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Fasted versus fueled training has become a popular topic as exercisers want to maximize their goals in both performance and body composition. Does training on an empty stomach lead to increased fat burning? What about the effects on performance? Here we navigate when fasted versus fueled training may be most appropriate.

Defining the Fasted State:

A true fasted state means that the body has been unfed (no calories at all) for at least 8 hours. This typically occurs naturally over the course of sleep. If following an intermittent fasting plan, typical plans advise a minimum of 12 hours unfed.

What Does the Research Say About Training In Fasted vs. Fed States?

In general, it has been shown that fasting is associated with weight loss, however both lean mass (muscle) and fat mass decrease as a result. Research has investigated the outcomes of fasted versus fed state training in relation to variables such as short term endurance performance, weight changes, and substrate utilization. Most research shows that fasted training may be okay for short duration (<60 min), submaximal training. Fasted training may limit performance or cause muscle breakdown if training at high intensities or long durations (>90 min).

This 2020 review concluded that performing endurance training while fasted increases fat oxidation due to increased adipose tissue lipolysis, increased plasma adrenaline and cortisol levels and lower insulin levels. However, this study found that changes in body composition were predominantly linked to a calorie deficit diet versus the presence or absence of fueling prior to training. Both studies found that training in the fasted state is associated with decreased physical performance.

Another study shows that the downfall of fasted endurance training is the risk for protein degradation, or muscle breakdown. Training sessions lasting longer than 90 minutes without fuel is associated with increased amino acid (muscle) breakdown for fuel. Meaning that while weight loss may occur due to fasted training, it may be coming from muscle breakdown. Overall, studies show that more research is needed to identify the long term effects on fasted exercise.

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