and consideration for your body's needs. Begin by choosing a suitable timing that aligns with your schedule and preferences, such as in the morning before breakfast.

Aim to keep your fasted cardio sessions moderate in intensity and duration, typically not exceeding 60 minutes without consuming any food. Hydration is key, so be sure to drink water before, during, and after your workout to maintain optimal performance and prevent dehydration. 

Suitable fasted cardio workouts include low-intensity or moderate-intensity exercises like brisk walking, jogging, cycling, which can effectively elevate your heart rate without overexertion. Listen to your body's signals and adjust the intensity as needed, especially if you feel lightheaded or fatigued. If you have any health concerns, talk to your doctor before starting any new exercise routine.

Eating After a Fasted Workout

yellow background with beige text box and black text with the nutrisense logo and bulleted list for tips to Eating after a Fasted Workout: Aim to eat within 45-60 minutes of your workout, Consume a 2:1 ratio of carbs and protein to restore muscle glycogen levels, Stay hydrated with water and electrolytes

After a fasted workout, it's important to prioritize replenishing energy stores and promoting muscle recovery with a balanced post-workout meal. Aim for a combination of carbohydrates and protein in a ratio of approximately 3:1 to facilitate glycogen replenishment and muscle repair. 

The International Society of Sports Nutrition recommends consuming 0.5 grams of protein per kilogram of body weight in your post-exercise meal, which comes out to about 34 grams of protein for someone who weighs 150 pounds.

Keep in mind that your ideal post-workout meal may vary depending on the intensity and duration of your exercise session. Opt for carbohydrates with a low to moderate glycemic index to provide sustained energy, such as whole grains, fruits, and vegetables. Pair these with lean sources of protein, such as chicken, fish, tofu, or Greek yogurt, to support muscle repair and growth.

Additionally, don't forget to hydrate adequately by drinking water or electrolyte-rich beverages to replace fluids and minerals (mainly sodium) lost during exercise, especially if you tend to sweat a lot. By following these guidelines and choosing nutrient-dense foods, you can optimize post-workout recovery and support your fitness goals effectively.

The Bottom Line

While fasted cardio may offer potential benefits such as improved insulin sensitivity, its effectiveness can vary based on individual factors. It's essential to approach fasted cardio with caution, listening to your body's signals and adjusting intensity as needed. If you don't feel well with fasted workouts, fed cardio workouts can still be as effective to help you reach your health goals.

For those considering incorporating fasted cardio into their fitness routine, remember to start gradually and consult with a healthcare professional, especially if you have medical conditions or specific dietary needs. 

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Amanda Donahue, MS, RD, CD

Reviewed by: Amanda Donahue, MS, RD, CD

Amanda is a Nutrition Manager and Registered Dietitian at Nutrisense, with a Masters in Dietetics from Stephen F. Austin State University. Originally from south GA, she got her undergrad degree from Texas Tech University. Before joining Nutrisense, she worked at a hospital in Fort Worth, TX, for 4 years as a dietitian, counseling those living with HIV.

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