Fasted cardio is normally done first thing in the morning on an empty stomach. This is when insulin levels are low and your body is no longer processing food. Non-fasted cardio (FED) is performed when the body is still processing food; insulin levels will typically be higher depending on what you eat.
Are there benefits to fasted cardio?
Short answer: No
A study was done by the Journal of the International Society of Sports Nutrition, two experimental groups of females participated in the study as volunteers. Twenty healthy females participated altogether. The purpose of this study was to measure body composition changes associated with fasted versus non-fasted aerobic exercise. Their nutrition was self-reported on a regular basis for monitoring and they received nutrition counseling throughout the study period. Both groups did steady-state cardio three days a week for one hour. The study period lasted for four weeks.
According to the Journal of the International Society of Sports Nutrition (2014), a meal replacement was provided prior to exercise for the FED group or immediately afterward for the fasted group. All the volunteers received individual meal plans. The macronutrient consumption and total energy were the same for both experimental groups. Both experimental groups consumed around 1,250 calories/day; consuming only small differences in their percentage of dietary macronutrient intake.
After the study was performed, the findings indicated that body compositional changes between the two groups were very similar; there were no significant differences between the two study groups. Although I’m only citing one specific study for the purposes of this blog; there are many more showing the same results.
Cons of Fasted Cardio
1 Lower quality and less intense workouts. Fasting prior to training is like trying to drive your car 100 miles on an empty fuel tank; it just doesn’t work! Our bodies do not burn fat at the same rate as it oxidizes carbohydrates. Workout intensity declines as you burn through your fuel sources. If your goal is to lose body fat, training on an empty tank might not be the best approach. Recent studies show that high-intensity interval training (HIIT) is associated with a greater percentage of fat loss compared to longer duration cardio such as aerobics. Training on an empty stomach could negatively impact your ability to give 100% effort as you perform high-intensity cardio. Diminished effort results in fewer calories burned and negatively impact your metabolic rate.
2 You burn fewer calories. Exercise research explains this metabolic response as the body perceiving exercise coupled with lack of food as a ‘threat,’ whereby it preserves calories and reduces total caloric expenditure. When you eat the proper nutrients pre-workout you burn more calories during the post-exercise recovery period.
3 No extra fat loss benefits. This is supported through exercise research. Exercising before you eat is a nice idea that just doesn’t pan out because it ignores the dynamic nature of the human body.
4 High-intensity training on an empty stomach is a good way to lose the muscle you have worked so hard for! In order to maintain lean muscle mass, pre-workout nutrition is important. Fasting prior to exercise deprives your body of the nutrients that help your muscles perform and can have negative metabolic effects.
Ultimately, you want to do what is best for you and your body. All of us react differently to different training programs, foods, and supplements; what works for me may not work for you, vice versa. If eating before your morning session is not your thing because of your stomach or your schedule… that is okay! As long as you are aware that fasting prior to cardio could decrease your performance and has no added fat loss benefits. Take notes and pay attention to the potential side effects that fasted cardio may cause and keep track of your fitness progress as you go.
Working out on an empty stomach to maximize your fat burning potential works well ‘in-theory’…but unfortunately the body just doesn’t work that way. This is good news! Instead of depriving yourself of food before you exercise, focus on what you eat! Try consuming fewer carbohydrates while increasing your protein and dietary fats. Consuming the right foods prior to your cardio sessions has the potential to make you feel better, burn more fat, boost your performance, and speed up your metabolism.
Schoenfeld, Brad, et al. “Body composition changes associated with fasted versus non-fasted aerobic exercise.” Journal of the International Society of Sports Nutrition 11.54 (2014)