When exercising, your body relies primarily on glycogen (stored carbohydrates) to fuel exercise. However, there is an exception to this rule when glycogen reserves are depleted.
The premise of fasted cardio or fasted conditioning is that depleting your glycogen stores, by not eating for long periods, will cause the body to burn other sources of energy, such as fat. The rationale for the theory is that low glycogen levels cause your body to shift energy utilization away from carbohydrates, thereby allowing greater mobilization of stored fat for fuel. However, although the prospect of reducing the body fat by training in a fasted state may sound enticing, science does not support its efficacy (Schoenfeld, 2011).
Don't lose sight of what is being said here. Your body’s main source of energy (carbs) is gone, so it has to find other sources to utilize. While fat may be the source you want to be used, it isn’t always the case. Your body also reverts to breaking down protein for fuel. So while you may shed more fat when exercising on a fasting diet, you may lose more muscle as well.
Fasted cardio can also slow your metabolism, making losing weight more difficult in the long run. Your body begins to adapt to the calories it’s given, so frequent caloric cuts will cause your body to burn fewer calories as it latches on to the calories you've consumed to ensure that breathing and other every day functions continue normally. Since all your body systems have been slowed down, you won't burn as many calories as you would if you were properly fueled, and any results from strength training you do see can be greatly reduced.
In regards to a slowed metabolism, a small study from the Pennington Biomedical Research Center, after a group fasted every other day for 22 days, their resting metabolic rates (how many calories they burned each day by simply living), had dropped by 5%, or 83 calories. That's not exactly ideal for any exercise plan that's supposed to end in weight loss (of fat). While you, in particular, may not be fasting for an entire day, it still holds true that fasted exercise can cause the metabolism to slow, particularly when done over time.
Lastly, the concept of performing cardiovascular exercise on an empty stomach to enhance fat loss is flawed, even when examining its impact on the amount of fat burned in the exercise session alone. Cardiovascular exercise in a fasted state causes more fat to be broken down than that the body can use for fuel. Free fatty acids that are not oxidized ultimately become re-esterified in adipose tissue, nullifying any lipolytic benefits afforded by pre-exercise fasting (Febbraio et al., 2000).
Two Studies on Why Fasted Cardio is A No Go:
Lee et al. (1999) results showed that ingestion of glucose beverage resulted in a significantly greater excess postexercise oxygen consumption compared with exercise performed in a fasted state in both high- and low- intensity bouts. Other studies have produced similar findings, indicating a clear thermogenic advantage associated with pre-exercise food intake.
In other words: consumption of food before training increases the thermic effect of exercise. Put simply: the right food before a work out is a good thing.
Lemon and Mullin (1980) found that nitrogen losses were more than doubled when training while glycogen depleted compared with glycogen loaded. This resulted in a protein loss estimated at 10.4% of the total caloric cost of exercise after 1 hour of cycling at 61% V_ o2max. This would suggest that performing cardiovascular exercise while fasting might not be advisable for those seeking to optimize muscle mass.
In other words: Lemon and Mullin found that fasted cardio led to protein loss, which means muscle loss. Uh oh.
Cortisol, a word that strikes fear into the hearts of all, is highest in the morning times due to the spike that occurs when you wake up. If no caloric intake is had, those cortisol levels stay elevated and actually increase. Additionally, when coupled with steady state cardio, cortisol levels continue to increase. These high cortisol levels are the best way to lose muscle. Once these levels are increased so greatly, it may be difficult bringing them back down. This ultimately delivers a catabolic state due to the caloric deficit.
Fasting itself (intermittent fasting) is NOT a bad thing. In fact, majority of research supports it. However, fasted physical activity is not supported.
Findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training (Schoenfeld et al., 2014). Additionally, findings suggest that 60 min treadmill running induces a negative daily energy balance relative to a sedentary day but is no more effective when performed before or after breakfast (Deighton et al., 2012).
Put simply, when striving for fat loss, do it in the best manner possible. Weight loss and fat loss from exercise is more likely to be enhanced through creating a meaningful caloric deficit over time, rather than exercising in a fasted or an over-fed state. The 24-hour energy balance is the most important determinant in reducing body fat, whichever way you decide to analyze the situation.
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., and 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 YS, Ha MS, and Lee YJ. The effects of various intensities and durations of exercise with and without glucose in milk ingestion on postexercise oxygen consumption. J Sports Med Physical Fitness 39: 341–347, 1999.
Lemon, P., and 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.