Is Fasted Cardio Good for You: The Pros, the Cons, and How to Do it Safely

If you are looking to switch up your cardio session, you may want to consider fasted cardio. Learn more about how to safely engage in this workout.

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by
Sarah Zimmer, PT, DPT
— Signos
PT, DPT
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Updated by

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Science-based and reviewed

Published:
July 24, 2024
December 7, 2023
— Updated:

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Fasted cardio, an exercise trend gaining recent momentum, offers an intriguing approach to individuals striving for weight loss and looking to maximize their workout efficiency. It's an enticing prospect, seamlessly fitting into your daily routine while potentially aiding your journey towards shedding extra pounds. In a world where health is our ultimate goal, incorporating daily exercise is more than just a “healthy habit”—it's a commitment to our overall well-being and feeling our best selves for as long as possible.

So why fasted cardio? The allure of fasted cardio lies in its simplicity and ability to optimize fat burning by capitalizing on the body's overnight fast and tapping into stored fat as the primary energy source during physical activity. Harnessing this strategy within your daily routine may bolster your weight loss efforts, providing an edge in your quest for a healthier, fitter self. After all, the journey towards a healthier lifestyle begins with small, consistent steps, and fasted cardio might just be the morning ritual to propel you toward your fitness goals. 

However, like any fitness approach, it comes with its own set of advantages and potential drawbacks. Check out this article to learn about how fasted cardio may help or hurt you on your way to finally losing weight and feeling your healthiest self.

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What Is Fasted Cardio?

Fasted cardio is a fitness strategy involving cardiovascular exercises on an empty stomach, typically in the morning before eating breakfast. Most individuals find doing fasted cardio in the morning is easiest as you can leverage the body's overnight fasting state, where glycogen (stored carbohydrates) levels are lower, prompting the body to rely more on fat stores for fuel during exercise. It's a strategy to maximize fat utilization, specifically in an attempt to enhance weight loss and fitness outcomes. 

The origins of fasted cardio trace back to studies exploring the effects of different meal timings on exercise performance and fat metabolism. Then, studies conducted in the late 20th and early 21st centuries began examining the impact of exercising in a fasted state, specifically in the context of the utilization of glycogen stores. These studies aimed to understand how the body responds to exercise when insulin levels are low, leading researchers to investigate whether exercising on an empty stomach could enhance fat burning. 

For example, a study performed in 2016 by Vieira et al. in the British Journal of Nutrition found a significant increase in fat oxidation during exercise in the participants who exercised in a fasted state compared to those who exercised in a fed state.1 

Over time, fitness enthusiasts and athletes have been experimenting with morning workouts before eating breakfast, based on the hypothesis that the body might rely more on fat stores for energy in a fasted state. It has developed into a tool for optimizing fat-burning and improving overall fitness. Yet - is it truly worth all the hype? Does the research still support fasted cardio for weight loss?

Benefits of Fasted Cardio

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Fasted cardio workouts, especially in the morning, present a range of potential benefits that make them an intriguing addition to fitness routines. Some benefits target physiological changes such as weight loss, reduced body fat, and potentially improved blood sugar levels over time, while other benefits include convenience and enhanced athletic performance. 

Check out the list below of the potential benefits found in the current scientific literature about fasted cardio. 

  1. Possible Insulin Function Improvement: A literature review performed in 2016 by Sports Medicine found fasted workouts enhanced insulin sensitivity and glycemic control in patients living with type II diabetes, which led to improved long-term blood sugar management.2 
  2. Potential Greater Fat Loss: Some studies find that engaging in short durations of high-intensity exercise while in a fasted state may lead to greater fat loss due to an increase in fat utilization during exercise as compared to those who exercise in a carbohydrate-fed state.3,7 In addition, engaging in regular fasted cardio might help the body become more efficient at using fat as a primary fuel source during exercise long term (fat-adaptation).4
  3. Saves You Time Before Working Out: If you are waking up early to do your morning workout before a busy day, exercising in a fasted state can save you time and make early morning workouts more manageable and sustainable.  
  4. Performance Improvement for Athletes: Most studies observing fasting on strength and aerobic capacity in athletes find that exercising in a fasted state was more beneficial when the athlete still met daily caloric needs throughout the remainder of the day post-exercise. Most of these performance enhancements were believed to be because of noticeable body composition changes and utilization of fat during longer or higher-intensity training.5 Studies on fasted workouts with athletes show performance decrements if caloric needs were not met through the remainder of the day, so fasted or not - total calorie intake is still crucial for athletic performance. 
  5. Increased Metabolism: Some evidence suggests that exercising in a fasted state might increase your metabolism due to the “afterburn effect” for a period post-workout.6 This could lead to burning fat at an increased rate.

As you can see, these combined benefits make fasted cardio an interesting option for individuals seeking to maximize their fitness outcomes and weight loss efforts while doing so within their busy schedules. However, it's crucial to weigh these benefits against potential drawbacks and ensure engaging in fasted cardio is safe for your overall health and fitness goals.

<p class="pro-tip"><strong>Learn More: </strong> <a href="/blog/metabolic-advantages-of-cardio">The Metabolic Advantages of Cardio</a>.</p>

What Are The Risks of Fasted Cardio?

While fasted cardio presents numerous benefits, it also carries potential risks that individuals should consider before incorporating it into their fitness routine. Exercising on an empty stomach, despite the ability to tap into fat stores, might have adverse effects on performance, muscle mass integrity, and overall well-being. This is especially true depending on the intensity, duration, and type of workout you are doing (i.e., HIIT class, trail running, yoga class, etc.). Understanding these risks is crucial for maintaining a balanced and safe approach to fitness. 

Here are a few negative side effects of fasted cardio to consider:

  1. Dizziness, Lightheadedness, or Fainting: For individuals prone to hypoglycemia or diabetes, fasted exercise might lead to dangerously low blood sugar levels during activity, causing dizziness, feelings of being lightheaded, shakiness, or fainting.
  2. Dehydration: Without pre-workout hydration or fuel, the risk of dehydration during fasted cardio increases, impacting exercise performance and overall health.
  3. Weakness: Exercising on an empty stomach may result in the breakdown of muscle tissue for energy, especially during prolonged sessions, potentially counteracting muscle-building efforts.
  4. Increased Risk of Overtraining: Insufficient energy levels from fasting could lead to overtraining, causing fatigue, decreased recovery, and potential injury risk.
  5. Possible Hormone Imbalance: Fasting before exercise might disrupt hormone levels, potentially affecting thyroid function, reproductive hormones, and metabolic regulation.
  6. Increased Cortisol Levels: Fasted exercise could elevate cortisol levels, a stress hormone, potentially contributing to muscle breakdown and hindering overall fitness goals.
  7. Risk of Nutrient Deficiency: Chronic engagement in fasted cardio without appropriate nutritional support may lead to nutrient deficiencies, impacting overall health and well-being.

Is Fasted Cardio Good for You?

Despite a need to lose weight or manage blood sugar levels, certain individuals may need to avoid fasted cardio to maintain their safety and well-being. For instance, those prone to hypoglycemia or diabetes should avoid fasted workouts to prevent dangerously low blood sugar levels. Additionally, individuals with a history of eating disorders, hypothalamic amenorrhea, or those in recovery from such conditions should opt for other exercise approaches that prioritize balanced nutrition and improve their relationship between food and exercise. 

Pregnant women or individuals with hormonal imbalances should also use caution, as fasting before workouts could potentially exacerbate hormone-related issues such as hypothyroidism, amenorrhea, relative energy deficiency in sport (RED-S), PCOS, etc. Anyone new to exercise or seeking performance gains in high-intensity workouts may benefit more from consuming pre-exercise meals to fuel their workouts effectively. 

Ultimately, anyone uncertain about their health status or fitness goals should consult a healthcare professional or fitness expert before incorporating fasted cardio into their routine to ensure it aligns with their specific needs and well-being.

Best Tips to Get Started with Fasted Cardio

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Embarking on a fasted cardio journey requires a thoughtful approach to ensure a safe and effective workout routine. Incorporating cardiovascular exercises in the morning, sans breakfast, demands a gradual and cautious initiation. 

Here are several tips tailored to help you ease into fasted cardio workouts, allowing your body to adapt and maximize the benefits of exercising in a fasted state. These suggestions aim to prioritize safety, hydration, and a gradual progression, ensuring a positive and sustainable start to your fasted cardio regimen.

  • Start Gradually: Begin with shorter sessions (20 to 30 min) to allow your body to adapt to exercising in a fasted state. Starting slow helps prevent exhaustion, over-exertion, over-training, or symptoms of low blood sugar levels (i.e. dizziness).
  • Prioritize Hydration During Your Workout: Before and during your fasted workout, ensure you're well-hydrated. Water intake is crucial to maintain performance and prevent dehydration.
  • Have Snacks Before Going to Bed: Consuming a light snack rich in protein or healthy fats before bedtime can provide sustained energy for your morning workout without causing discomfort.
  • Eat Well After Working Out: After your fasted workout, prioritize a balanced meal containing protein and carbohydrates to replenish energy stores and support muscle recovery.
  • Exercise Lightly During Your Fasting Period: Opt for low-intensity or moderate-intensity workouts initially. This approach can help prevent fatigue and dizziness while your body adjusts to exercising without immediate fuel.

Learn More About Fitness and Healthy Habits with Signos’ Expert Advice

If you have more questions on improving your health, fitness, and nutrition, seek the expert advice of the Signos continuous glucose monitor and Signos team. A continuous glucose monitor (CGM) can give you the insights to make smarter nutrition and exercise choices. The Signos app provides a unique, personalized program to help you lose weight and reach your health goals. Take this quiz to see if Signos is a good fit for you and reach your goals faster than ever before.

<p class="pro-tip"><strong>Also Read:</strong> <a href="/blog/cardio-vs-weights">Cardio vs. Weights for Weight Loss: Benefits and Key Differences</a>.</p>

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References

  1. Vieira, A. F., Costa, R. R., Macedo, R. C. O., Coconcelli, L., & Kruel, L. F. M. (2016). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. British Journal of Nutrition, 116(7), 1153-1164.
  2. Hansen, D., De Strijcker, D., & Calders, P. (2017). Impact of endurance exercise training in the fasted state on muscle biochemistry and metabolism in healthy subjects: can these effects be of particular clinical benefit to type 2 diabetes mellitus and insulin-resistant patients?. Sports Medicine, 47, 415-428.
  3. Horowitz, J. F., Mora-Rodriguez, R., Byerley, L. O., & Coyle, E. F. (1997). Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. The American journal of physiology, 273(4), E768–E775. https://doi.org/10.1152/ajpendo.1997.273.4.E768
  4. Van Proeyen, K., Szlufcik, K., Nielens, H., Ramaekers, M., & Hespel, P. (2011). Beneficial metabolic adaptations due to endurance exercise training in the fasted state. Journal of applied physiology.
  5. M. Correia, J., Santos, I., Pezarat-Correia, P., Minderico, C., & V. Mendonca, G. (2020). Effects of intermittent fasting on specific exercise performance outcomes: a systematic review including meta-analysis. Nutrients, 12(5), 1390.
  6. Bahr, R. O. A. L. D., & Sejersted, O. M. (1991). Effect of feeding and fasting on excess postexercise oxygen consumption. Journal of Applied Physiology, 71(6), 2088-2093.
  7. Aird, T. P., Davies, R. W., & Carson, B. P. (2018). Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scandinavian journal of medicine & science in sports, 28(5), 1476–1493. https://doi.org/10.1111/sms.13054

About the author

Sarah is a Doctor of Physical Therapy, graduating from the University of Wisconsin Madison in 2017.

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