Fasted Cardio? Probably not the BEST choice

In recent years, fasted cardio has gained popularity as a method to accelerate fat loss. The premise behind this trend is simple: exercising on an empty stomach forces your body to use stored fat for energy, potentially enhancing fat loss. However, while the concept may sound appealing, scientific research does not support its efficacy. In fact, exercising in a fast state might not be the best choice for those looking to optimize their health and fitness outcomes. As an athletic performance trainer in Atlanta, let’s dive into the science and examine why fasted cardio might not be the best approach for achieving your fitness goals.

The Science Behind Fasted Cardio

When you exercise, your body primarily relies on glycogen, which is stored carbohydrates, to fuel your workout. Glycogen is the most readily available source of energy for your muscles during exercise. However, when glycogen stores are depleted, your body needs to find alternative energy sources.

The idea behind fasted cardio is that by not eating for an extended period, your glycogen stores will be depleted, causing your body to shift its energy utilization from carbohydrates to fats. This shift theoretically allows for greater mobilization and utilization of stored fat for fuel. While this sounds like a promising strategy for fat loss, scientific evidence suggests otherwise.

Muscle Loss and Metabolism Slowdown

One major drawback of fasted cardio is the potential for muscle loss. When glycogen stores are low, your body may turn to protein as an energy source. This means that instead of burning fat, your body could start breaking down muscle tissue to meet its energy needs. Losing muscle mass is counterproductive to weight loss efforts because muscle is metabolically active tissue, meaning it burns more calories at rest compared to fat tissue.

Furthermore, fasted cardio can slow down your metabolism. Your body adapts to the reduced caloric intake by becoming more efficient at conserving energy, which means it will burn fewer calories. A study from the Pennington Biomedical Research Center found that participants who fasted every other day for 22 days experienced a 5% reduction in their resting metabolic rate. This reduction equates to burning 83 fewer calories per day, making it harder to lose weight in the long run.

Ineffectiveness in Fat Utilization

The concept of performing cardiovascular exercise on an empty stomach to enhance fat loss is flawed, even when considering its impact on the amount of fat burned during the exercise session. Research by Febbraio et al. (2000) demonstrated that cardiovascular exercise in a fasted state leads to more fat being broken down than the body can use for fuel. The excess free fatty acids that are not oxidized are ultimately re-esterified in adipose tissue, nullifying any potential fat-burning benefits of pre-exercise fasting. In other words, your body can only use a limited amount of the fat it breaks down, and the rest gets stored back as fat.

The Thermogenic Advantage of Pre-Exercise Food Intake

Studies have shown that consuming food before exercise increases the thermic effect of exercise, which refers to the amount of energy expended during and after physical activity. Lee et al. (1999) found that ingestion of glucose beverages resulted in significantly greater excess post-exercise oxygen consumption compared to exercising in a fasted state. This means that eating before a workout can enhance calorie burning and improve overall exercise performance.

Impact on Protein and Muscle Preservation

Another study by Lemon and Mullin (1980) found that training in a glycogen-depleted state led to more than double the nitrogen losses compared to training with adequate glycogen levels. This increased nitrogen loss indicates a higher rate of protein breakdown, which can lead to muscle loss. Preserving muscle mass is crucial for maintaining a healthy metabolism and achieving long-term weight loss goals.

The Role of Cortisol

Cortisol, a hormone associated with stress, is naturally higher in the morning. Exercising in a fasted state can cause cortisol levels to remain elevated or even increase. High cortisol levels can lead to muscle breakdown and hinder recovery, making it difficult to achieve fitness goals. Additionally, elevated cortisol levels can promote fat storage, particularly in the abdominal region.

Fasting and Exercise: What the Research Says

Several studies have examined the effects of fasting on exercise performance and body composition. For instance, Schoenfeld et al. (2014) found that changes in body composition associated with aerobic exercise and a hypocaloric diet were similar regardless of whether individuals were fasted before training. This suggests that the timing of food intake relative to exercise may not significantly impact fat loss.

Deighton et al. (2012) also investigated the effects of fasted versus fed exercise on energy balance and found that 60 minutes of treadmill running-induced a negative daily energy balance regardless of whether it was performed before or after breakfast. This indicates that the overall energy balance throughout the day is more important for fat loss than the timing of food intake relative to exercise.

Practical Recommendations for Effective Fat Loss

Given the evidence, it’s clear that fasted cardio is not the most effective strategy for fat loss. Instead, consider the following practical recommendations for achieving your fitness goals:

1. Balanced Diet: Focus on a balanced diet that includes a mix of carbohydrates, proteins, and healthy fats to fuel your workouts and support recovery.
2. Pre-Workout Nutrition: Consume a small meal or snack containing carbohydrates and protein before your workouts to enhance performance and promote muscle preservation.
3. Consistent Exercise Routine: Follow a consistent exercise routine that includes a combination of cardiovascular exercise, strength training, and flexibility exercises.
4. Caloric Deficit: Aim to create a moderate caloric deficit through a combination of diet and exercise to promote sustainable fat loss.
5. Hydration: Stay hydrated before, during, and after your workouts to support optimal performance and recovery.
6. Sleep and Stress Management: Prioritize sleep and stress management techniques, such as mindfulness and relaxation exercises, to support overall health and fitness goals.

Conclusion

While the idea of fasted cardio may seem appealing, the scientific evidence does not support its efficacy for fat loss. Exercising in a fasted state can lead to muscle loss, slow down your metabolism, and fail to provide significant fat-burning benefits. Instead, focus on a balanced diet, pre-workout nutrition, and a consistent exercise routine to achieve your fitness goals effectively and sustainably.

Ultimately, the most crucial factor in reducing body fat is creating a significant caloric deficit over time, regardless of whether you exercise in a fasted or fed state. Prioritize overall energy balance, proper nutrition, and a well-rounded exercise regimen to optimize your health and achieve long-term success.

References

• Deighton, K., Stensel, D., & Zahra, J. (2012). Appetite, energy intake, and resting metabolic responses to 60min treadmill running performed in a fasted versus a postprandial state. Appetite, 58(3), 946-954.
• Febbraio, M., Chiu, A., Angus, D., Arkinstall, M., & Hawley, J. (2000). Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance. Journal of Applied Physiology, 89, 2220–2226.
• Hackett, D., & Hagstrom, A. (2017). Effect of overnight fasted exercise on weight loss and body composition: a systematic review and meta-analysis. Journal of Functional Morphology and Kinesiology, 2(4), 43.
• Lee, Y. S., Ha, M. S., & Lee, Y. J. (1999). The effects of various intensities and durations of exercise with and without glucose in milk ingestion on post-exercise oxygen consumption. Journal of Sports Medicine and Physical Fitness, 39, 341–347.
• Lemon, P., & Mullin, J. (1980). Effect of initial muscle glycogen levels on protein catabolism during exercise. Journal of Applied Physiology, 48, 624–629.
• Pacy, P., Barton, N., Webster, J., & Garrow, J. (1985). The energy cost of aerobic exercise in fed and fasted normal subjects. The American Journal Of Clinical Nutrition, 42(5), 764-768.
• Schoenfeld, B. (2011). Does cardio after an overnight fast maximize fat loss? Strength & Conditioning Journal, 33(1), 23-25.
• Schoenfeld, B., Aragon, A., Wilborn, C., Krieger, J., & Sonmez, G. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1), 54.
• Stannard, S., Buckley, A., Edge, J., & Thompson, M. (2010). Adaptations to skeletal muscle with endurance exercise training in the acutely fed versus overnight-fasted state. Journal of Science and Medicine in Sport, 13(4), 465-469.

By prioritizing a balanced approach to nutrition and exercise, you can achieve your fitness goals more effectively and sustainably.

Zach Bettis

-CSCS