Giulia Ralph is a qualified Nutritionist, Strength & Conditioning Coach, Weightlifting Coach, and Personal Trainer. She has a Bachelor of Health Science in Nutrition and a Post-Graduate Certificate in Human Nutrition.
Fasting has become a popular topic in recent years due to its potential health benefits and influence on body composition (6). Intermittent fasting can be followed through a range of different methods. A standard method is the 16/8 fast, which means you fast for 16 hours and have an 8-hour eating window. Fasting typically involves alternating between periods of eating and fasting. During fasting periods, no food is consumed, while during eating windows, food intake is allowed.
The most common approach is generally an overnight fast, or 18:6 fasting, defined as time-restricted eating or going for a certain amount of time without consuming food. Water is allowed during this time, as are other fluids such as coffee and tea that don’t contain any calories (13).
The rise of fasting has introduced various protocols, providing adaptable and versatile options. For example, the 15/9 intermittent fasting method, with its longer eating window, may be preferable for some individuals. While many people adopt fasting for weight loss, it’s important to note that the results often stem from a reduction in overall calorie intake.
However, whether long-term fasting and exercise are a good mix is a different area of the topic that deserves attention. Further, whether or not the benefits are all that people perceive is an area worth exploring.
Many factors can significantly influence physical performance, capacity, energy uptake, and recovery when working out. With this in mind, it’s essential to ensure that efforts taken around working out support the workout rather than diminish it. Intermittent fasting can be followed in various ways, from subtle to more intense, prolonged periods.
While it can be okay to work out while fasting, several factors will determine what kind of impact, whether positive or negative, will be experienced. Some key factors that should be considered include:
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While there may be a lot of hype around the potential benefits of exercise while fasting, it ultimately depends on how long you choose to fast, the time of day you’ll be working out, and the type of workout. Duration and intensity determine whether it’s wise to fast while working out.
For example, a 16:8 fasting schedule, where you fast for 16 hours and eat over 8 hours, can support energy levels. This fasting method works well as the fasting period is overnight, which allows a light to moderate workout in the morning. Easy exercises before breakfast, such as walking or jogging, should be fine without seeing a noticeable drop in energy levels.
However, there may be better options than fasting for more demanding physical activities, particularly strength training. Weightlifting or resistance training on an empty stomach may reduce energy availability. Furthermore, this could impact your ability to lift heavy weights or perform as many repetitions and sets. This limitation could impede your ability to achieve maximal strength gains and muscle adaptation.
Timing your workout after consuming your first meal can be important for activities that demand high intensity or endurance, such as high-intensity interval training (HIIT) or prolonged aerobic exercises. Ideally, this meal should be balanced, containing carbohydrates for energy and protein to support muscle growth and repair. This strategy enhances performance and supports efficient recovery, optimizing the benefits of your training efforts.
A study compared fasted vs. non-fasted exercise and its influence on body composition. Interestingly, results showed that maintaining a calorie deficit is the most significant influence on weight loss regardless of method (1).
When fasting, the body depletes its glycogen stores, the primary energy source during short-term, high-intensity activities. This depletion prompts the body to switch its energy source to stored fat, a process that is known as metabolic switching (9). However, this can take 12 to 36 hours to start after eating your last meal. While this may seem promising, it may not be realistic or effective as a stand-alone approach to burning fat.
Long-term fat loss is dependent on consistent dietary management and the integration of healthy lifestyle practices beyond fasting alone. Factors such as caloric intake, diet quality, physical activity, stress, and sleep all influence the success of fat loss and long-term weight management.
Although fasting has been acknowledged to support weight loss, more research on fat loss is needed. It’s uncertain whether fasting directly burns fat or if weight loss primarily stems from reduced calorie intake due to established eating windows. The multitude of fasting methods, together with a variety of influencing factors and study variables, complicates determining whether fasting directly impacts fat burning or results from a combination of factors (2).
Read more: Green Tea Intermittent Fasting: Benefits and Side Effects
The impact fasting has on muscle mass largely depends on the type of fasting, your overall diet, and your exercise routine. An adequate protein intake, together with carbohydrates and energy, is essential for muscle maintenance and growth. Optimally, protein should be distributed evenly throughout your daily meals to support muscle synthesis and recovery. Research supports the favorable effect of evenly distributed protein on muscle mass instead of skewed or inconsistent intakes (15).
Following a fasting schedule with a small eating window can make it challenging to consume enough total protein, potentially compromising muscle mass. In addition, a high-protein intake is generally encouraged to preserve muscle mass if in a calorie deficit or on a low-calorie diet. These points highlight the potential difficulties when considering fasting and maintaining muscle mass.
Regular resistance or strength training is another important aspect of stimulating the muscles. Therefore, muscle mass development and maintenance require nutrition considerations and appropriate physical stress applied through resistance training.
Muscle loss should not be a significant concern if you’re fasting for seven days using a schedule such as the 16:8 method. This approach means you’ll fast for portions of each day rather than continuously for an entire week. In summary, you may fast overnight and then consume enough food during your eating window each day to support muscle maintenance.
Fasting duration should always prioritize health and safety. Generally, extended or prolonged fasting such as over 24 hours is not recommended due to the potential health risks. Furthermore, individuals with specific health conditions may be at a heightened risk from fasting and should consult a healthcare professional. The evidence supporting the benefits of multi-day fasting varies, and it’s essential to evaluate the sources of such health claims critically. Specific populations, including pregnant or breastfeeding individuals, those who are underweight, and people with a history of eating disorders, diabetes, or who are undergoing medical treatment, should avoid fasting. Athletes should also approach fasting with caution to avoid negatively impacting or compromising their performance and recovery (3).
Potential side effects, such as headaches, dizziness, and bloating, can occur with fasting periods of 12-24 hours. You can learn more about intermittent fasting and bloating here. Overnight fasting, ranging from approximately 12-16 hours, is generally safe and can be incorporated without compromising diet quality or nutritional intake. It’s an effective and convenient way to incorporate fasting into your routine while also promoting optimal health.
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Many individuals may ask the question, should I work out fasted in the morning? Exercising on an empty stomach utilizes the body’s stored carbohydrates (glycogen) for energy. An important concept to be aware of is that carbohydrates are the body’s preferred and predominant energy source. Working out on an empty stomach is generally fine for low-intensity exercise, but it may not be ideal for longer sessions or high-intensity exercise.
As the body stores a limited amount of carbohydrates, high-intensity or endurance workouts may not be optimally supported once these sources are depleted. This limitation can lead to decreased performance, including the ability to sustain efforts, work rate, and power output. Fatigue may also be experienced earlier on, further impeding performance. This could also negatively affect the quality of the workout session and impair the achievement of physical adaptations or desired fitness goals.
Exercising during or immediately following a 72-hour fast is generally not recommended. Fasting for a prolonged period significantly depletes the body’s energy, as no food is consumed to replenish these stores. While some people may fast for extended periods as they seek health benefits, weight loss, or digestive rest, attempting physical activity during this time can be particularly challenging. This is due to the lack of energy and potential adverse effects on mood, cognition, and decision-making abilities (4). These variables highlight that extended fasting can compromise the safety and effectiveness of your workout. It’s sensible to avoid strenuous exercise until you’ve broken your fast and replenished your body’s nutrients.
Read more: Intermittent Fasting and Running: A Winning Combination or a Terrible Mistake?
While there are mixed opinions on this, recent research has highlighted that fasting may decrease testosterone levels. An 8-week study of resistance-trained males compared normal eating patterns to a time-restricted eating window. It aimed to determine the effects of intermittent fasting when combined with resistance-trained individuals. While the time-restricted group displayed a reduction in body fat mass, results also showed a reduction in testosterone levels. In addition, there was a decrease in insulin-like growth factor, another hormone that plays an important role in muscle growth and development (5).
Four other trials observed decreased testosterone levels in 8-hour time-restricted fasting interventions combined with regular resistance training (7). While more research is required, recent research has highlighted the potential negative link between fasting and testosterone levels.
While fasting can support weight loss, effectively burning and losing fat requires a comprehensive approach. Achieving sustainable fat loss requires maintaining a calorie deficit, getting adequate nutrition, and staying well-hydrated. Ensuring you get enough sleep and manage your stress levels are also influencing factors. Finally, regular exercise can help accelerate progress by boosting your metabolism.
How many minutes you work out while fasting will ultimately depend on the type of fast you’re doing and the intensity of the session. For example, a 16/8 intermittent fasting workout could still provide ample energy throughout the day, allowing high-intensity and longer-duration workouts to be performed safely. Other methods, such as the 17/7 fast, could also enable ample food intake and support moderate and high-intensity efforts.
The general guidelines recommend that adults should aim for at least 150 minutes of moderate-intensity exercise or 75 minutes of high-intensity activity each week. These activity targets can be distributed throughout the week for convenience. Considering this, engaging in a 30-60-minute fasting workout, such as a morning walk or a light jog, is entirely manageable. However, exercising after consuming a suitable meal for moderate to high-intensity workouts is advisable. Doing this supports adequate energy for the work required, supporting optimal performance and recovery (8).
Fasting is a versatile strategy that can be incorporated into a variety of different lifestyles. Weight loss can be experienced, but it should be noted that it is generally a result of reduced calorie consumption. While the effects of long-term fasting and exercise require more research for a thorough understanding, there are some key points to follow. If you choose to incorporate fasting into your schedule, you should choose a viable method that allows for adequate nutrition.
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