It sounds perfect: Work out on an empty stomach to maximize your fat burning potential. But does delaying breakfast until after your workout pay off in real life?
This article will address the pros and cons of fasting before your workout in an effort to help you answer that question.
Pros of Fasted Cardio
#1: Increased Fat Burning
Good news is that anytime you fast, whether simply by skipping meals or by pairing it with exercise, fat burning increases. After all, your body needs to get energy from somewhere and body fat is a great source. Unfortunately, the body will also degrade muscle mass for energy, so you lose some of that at the same time, which is never a good thing. Then there’s the fact that exercise, especially interval training, is a known catalyst for helping overweight people become metabolically flexible and fasting beforehand isn’t necessary to get the benefit.
Bottom Line: If an increased ability to burn fat is your goal, fasted cardio is not really the best way to do it. Try intermittent fasting or interval training instead.
#2: Better Insulin Health & Glucose Tolerance
A recent study found an increase in gene signaling that is associated with better blood sugar regulation and insulin levels in response to a walking protocol that was preceded by an overnight fast. The authors concluded that eating before exercise is likely to blunt long-term adaptations induced within fat tissue in response to regular exercise.
This is one of the first studies of its kind, so results should be interpreted with caution.
The Bottom Line: In light of the diabetes epidemic and all the problems people are having with poor insulin sensitivity, this could be a game changer by improving metabolic health.
#3: Increased Nutrient Absorption
After exercise, muscles are “hungry” for glucose, insulin sensitivity is maximized, and nutrients are better absorbed. Pair this with the fact that no one wants to exercise on a full stomach and it may make more sense to eat after a workout if you don’t have time to allow for digestion in between your meal and your exercise session.
The Bottom Line: If you have the option of eating either before or after, the argument could be made to save your meal for post-exercise, assuming the quality of your workout doesn’t suffer too much.
#4: Increased Blood Flow To “Stubborn” Fat Areas
A healthy blood flow is necessary for the first two stages of fat burning: Mobilization and transport of fatty acids. One possible reason that people who are lean have a trouble getting rid of “stubborn” fat in the upper leg and abdominal area is due to decreased blood flow to these regions.
The Bottom Line: Fasted training may increase blood flow to “stubborn” body parts and allow for oxidation of these remaining fat stores, allowing for you to get super lean.
Cons of Fasted Cardio
#1: Lower Total Calorie Burn
When you eat pre-workout you burn more calories during the post-exercise recovery period in both moderate- and high-intensity ranges. Scientists explain this as the body undergoing metabolic adaptation whereby it senses the combination of exercise coupled with lack of food as a “threat,” reducing total energy expenditure.
The Bottom Line: The ability to create a calorie deficit is what dictates whether you lose body fat. Therefore, the amount of calories you burn over the long run is what really matters, not how much fat you burn during exercise.
#2: Lower Quality Workouts
The body is not able to burn fat at the same rate as it can oxidize carbohydrates. This isn’t such a problem for your average person getting on the treadmill and walking for an hour, but it’s not a great idea for athletes or most people who actually want to accomplish something with their training.
The Bottom Line: Workout intensity decreases as you burn through your carb stores. Training on an empty stomach makes this happen that much faster, which is a major drawback since higher training intensities are associated with greater fat loss.
#3: No Greater Fat Loss
A series of studies show no greater fat loss when comparing training fasted or fed. Why? If you fast before exercise, you don’t burn as many calories over time. It’s the number of calories you burn that determines how much fat you lose.
The Bottom Line: Advice to exercise before you eat is a nice idea that doesn’t pan out because it ignores the dynamic nature of the human body.
#4: High Cortisol & Excess Stress
Cortisol is a hormone that releases energy stores when you are running low. Exercise elevates it as does fasting. Combine the two together and you have the perfect recipe for a sky-high cortisol level, which has seriously negative effects on anxiety, fat loss, and health.
The Bottom Line: If there’s any chance that stress is an issue for you, avoiding fasted exercise is a top priority.
Deighton, K., et al. Appetite, energy intake and resting metabolic responses to 60 min treadmill running performed in a fasted versus a postprandial state. Appetite. 2012. 58(3), 946-54.
Gillen, J., et al. Interval Training in the Fed or Fasted State Improves Body Composition and Muscle Oxidative Capacity
in Overweight Women. Obesity Journal. 2013. 21(11).
Paoli, A., et al. 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. International Journal of Sport Nutrition and Exercise Metabolism. 2011. 21, 48-54.
Schoenfeld, B., et al. Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition. 2014. 11(1):54.