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Does Fasting Cause Muscle Loss?


Intermittent fasting has revolutionised the way we approach weight management, insulin sensitivity, blood sugar regulation, and ketosis. Fasting is a free tool in which you need absolutely nothing to implement it, except the commitment to it. Intermittent fasting is not a diet, it is a lifestyle change which you can take anywhere with you.

However, one aspect that continues to confound many individuals is the question of whether or not fasting will cause lean muscle mass loss or not? This article aims to provide clarity on this question and offers comprehensive guidance on how to exercise during a fast to maximise benefits while also mitigating muscle loss. If we are talking about muscles, then we need to talk about protein intake, hormones, and exercise. All the key drivers for muscular hypertrophy.

The Crucial Role of Exercise During Fasting

Before we delve into the specifics of exercising during fasting, it's essential to emphasise that maintaining physical activity is of utmost importance during a fast, I mean probably as important as not eating during fasting! When you remain sedentary during a fast, your body interprets this as a signal of weakness and begins to consider your muscle tissue as a potential source of amino acids for glucose production (gluconeogenesis). This can lead to muscle loss, a common concern during calorie-restricted diets when performed alongside fasting.

Exercise is not just recommended during fasting; it's a non-negotiable!

Why You Should Exercise During a Fast

  • Signalling Strength: In our evolutionary history, fasting was closely tied to the necessity of hunting and gathering food. Remaining sedentary during a fast would have been a sure path to starvation. By engaging in physical activity, you signal to your body that you are capable and resilient enough to endure the fast effectively.

  • Preserving Muscle Mass: Without exercise, your body may perceive your muscles as expendable sources of amino acids for glucose production, leading to muscle loss. This phenomenon is a significant concern during fasting, given the inherent calorie reduction.

We have THREE GOLDEN RULES when fasting to avoid muscles loss:

1. Exercise when fasting – This involves any movement but especially weight training and cardio.

2. Consume optimal amounts of protein – 1.6 – 2.2g of protein per kg of body weight.

3. Optimise minerals - especially Magnesium and Zinc intake.

Benefits of Exercising During Short-Term Fasts

For short-term fasts, which include time-restricted eating with eating windows of 16 to 18 hours or fasts lasting 24 to 36 hours, continuing with your regular exercise routine is recommended.

Here are some key findings from human trials:

  • Muscle Preservation: In an 8-week trial with young men, resistance training (RT) was done with and without time-restricted feeding (TRF). TRF reduced daily calorie intake by 650 kcal but didn't significantly change body composition. The non-TRF RT group gained lean tissue. Both groups improved upper/lower body strength and lower body endurance, with TRF showing greater gains. Overall, TRF reduced calorie intake without harming lean mass or strength gains during short-term RT in young men.

  • Resistance Training Benefits: Women who engaged in resistance training on a time-restricted eating schedule (16/8) experienced significant muscle gains when they maintained adequate calorie and protein intake.

  • Enhanced Anabolic Response: Exercising in a fasted state, particularly in the morning, can lead to an enhanced anabolic response to post-workout nutrition. The results indicated that prior fasting may stimulate the intramyocellular anabolic response to ingestion of a carbohydrate/protein/leucine mixture following a heavy resistance training session.

  • Improved Power-to-Weight Ratio: A study with 16 elite under-23 cyclists found that a Time-Restricted Eating (TRE) group, consuming all daily energy in an 8-hour window, experienced weight loss, improved body composition, and increased peak power output per body weight. This approach may reduce inflammation and support the immune system, suggesting TRE's potential as part of a nutrition plan for endurance athletes.

Intermittent Fasting, Exercise and Its Impact on Hormones for Muscles and Health.

To further enhance the benefits of fasting and exercise, consider extending your fast for a few more hours after your workout. I would advise doing this a few times a week to make sure the hermetic spike does not become blunted. Extending your fast past your workout can amplify the release of growth hormone, a crucial element for fat burning and tissue maintenance. The relationship between growth hormone and testosterone is also a topic of interest as they are all key drivers for muscle hypertrophy.

Intermittent fasting can have an impact on hormonal balances in the body, usually when practiced in a caloric deficit, strenuous exercise, low protein intake or at the wrong time during a women's cycle. Intermittent fasting can counteract factors that can reduce testosterone levels, can also optimise the functioning of key hormones involved in testosterone production.

When discussing hormones like testosterone, many people assume it's primarily a concern for men. However, it's important to recognise that testosterone also plays a significant role in women's health.

Symptoms of low testosterone in women can present as:
  • Depression

  • Mood Swings

  • Low Libido

  • Difficulties In muscle gain.

Back to the hormones during intermittent fasting. One such hormone is luteinizing hormone (LH), which plays a pivotal role in testosterone synthesis in men and women. It triggers the Leydig cells within the testicles and ovaries to manufacture and release testosterone into the bloodstream. Consequently, regulating LH levels is essential for maintaining or elevating testosterone levels, and intermittent fasting can contribute to this process.

A study from 1989, published in the National Library of Medicine, demonstrated that intermittent fasting over short intervals can boost LH activity by 67% and post-testosterone response by 180%. You can make causative inference between how intermittent fasting indirectly benefits testosterone levels by enhancing the actions of hormones tied to testosterone production. We will get more into this in later after we deconstruct testosterone production.

Understanding Testosterone Production

We have briefly mentioned similarities in both men and women when it come to initiate testosterone production through signals from the brain. The Hypothalamus – Pituitary – Gonadal (testes or ovaries) Axis creates signals to instruct the ovaries in women and the testes in men to produce testosterone. However, the process differs between the sexes.

Testosterone Production In WOMEN
  • 25% of testosterone is produced in the ovaries.

  • 25% coming from the adrenal glands.

  • 50% originating from peripheral tissues.

These peripheral tissues, which can also cause inflammation and insulin resistance, play a crucial role in testosterone production. Hence, reducing overall inflammation becomes vital for women to maintain healthy testosterone levels by having optimal hormonal conversion into testosterone in the peripheral tissues.

Testosterone Production In MEN
  • 95% of testosterone is produced in the testes

  • 5% contribution from the adrenal glands.

This makes the testosterone production process more straightforward for men.

The Significance of Adrenal Health:

The role of adrenal glands in testosterone production varies significantly between men and women. With 25% of a women’s total testosterone being generated in the adrenals compared to just 5% in men, stress can have a more pronounced impact on testosterone production in women. Men, with their minimal adrenal contribution, are less susceptible to stress-related hormonal disruptions.

Menopausal women face additional considerations, as their ovaries shift their role during this period. Since a quarter of testosterone production occurs in the ovaries, menopausal women must take extra steps to support testosterone production from other organs during and after menopause.

Fasting for Hormonal Health:

Dr Mindy Pelz is a great resource when it comes to fasting for women’s health. I am going to share with you some of her fasting protocols to make sure optimal hormonal support.

Dr. Mindy's 3-3-1 Protocol for Testosterone:

- 3 days a week of Intermittent Fasting with Ketobiotic meals (50g protein, 50g net carbs, 60% calories from good fats).

- 3 days a week of Autophagy Fasting.

- 1 day a week dedicated to a feast day, focusing on foods that support progesterone, a hormone linked to testosterone. Options include pumpkin, squash, beans, seeds, and tropical fruits.

Intermittent Fasting and Its Impact on Hormones

We are going to move into how intermittent fasting also influences several other hormones that can positively impact testosterone production.

  • Human Growth Hormone (HGH): HGH and testosterone share a close relationship in terms of production and their influence on male and female physiology. HGH is decreases with age, as does muscle mass, collagen levels etc. HGH acts as a gonadotropin, stimulating the release of testosterone from Leydig cells. Research indicates that brief 24-hour intermittent fasts can lead to a substantial increase in HGH production, potentially resulting in a significant rise in testosterone levels.

Women typically make MORE HGH than men due to the oestrogen hormone. The more oestrogen levels in a woman, the higher the growth hormone. This means that when oestrogen levels decline significantly in women during menopause, HGH also reaches lower levels. HGH production is also high during the phases of menstrual cycles in women, where oestrogen is highest.
  • Insulin: Intermittent fasting can help mitigate the issue of elevated insulin levels upon waking in the morning. It also improves insulin sensitivity, reducing the amount of insulin required to regulate glucose and thereby preventing complications like Type 2 diabetes that can adversely affect testosterone levels.

  • Leptin: Often referred to as the "satiety hormone," leptin signals the brain to halt food consumption after meals. Higher body fat levels typically lead to increased leptin, which instructs the brain to stop eating. However, excess body fat can lead to leptin resistance, causing persistent hunger signals and overeating. Intermittent fasting can reduce leptin resistance, improving leptin sensitivity and curbing food cravings. This aids in weight loss and indirectly supports higher testosterone levels. Leptin rises in response to food like insulin and therefore this rise and fall is important to not get ‘leptin resistance’ cells.

  • Ghrelin: Ghrelin, known as the "hunger hormone," stimulates appetite and cravings when levels rise. Intermittent fasting helps decrease ghrelin levels, reducing appetite and making it easier to adhere to a diet and avoid overeating. This promotes healthy weight management, indirectly contributing to increased testosterone levels.

  • Adiponectin: Like leptin, adiponectin, released by fat cells, plays a role in metabolic functions, including insulin sensitivity. By enhancing insulin sensitivity, adiponectin encourages glucose absorption by cells and lowers blood sugar levels. This supports weight loss and boosts testosterone production. Intermittent fasting has been shown to enhance adiponectin activity, further improving insulin sensitivity, and indirectly fostering increased testosterone production.

In summary, intermittent fasting's positive impact on hormones like LH, HGH, insulin, leptin, ghrelin, and adiponectin contributes to the overall enhancement of testosterone levels, making it a potentially valuable strategy for those seeking to optimise their hormonal balance. This hormonal balance is also essential for muscle growth and maintenance.

Exercise During Fasts

  • Walking: During extended fasts, walking is a simple yet effective form of exercise. It keeps you occupied, especially when hunger strikes, and research suggests that fasted walking is not more stressful than walking on eating days. It's a low-impact way to stay active and maintain energy levels.

  • Weightlifting: Lifting weights is paramount for preserving muscle mass during long-term fasting. However, your approach should be adjusted:

    • Higher Intensity, Lower Reps: Lift at a higher intensity for lower reps to stimulate muscle without exhausting it. Need to leave reps in reserve.

    • Avoid Failure: Avoid lifting to failure or maxing out, as this can be counterproductive during fasting.

    • Full-Body Compound Movements: Focus on full-body compound movements to send a powerful stimulus to your entire body.

  • Zone 2 cardio: If you incorporate cardio during an extended fast, keep the intensity low to stay in the aerobic zone, primarily burning body fat. To determine your aerobic heart rate, subtract your age from 180. This ensures that you're not depleting glycogen stores or increasing sugar cravings.

  • Avoid High-Intensity Workouts: High-intensity endurance training, HIIT, or sprinting is best reserved for the end of your fast, with plans to break the fast shortly afterward. These activities can make fasting significantly more challenging and may lead to undue stress on the body, especially during longer fasts.

The Bottom Line

While there are no hard-and-fast rules for exercising during fasting, it's important to remember that you are still an individual with unique needs and preferences. However, here are some general recommendations:

  • For fasts lasting less than 48 hours, you can continue with your regular workouts.

  • For longer fasts, consider daily walks and weightlifting at least once or twice a week.

Naru Nutrition’s THREE GOLDEN RULES when fasting to avoid muscles loss:

1. Exercise when fasting – This involves any movement but especially weight training and cardio.

2. Consume optimal amounts of protein – 1.6 – 2.2g of protein/kg of body weight.

3. Optimise minerals - especially Magnesium and Zinc intake.

Remember that these are guidelines, and the key is to listen to your body. Everyone's response to fasting and exercise can vary, so adapt your routine to what works best for you.

In conclusion, exercising while fasting is not just safe but also highly beneficial for preserving muscle mass and optimising the effects of fasting. Walking, weightlifting, and low-intensity cardio are effective strategies for staying active during fasting periods. The profound connection between fasting, growth hormone, and testosterone further highlights the potential benefits of combining fasting and exercise for overall health and well-being.


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