Is Fasted Cardio Necessary to Lose Weight?

One of the most frequently asked questions a person has when looking to transform their physique is if it’s necessary to perform fasted cardio in order to lose weight. 

You’ll find all sorts of answers across the cesspool that is social media fitness, but we’re here to separate fact from fiction and given you the actual science-based answer. 

The Theory of Fasted Cardio for Weight Loss 

The theory of why fasted cardio is better for fat loss is based around the concept that your body maximizes fat burning when it is in a fasted state. 

Now, most people assume “fasted” simply means they have an “empty feeling” in their stomach, but just because you don’t feel full does not mean that your body has completed the digestion and absorption of nutrients from your previous meal. 

In fact, it takes between 3-6 hours to fully digest and absorb all the nutrients from a meal and insulin levels to return to baseline. The amount of time it takes for this to occur depends on the size of the meal as well as the macronutrient composition of the meal. The larger a meal is in calories as well as the higher amounts of fiber, fat, and protein it has, the longer it will take to digest. 

During this time of digestion and absorption, insulin levels are elevated, and when insulin levels are elevated the body is using the food you just consumed for energy and storing any unused energy in muscle and fat tissue. That also means during this period of time, you’re not relying on stored energy, and thus fat burning is minimal. 

Based on this, it was believed that if you wanted to maximize fat burning (and subsequently weight loss) you should exercise in the fasted state, and since carbohydrates are highly insulinogenic (meaning they create the largest release of insulin in the body), consuming them prevents you from burning fat. 

Therefore, you should avoid carbohydrates or severely limit them if you want to lose fat as quickly as possibly. 

Of course this theory only holds water if you agree with the keto keyboard warriors who believe that carbohydrates and insulin are the causes of fat gain and the inhibitors of fat loss...which they’re not. 

Realize this, carbohydrates or not, if you are in an energy deficit consistently (regardless if you consume carbohydrates or not) you will lose weight. 

That is because, an individual’s energy balance (calories in vs calories out) is the biggest determinant in weight loss, not whether you had carbohydrates before your workout or not. 

The Truth About Fasted Cardio 

In an effort to put the fasted cardio myth to bed once and for all, “the hypertrophy doc” Brad Schoenfeld, PhD, conducted a study where subjects performed 1 hour of steady-state cardio three times per week in either a fed or fasted state.[1] 

At the end of the trial, researchers concluded: 

“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.”[1] 

The reason for this is that our bodies have various feedback mechanisms that regulate substrate utilization in the body. For instance, if you burn a greater amount of energy from fat during exercise (i.e. fasted cardio), the body compensates by oxidizing less fat for energy the rest of the day and burning a higher amount of carbohydrate.[2] 

Remember, calories burned from exercise is only a small percentage of the total energy burned in a day. How many calories you eat and the composition of those calories has a far greater impact on weight loss and body composition than whether or not you perform cardio fed or fasted. 

So long as you maintain a moderate and consistent energy deficit, you will lose weight. 

References 

1. Schoenfeld, B. J., Aragon, A. A., Wilborn, C. D., Krieger, J. W., & Sonmez, G. T. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1), 54. https://doi.org/10.1186/s12970-014-0054-7 

1. Paoli, A., Marcolin, G., Zonin, F., Neri, M., Sivieri, A., & Pacelli, Q. F. (2011). 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, 21(1), 48–54.