I’ve been practicing intermittent fasting for around 9 years, and I work out most days. Training in a fasted state feels great for me—as long as the workout isn’t too intense. For higher-intensity sessions, like HIIT & strength training, I make sure to hydrate with electrolytes, especially those rich in salt, and then recover with a high-protein meal a few hours later. This approach works well for me, though I’m an exercise enthusiast, not an elite athlete. I exercise mostly for health, but sometimes I do train for performance—like when we recently SUP across the English Channel to set a new Guinness World Record.
One thing I’ve learned? Despite what “the literature” tells you, it’s crucial to do your own research and listen to your body before committing to any approach.
I believe in sustainable habits, so I only adopt practices I can see myself sticking with over a 10-year period.
For performance-focused periods, I might shorten my fasting window to 12–16 hours daily to balance recovery and energy.
However most science does point towards not training in the fasted state for intense workouts and at an elite level. So this blog explores the benefits and potential downsides of fasted workouts, helping you decide if they might be a good fit for your lifestyle and goals.
When you exercise during a fasted state, your body will become better at using fat and ketones for fuel. When you exercise in a fed state, your body will become better at utilising carbohydrates for fuel. You should be metabolically flexible and be able to do both, which will improve your body’s ability to use each fuel. Key take-aways Train in both a fasted and a fed state to become metabolically flexible Going in and out of ketosis, known as “metabolic flexibility”, will improve your body’s ability to utilise more than one fuel
Why Train Fasted?
The core idea behind fasted training is that exercising on an empty stomach—often after an overnight fast—may help the body burn stored fat more effectively. Since glycogen (stored carbohydrates mostly in the muscles and liver) is lower after fasting, the body can turn to fat as a primary fuel source during low- to moderate-intensity exercise, leading to improved fat oxidation.
However, for higher-intensity workouts like strength training or intense cardio, the body needs a quicker fuel source, which is why some people might feel low on energy during these sessions if they haven’t eaten. If you find yourself dragging through high-intensity workouts or struggling to hit your performance goals, fuelling up beforehand may be a better approach.
For moderate or intense cardiovascular workouts, it’s a different story. Fat burning during these higher-intensity exercises tends to be similar whether you’re fed or fasted. That’s because intense aerobic exercise releases more fat into circulation than your body can actually use.
One study illustrated this by comparing a group of men who consumed either a placebo or carbohydrates before a moderate-intensity cycling session. The results showed that the group who consumed carbs not only performed better but also burned fat at rates similar to those who were fasted.
So, while fasted training may boost fat burning during low-intensity activities, it doesn’t offer the same advantage at higher intensities. Additionally, if you’re already on a low-carb or ketogenic diet, your body is likely using fat as its primary fuel, so fasted training may not provide much extra benefit in that context.
Benefits of Fasted Training
Increased Fat Oxidation: Training fasted can encourage fat utilisation, particularly for low-intensity or longer-duration activities. Low insulin levels after an overnight fast allow the body to access stored fat, which can be beneficial for those looking to burn fat.
Metabolic Flexibility: The ability to switch between fat and carb burning—known as metabolic flexibility—can be a valuable fitness tool. Fasted training helps train the body to tap into fats and ketones, while training fed improves carbohydrate utilisation. I find this balance helpful, especially as I alternate between health-focused workouts and performance-based training.
Endurance Benefits: Improving VO2 max, a key measure of aerobic capacity, has broad health implications, particularly as higher VO2 max is closely associated with reduced mortality risk. Studies underscore that VO2 max, along with grip and leg strength, is among the most powerful predictors of longevity, as it directly reflects cardiovascular and respiratory efficiency. Research highlights the importance of aerobic fitness and muscular strength in reducing mortality risk, making the boost in VO2 max from fasted cardio even more impactful for long-term health. Increasing VO2 max not only enhances endurance but may contribute to better health outcomes and extended lifespan, highlighting the potential of fasting combined with cardio for comprehensive fitness benefits.
Hormonal Benefits: Research shows that fasting significantly enhances growth hormone (GH) secretion by increasing both the frequency and amplitude of GH pulses, an adaptation that aids in fuel mobilisation and conserves muscle tissue during caloric restriction. One study found that fasting nearly doubles GH pulse frequency and peak amplitude, supporting tissue preservation by reducing muscle protein breakdown, while another highlights GH’s critical role in protein conservation under fasting conditions. When combined with exercise, particularly high-intensity or resistance workouts, the GH response is further magnified, as intense physical activity triggers even greater GH release. This compounded GH boost in a fasted, exercised state has substantial implications for metabolism and muscle preservation, potentially enhancing fat metabolism and supporting muscle maintenance during endurance training.
Minerals and Protein Timing - Essential Support for Fasted Training.
Mineral Loss Through Fasting
When fasting, the body begins to draw on stored minerals to sustain essential functions, leading to a gradual depletion. Daily mineral losses from fasting include:
Magnesium: Approximately 125 mg of magnesium is lost from bone each day.
Calcium: Around 100 mg of calcium is released from bone reserves daily.
Sodium: Roughly 2,000 mg of sodium is excreted in urine each day for the first 10 days, with additional chloride loss over a 30-day period. Even a daily intake of 2 grams of sodium may be insufficient to maintain balance, especially as more sodium is also lost through sweat.
Potassium: About 587 mg of potassium is lost each day from bodily reserves.
Mineral Loss Through Exercise
Exercise further increases mineral loss due to sweating, especially during intense or prolonged sessions. Typical mineral losses after one hour of exercise are:
Sodium and Chloride: Approximately 1,150 mg of sodium and 1,725 mg of chloride are lost through sweat.
Potassium: Roughly 140 mg of potassium is lost in sweat.
Magnesium: About 8-10 mg of magnesium is depleted through perspiration.
Calcium: Around 40 mg of calcium is also lost in sweat.
Total Daily Mineral Loss
When combining the effects of fasting and exercise, daily mineral depletion becomes significant:
Sodium and Chloride: Up to 3,150 mg of sodium (from fasting and exercise) and additional chloride losses.
Potassium: Roughly 727 mg per day.
Magnesium: Up to 135 mg per day.
Calcium: Approximately 140 mg per day.
Additionally, protein timing is crucial if you’re looking to build or maintain muscle. Studies suggest eating protein before 10:00 AM supports muscle synthesis and may improve hypertrophy, as early protein intake aligns with our body’s natural rhythm for muscle building. For me, I aim to get a protein-rich meal a few hours after intense workouts to kick-start recovery.
Downsides of Fasted Training
I have successfully been able this year prior to our SUP World Record attempt to add 6kg of muscle in 6 months of training whilst fasting 6 days a week. Great for me you might be thinking, however, like I mentioned before I have been doing this for 9 years now. So always work with your body, your goals and not what the ‘evidence’ shows too much.
Muscle loss:
Muscle loss can indeed be a concern for those training in a fasted state, especially when aiming to build muscle. Fasted cardio, for instance, has been shown to increase proteolysis, or muscle protein breakdown, as the body turns to protein stores for energy once glycogen is depleted. For high-performance athletes, ‘carbohydrate loading’ before training can slow this breakdown by maintaining glycogen levels. However, recreational athletes or those focused on fat loss do not necessarily need to load up on carbs before exercise, as other strategies can mitigate muscle loss without a high carbohydrate intake.
For those considering fasted strength training, the situation is more nuanced. While it’s true that fasted training can increase growth hormone (GH) levels, GH is primarily involved in energy redistribution rather than directly promoting muscle growth. In the absence of amino acids—particularly leucine, which is crucial for signalling muscle protein synthesis—fasted strength training can lead to muscle breakdown. Therefore, consuming a protein source rich in leucine, like whey, around your workout can prevent muscle catabolism and support muscle building, even without added carbohydrates. The insulin response from protein alone is sufficient to help preserve muscle tissue and avoid catabolism.
Studies indicate that while timing within each meal can influence short-term muscle protein synthesis, an overall high protein intake allows for flexibility. Moore (2019) supports the idea that the total quantity of protein plays a more critical role, showing that achieving a daily intake around 3 g/kg may alleviate the need for meticulous timing to optimise muscle gains
Importantly, while timing and distribution of protein intake can reduce muscle loss, as research suggests, the most effective strategy is often the one that aligns with an individual’s unique needs and goals. Some may thrive on fasted training with protein adjustments, while others might prefer pre-training nutrition to maximise performance and recovery. The key is to find what sustains your muscle growth, performance, and overall health best.
Stress on Stress
Fasting, exercise, and even caffeine are all forms of stress that, when managed properly, can induce a hormetic response—a beneficial adaptation where the body becomes stronger and more resilient. For example, stressing muscle fibres resistance exercise prompts them to repair, adapt, and grow back stronger. However, the body has a limited capacity to handle stress, and overloading it can lead to negative consequences.
Research shows that fasting significantly amplifies cortisol release, a marker of the body's heightened stress response to nutrient deprivation. One study found that fasting increased cortisol secretion nearly twofold, adding stress on top of any physical activity done in a fasted state. Exercise itself is another stressor, especially intense or prolonged workouts, which raise cortisol as the body shifts to handle the increased physical demand. This accumulation of cortisol can impede recovery, particularly in a fasted state. However evidence suggests that aligning TRE with the body’s circadian rhythm, particularly through morning eating windows, may help in maintaining a more balanced cortisol rhythm.
Wearable tech like Garmin and Whoop can be invaluable for monitoring the body’s recovery status by measuring heart rate variability (HRV), which reflects stress levels. HRV can decrease significantly during fasting, as shown in a study where a 48-hour fast led to parasympathetic withdrawal and sympathetic activation—clear markers of physiological stress. Additionally, exercising in a fasted state increases fat utilisation but may reduce endurance and deplete muscle glycogen, which can lead to fatigue if not managed carefully. Together, these studies emphasise the importance of balancing fasting and exercise, adjusting training intensity based on recovery data, and considering electrolyte and protein intake to prevent negative effects.
The goal is sustainable health and resilience—finding what best supports your individual needs while managing these stressors effectively.
Personalising Your Approach
Everyone responds differently to fasted training, so experimenting with both fasted and fed workouts is key. For most days, I stick with fasted, moderate-intensity workouts that support my health and fitness. But when training for an event or higher performance, I’ll often shorten my fasting window to ensure I’m getting enough fuel and recovery.
Final Thoughts
Fasted training offers unique benefits, from improved fat burning and endurance to greater metabolic flexibility. However, it’s essential to listen to your body, manage mineral intake, and time protein properly to get the most out of it. As with any approach, consider what feels sustainable and beneficial for your own goals.
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