The Truth about "Fasted Cardio"

As many of us know, a combined diet and exercise program is more effective in promoting long-term weight loss than a diet program alone.(1) Science has supported this theory, in that weight management is predicted based on the balance between energy intake and expenditure. Thus, your body survives on a set number of calories to carryout all basic functioning (breathing, digestion, spontaneous movement, etc.). If one consumes more calories (or energy) than one burns, the person will gain weight. Likewise, if one burns more calories than consumed, the person will lose weight. Due to this phenomenon, many have implemented different strategies to increase weight loss with exercise. More specifically, scientists have questioned whether the effect of timing of nutrient intake has had an effect on fat loss.

Many athletes, bodybuilders, and fitness enthusiasts promote the performance of “fasted cardio.” “Fasted cardio,” or performing cardiovascular exercise on an empty stomach, is believed to maximize fat burning. Bill Philips initially popularized this idea in his book, “Body for Life.”(2) According to Philips, performing 20 minutes of intense aerobic activity after an overnight fast has greater effects on fat loss than performing an entire hour of cardio in the postprandial state (after eating). Wow! That's a huge difference...However, this theory lacks scientific support.

As a reminder, glycogen is the storage form of glucose; the body uses glycogen as energy as needed. The rationale for performing “fasted cardio” is that after an overnight fast, the body’s glycogen stores are low. It is believed that due to low glycogen availability, the body no longer uses energy from carbohydrates, but shifts to the use of stored fat as fuel. In contrast with this view, it is well known that physiologically, the body does not use fat as a primary fuel. In fact, the body uses stored fat as a last resort. The body is designed to use carbohydrate as a primary source, and then protein sources.

Scientists have determined substrate utilization during exercise by measuring one’s respiratory-exchange ratio (RER) and oxygen consumption (VO2). RER is measured by the breath and helps determine which substrate (fat, carbohydrate, or protein) is being used by the body at a given time. Generally, lower RER values indicate that fat is being utilized; a higher RER (closer to 1.00) indicates that carbohydrates are being used. In a study completed by Paoli et al.(3) researchers compared the effect on oxygen consumption (VO2) and substrate utilization (estimated by the RER) in eight healthy young men who performed the same moderate-intensity training session in the morning. In this study, participants underwent two tests in random sequence: exercise while FASTED or exercise while FED. The same amount and quality of food was consumed before and after the testing for both groups. Results show that the consumption of breakfast for the FED condition resulted in a significant increase in RER compared to fasting (0.96 vs. 0.84, respectively). RER was increased by the food intake, suggesting a shift of substrate utilization from lipids to carbohydrates. However, at 12 hours after exercise RER was significantly lower in the fed versus fasting condition (P<0.01). Furthermore, this difference remained significant after 24 hours (P<0.05). This indicates that fat is being utilized 12 and 24 hours after exercise in FED conditions.

To test this same theory, Schoenfeld et al.(4) examined 20 healthy females, randomly assigned to one of two experimental groups: a FASTED training group that preformed exercise after an overnight fast (n=10) or a post-prandial training FED group that consumed a meal prior to exercise (n=10). The training intervention lasted four weeks; each participant was instructed to consume a caloric deficit diet (much lower than they normally would have consumed). Results revealed that both groups significantly lost weight (P=0.0005) and fat mass (P=0.02) from baseline. Although both groups lost a significant amount of weight and fat mass (due to consuming a caloric deficit diet), no differences were seen between the conditions in any outcome measure, regardless of exercising at a fed- or fasted-state. Fasted conditions did not show any significance on fat loss when compared to fed-state conditions.

Scientific research does not support benefits in fat utilization in performing “fasted cardio.” In fact, glucose is the primary fuel for exercise. Research has shown that the ability to sustain aerobic exercise is mainly determined by carbohydrate availability. Thus, carbohydrate intake before or during exercise may be key to prolonging the duration of aerobic exercise. Consuming glucose prior to exercise is beneficial because it spares muscle glycogen, thus allowing you to exercise for a longer duration. If someone exercises in a fasted state, one can become hypoglycemic and can become fatigued, not only physically, but also mentally (since glucose is the primary fuel our brains utilize). This will obviously affect one’s performance as well. Furthermore, if one exercises in a fasted state, the body utilizes protein as fuel. For bodybuilders, using protein for fuel rather than building muscle is not ideal.  

Although the media has popularized “fasted cardio” and its ability to increase fat loss, the science is insignificant. In order to perform at an optimal level, it is recommended to eat before exercise. If you don't, the fuel source comes from your precious muscles (protein), not fat. Rather than worrying whether to eat or not, focus on what you should be eating. Lastly, research shows that eating before moderate-intense exercise may even prolong fat utilization up to 24 hours after exercise! 

Moral of the story: Wake up and MUNCH! For those of you wondering WHAT to eat, leave me a comment and I will plan to write all about it!





1.     Wu T, Gao X, Chen M, van Dam RM. Long-term effectiveness of diet-plus- exercise interventions vs. diet-only interventions for weight loss: a meta- analysis. Obesity Reviews 2009, 10(3):313–323.

2.     Phillips B. Body for Life. New York, NY: Harper Collins, 1999.

3.     Paoli A, Marcolin G, Zonin F, Neri M, Sivieri A, Pacelli QF: Exercising fasting or fed to enhance fat loss? influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training. Int J Sport Nutr Exerc Metab 2011, 21(1):48–54.

4.     Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger JW, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition 2014, 11:54.