Fasted Weight Training: Is It Optimal?

Fasting – How it works

Fasting can be described as a dietary strategy that consists of small periods of eating separated by larger periods of not eating (fasting). The absence of calories (in such periods) can encourage alternative forms of cellular energy production (i.e., gluconeogenesis) and for cells to become more energy efficient. This physiological cascade can trigger additional processes such as autophagy (cleaning out damaged cells and regenerating others) which is why fasting grew in popularity rapidly despite the body of research on the topic still in its preliminary stages. 

In recent times, the proposed goal of fasting has drifted drastically, with many adopting fasting as a purported fat loss aid as opposed to longevity enhancement. With the explosion of fasting across the fitness community, fasted training quickly followed suit.

Why fasted weight training?

There are 3 main reasons why one would prefer to train in a fasted state.

What does the research say about fasted weight training?

Trabelsi and colleagues (2013) conducted a study with the purpose of evaluating the effects of resistance training in a fasted versus a fed state during Ramadan on body composition & metabolic parameters in a group of bodybuilders.

This study randomly allocated the bodybuilders into two groups (fasted or fed) during Ramadan. All individuals followed a hypertrophy-based training program at least 3 times per week for the entire duration of Ramadan (29 days). The researchers saw no significant differences on body mass or body composition between the groups of bodybuilders after 4 weeks. The researchers went on to conclude that resistance training did not significantly affect body composition nor immune or inflammatory markers when done in a fasted or a fed state.

Another study by Frawley and colleagues in 2018 examined the effects of a bout of resistance training in a fasted state compared to ingestion of a standardized meal on fat and carbohydrate utilization. In this study they found that individuals who performed fasted resistance training did not perceive the training sessions to be any more challenging compared to a fed state. Moreover, the researchers determined that fasted training happened at a significantly lower respiratory exchange ratio (RER), indicating that during fasted training individuals use more energy for fuel from fats than carbs to meet energy demands. Important to note, this doesn’t necessarily translate to greater fat loss since the body tends to compensate for substrate utilization, using more of the other substrate that wasn’t used as fuel throughout the day!

While the above studies suggest that intermittent fasting is not superior nor worse than fed training for overall body composition change, it Is important to note that other research does show that training in a fasted state can at times hinder performance. Several studies conducted on elite athletes (judo & soccer players) have shown how performance capabilities (speed, agility, endurance) become significantly impaired during fasted regimes. According to a review paper by Zouhal and colleagues, “fasting can elicit negative effects on performance in some events, and hence modifications to the training schedules may be required to minimize such effects.”

Conclusion

For most people (i.e., gen pop), training in the fasted or fed state isn’t going to matter too much. You’re unlikely to burn more fat training fasted, but you’re also not going to lose any more muscle either, compared to training when fed. However, for the athletes among us, we should be weary of the performance detriment that has been observed when training in the fasted state. While it might not matter too much for people just wanting to look better naked…a 5% performance drop off from training fasted could be the difference between a 1st and 5th place for a high-level athlete.

So… if you love fasted training go right ahead, this strategy won’t stop you from making progress (although it won’t enhance it either), but if you consider yourself an elite athlete and are looking to optimize the training process and your results, you might be better off having some food prior to hitting the iron!