Some athletes swear by fasted training, with others staying far away. Does skipping breakfast before a run really give you an edge? We’re here to help you dig into the science to determine whether hitting the pavement on an empty stomach is a smart move or just another fad.
Fasted training involves running without eating beforehand, typically in the morning after an overnight fast. The idea is that by training in a low-glycogen state, your body will adapt to use fat (rather than glycogen, or stored carbohydrate) as a fuel source more efficiently. Some studies have shown that fasted training can increase mitochondrial efficiency and enhance fat oxidation, which may be beneficial for endurance athletes. However, it’s important to note that increased fat oxidation doesn’t necessarily translate to better performance in shorter races where speed and glycogen stores are crucial. (Click here for a fuller explanation of the role of fat and carbohydrate in fuelling exercise performance.)
The pros: fat adaptation and mental toughness
One of the potential benefits of fasted training is improved fat metabolism, which can help runners become more “fat-adapted.” Some athletes feel this is useful in long-distance events, where preserving glycogen stores (i.e., fuel derived from carbohydrate, rather than fat) is key. Fasted runs may have the potential to enhance mental toughness. Running on an empty stomach can be challenging, and training in a state of discomfort may help you develop the grit needed for race day. However, these benefits come with a caveat: fasted training should be done strategically, and not as a daily routine.
The cons: performance and recovery risk
There are some risks to fasted training. Running without proper fuelling can lead to decreased performance, especially during high-intensity workouts. Fasted training might impair your ability to hit the necessary intensities during speed or tempo sessions, which are crucial for improving race performance. Fasted training can also increase the risk of muscle breakdown and hinder recovery, particularly if you’re not replenishing with adequate nutrition immediately after your run.
Women, be wary
Dr. Stacy Sims, a renowned exercise physiologist, warns that fasted training can be particularly challenging for women: “Women are more sensitive to calorie restriction, and fasted training can lead to hormone disruption and decreased performance,” she says. This is because women’s bodies respond differently to stress and energy deficits, making fasted training less beneficial for them than for men.
The verdict
Should you incorporate fasted training into your routine? The answer is maybe—with caution. Fasted runs can be a tool for building endurance and mental resilience, but they should be used sparingly. Reserve fasted training for low-intensity runs, and avoid it on days when you need to perform at your best by having at least a light snack before heading out for your morning run. Always listen to your body and make sure to refuel properly afterward to support recovery. It’s also great idea to check in with your favourite sports medicine professional (or ideally, a registered dietitian) to help you determine whether the occasional fasted run is a good fit for you.
