Sports Nutrition Fasted Cardio: Does It Lead to Greater Weight Loss? By Darla Leal Updated on June 18, 2024 Medically reviewed Verywell Fit articles are reviewed by board-certified physicians and nutrition and exercise healthcare professionals. Medical Reviewers confirm the content is thorough and accurate, reflecting the latest evidence-based research. Content is reviewed before publication and upon substantial updates. Learn more. by Mia Syn, MS, RDN Print Cecilie_Arcurs / Getty Images Table of Contents View All Table of Contents How Fasted Cardio Works Does Fasted Cardio Burn More Fat? Are Fasted Cardio Workouts Effective? Benefits Risks Frequently Asked Questions Fasted cardio is when you exercise on an empty stomach, known as a "fasted" state, usually in the morning before your first meal. The theory is that working out on an empty stomach or after a prolonged period of fasting can lead to greater fat loss. While some fitness professionals adhere to this theory, it is still a hotly debated topic in the fitness world and lacks scientific evidence. The efficacy of the fasted cardio theory remains contested, and ultimately the decision to exercise on an empty stomach is up to you. If you embrace the fasted cardio theory, there are drawbacks to be aware of. Here's a look at research that tests the ideas behind fasted cardio. How Fasted Cardio Works Most people who engage in fasted cardio do their workout first thing in the morning, before eating breakfast. A 30-45 minute moderate-intensity workout, such as brisk walking on a treadmill, is recommended if you can endure for that long. Shorter workouts also contribute to the recommended amount of exercise per week, which is currently 150-300 minutes of moderate-intensity exercise, according to the U.S. Department of Health and Human Services. Eat After A Fasted Workout After your workout, you should eat protein and carbohydrates within an hour. Options include: Eggs and whole-wheat toast Oatmeal and yogurt Chicken and rice Salmon with quinoa Does Fasted Cardio Burn More Fat? Bill Phillips, bodybuilder and author of "Body-for-LIFE," has been credited with introducing the fasted cardio theory in 1999. In his book, he suggests that the human body maximizes fat-burning while exercising in a fasted state. The book was a success, and his concept of fasted cardio has remained popular ever since. According to Phillips, fasting overnight reduces blood sugar, insulin levels, and glycogen stores. Typically, our bodies rely on these glycogen stores, which come from carbohydrates in our diet, for energy. However, if you exercise in a fasted state (i.e., with reduced glycogen), the theory is that your body relies on body fat for energy to fuel your workout. Are Fasted Cardio Workouts Effective? While some fitness industry professionals follow the fasted cardio method and note it has some fat-burning benefits, scientific research has not fully supported the theory's effectiveness. One scientific review by researchers in Australia looked into five separate studies and a total of 96 participants to measure the effectiveness of exercising after an overnight fast. In their research, the review authors determined that working out post-fast had little—if any—effect on body mass. Fasting may make it challenging to complete a workout, whereas eating could help you complete your exercise plan. One review from the University of Limerick in Ireland looked into 46 previously published studies and found when study participants ate before a cardio session, they could work out for longer periods. Working out after eating led to longer aerobic workouts, showing that while fasted cardio may have other benefits, it does not typically lead to longer workouts. It's important to remember that exercise offers benefits whether it is done in a fasted state or after eating. The fasted cardio theory argues that if you can exercise on an empty stomach, you may be able to leverage greater fat-burning effects. Overall, weight loss and reduction of body mass are likely a result of an overall calorie deficit, whether exercise is completed on an empty stomach or not. Benefits of Fasted Cardio While scientific studies have found limited support for the theory behind fasted cardio, other research has found that athletes who fast have experienced measurable fat loss. One study looked specifically into the effects of a fasted state on resistance training. The researchers studied 12 female NCAA Division I athletes, each of whom completed two workouts. One resistance workout session happened after a 10-hour fast, while another happened after a fat- and carb-loaded meal. Following the two workouts, the researchers found that the fasted session allowed the body to use more fat as energy than carbohydrates. Body Mass Index (BMI) is a dated, biased measure that doesn’t account for several factors, such as body composition, ethnicity, race, gender, and age. Despite being a flawed measure, BMI is widely used today in the medical community because it is an inexpensive and quick method for analyzing potential health status and outcomes. Although this study looked at weight training instead of cardio, it provides further insight into the effect of fasting on exercise and body composition. For athletes who aren't early risers, overnight fasting and fasting before a morning workout may also save you time after waking up. With a no-meal-before-workout schedule, you don't need to wake up early to build time for eating before a session. Instead, enjoy those extra few moments of sleep. Risks of Fasted Cardio While body-fat reduction may be one benefit of fasted cardio, it's important to note some potential drawbacks of this nutrition and fitness approach. When you know you are working out in a fasted state, it is possible to compensate by eating more at other times. One study showed that participants who knew they would work out without eating in the morning consumed more calories the evening before. Fasted Cardio Safety Precautions Speak to a healthcare provider if you have any conditions that might lead to dizziness or lightheadedness in a fasted state, or before starting any new workout regimen. If you feel unwell during fasted cardio, stop and eat a balanced meal. Follow up with a healthcare provider if needed. Other studies have shown that fasting generally can lead to protein and muscle loss. These findings show that if you're trying to build muscle mass, fasted cardio may not be the optimal choice. Similarly, fasted cardio may not lead to longer workouts. If endurance workouts—marathon training, for example—are your goal, fasted cardio may not be your best option. One study shows consuming food pre-workout can naturally lead to a longer endurance aerobic session. On the other hand, working out in a fasted state can result in a shorter workout. Finally, another study measured the workout efficiency of 20 male cyclists who performed both fasted and fed workouts. After each session, study authors found that fasted workouts negatively impact workout intensity and volume. Such research suggests that working out on an empty stomach can adversely impact how intensely you perform a workout. A Word From Verywell The choice to practice fasted cardio is a personal decision. Performing exercise consistently, regardless of whether or not you eat before, provides clear health benefits, including healthy, sustainable weight loss. If you have never done a workout before eating breakfast, it's best to start slow. Give your body a chance to adapt to any changes in your exercise and eating plan, and find ways to be active that work best for you and your lifestyle. Frequently Asked Questions Is fasted cardio good for weight loss? Any increase in exercise can help support weight loss. Some wonder if more weight can be lost if cardio is performed on an empty stomach. There is very little scientific research to support this theory. Fasted cardio may result in a shorter workout or overcompensating with caloric intake at other times of day. How long should fasted cardio be? Studies have shown that it is more difficult to do a longer cardio workout in a fasted state. If you choose to workout on an empty stomach, be careful not to overdo it. A 30- to 45-minute cardio session is a reasonable goal. You may need to work up to this length if you are beginning a new exercise regimen. Is 20 minutes of fasted cardio enough? Doing 20 minutes of fasted cardio will certainly give you the benefits of any exercise. Whether you will gain additional benefit from a workout because you have fasted beforehand is unclear. However any amount of activity will lead to increased fitness, whether you have fasted or not. 7 Sources Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. U.S. Department of Health and Human Services, Office of Disease Prevention and Health Promotion. Top 10 things to know about the second edition of the physical activity guidelines for Americans. Hackett D, Hagstrom AD. Effect of overnight fasted exercise on weight loss and body composition: A systematic review and meta-analysis. J Funct Morphol Kinesiol. 2017;2(4):43. doi:10.3390/jfmk2040043 Aird TP, Davies RW, Carson BP. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scand J Med Sci Sports. 2018;28(5):1476-1493. doi:10.1111/sms.13054 Frawley K, Greenwald G, Rogers RR, Petrella JK, Marshall MR. Effects of prior fasting on fat oxidation during resistance exercise. Int J Exerc Sci. 2018;11(2):827-833. PMID:29997729 Barutcu A, Briasco E, Moon J, et al. Planned morning aerobic exercise in a fasted state increases energy intake in the preceding 24 h. Eur J Nutr. 2021;60(6):3387-3396. doi:10.1007/s00394-021-02501-7 Laurens C, Grundler F, Damiot A, et al. Is muscle and protein loss relevant in long-term fasting in healthy men? A prospective trial on physiological adaptations. J Cachexia Sarcopenia Muscle. 2021;12(6):1690-1703. doi:10.1002/jcsm.12766 Terada T, Eshghi SRT, Liubaoerjijin Y, et al. Overnight fasting compromises exercise intensity and volume during sprint interval training but improves high-intensity aerobic endurance. J Sports Med Phys Fitness. 2019;59(3):357-365. doi:10.23736/S0022-4707.18.08281-6 Additional Reading Gonzalez JT, Veasey RC, Rumbold PLS, Stevenson EJ. Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males. British Journal of Nutrition. 2013;110(4):721-732. doi:10.1017/S0007114512005582 By Darla Leal Darla Leal is a Master Fitness Trainer, freelance writer, and the creator of Stay Healthy Fitness, where she embraces a "fit-over-55" lifestyle. See Our Editorial Process Meet Our Review Board Share Feedback Was this page helpful? Thanks for your feedback! What is your feedback? Helpful Report an Error Other Submit Advertiser Disclosure × The offers that appear in this table are from companies that partner with and compensate Verywell Fit for displaying their offer. These partnerships do not impact our editorial choices or otherwise influence our editorial content.