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Fasted Cardio: Unpacking the Science and Its Applications

November 1, 2023 / Uncategorized
Fasted Cardio: Unpacking the Science and Its Applications

Fasted cardio is a hot topic in the fitness world, but does it really offer an edge when it comes to fat loss? In this article, we’ll dive deep into what fasted cardio is, examine the science behind it, and explore its relevance in different training programs including contest prep, off-season, and lifestyle routines.

What is Fasted Cardio?

Fasted cardio is a workout strategy where aerobic exercises are performed on an empty stomach, usually after a period of fasting that most commonly lasts overnight. This approach is based on the principle that glycogen stores are low after an extended period without food, leading the body to utilize stored fat as the primary source of energy.

Mechanism of Action

In a fed state, your body has readily available glucose and glycogen (stored form of glucose) to fuel workouts. However, in a fasted state, these readily available energy sources are depleted. As a result, the body is thought to shift its energy source from carbohydrates to stored fat, increasing fat oxidation.

Timing and Duration

The most common time to perform fasted cardio is in the morning after waking up, following an overnight fast. The fasting period should ideally be at least 8 hours to ensure a shift towards fat metabolism. The duration of the cardio session can vary but often ranges from 20 to 60 minutes, depending on individual goals and tolerance levels.

Types of Exercises

The types of exercises performed during fasted cardio are typically low to moderate intensity to allow for sustained effort without the need for immediate energy from carbohydrates. Common exercises include walking, jogging, cycling, and elliptical workouts.

Who Uses It?

Fasted cardio is often popular among bodybuilders, fitness models, and athletes who are aiming for reduced body fat levels. However, it’s also used by regular fitness enthusiasts seeking fat loss benefits. Its popularity arises from the belief that it’s more effective for fat loss compared to cardio performed in a fed state.

By understanding the intricacies of what fasted cardio is, one can make a more informed decision about whether to incorporate it into different training programs, from contest prep to off-season and lifestyle-focused routines.

The Science Behind Fasted Cardio

Fat Oxidation vs. Overall Fat Loss

The concept that fasted cardio increases fat oxidation during exercise stems from the physiological shift toward using stored fat as fuel due to depleted glycogen levels. Research shows an increase in fat oxidation rates during fasted cardio when compared to fed-state cardio (Horowitz, Mora-Rodriguez, Byerley, & Coyle, 1997).

However, the critical point here is that fat oxidation does not necessarily equate to long-term fat loss. Fat loss is more influenced by your overall caloric balance—calories in vs. calories out—over an extended period. The acute increase in fat oxidation during a single session may not have a significant impact on overall body composition, particularly if the calorie intake later in the day compensates for the morning’s energy expenditure.

Meta-Analyses and Long-term Impact

Meta-analyses have been instrumental in providing a comprehensive view of the efficacy of fasted cardio. A meta-analysis by Schoenfeld et al. (2014) assessed the long-term effects and concluded that there were no significant differences in fat loss or body composition between fasted and non-fasted aerobic exercise. This implies that the overall energy balance is the dominating factor in determining fat loss, rather than the metabolic subtleties of fasted versus fed-state exercise.

Similarly, a study by Paoli et al. (2011) evaluated whether fasted cardio had any influence on weight loss and body composition and found negligible differences. The study concluded that when caloric intake and exercise intensity are matched, there’s little to no difference in the outcomes based on the timing of food intake relative to the workout.


The scientific evidence suggests that while fasted cardio may increase fat oxidation rates in the short term, this does not necessarily translate to greater overall fat loss over an extended period. Meta-analyses and long-term studies have generally found no significant difference between fasted and fed-state cardio for affecting body composition changes. Some athletes report higher energy levels and better performance when exercising on an empty stomach. However, this is more about personal preference and comfort rather than any metabolic advantage from being in a fasted state.