Home General Dr. Jason Fung: Does Fasting Burn Muscle?

Dr. Jason Fung: Does Fasting Burn Muscle?

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Hello and welcome to the second part of my series on the topic of intermittent fasting. In my first article, I explored the benefits and drawbacks of fasting. If you haven’t read the first article, I recommend you do, since it is a good overview of most of the benefits of fasting and a few of the drawbacks.

Today, I would like to present a few more benefits of intermittent fasting, as well as discuss how you should expect to feel while following a strict intermittent fasting diet. I stumbled upon this video of Dr. Jason Fung a while ago and have been curious about how it went. He’s a professor of medicine at the University of Toronto and has a Ph.D. in endocrinology.

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He writes a blog called “BeyondtheFad”, where he talks about various health and fitness topics. This video was from a lecture he gave at The Canadian College of Naturopathic Medicine in Toronto. His work is entrenched in many of the most important health studies of the past 20 years, and the audience was full of physicians, nutritionists, dieticians, and other health-care professionals.

Dr. Jason Fung, author of the controversial book The Obesity Code, has developed a unique approach to weight loss, based on a concept called autophagy. Autophagy is a process your body uses to maintain muscle mass and repair damaged cells, and it’s typically associated with longer term weight loss rather than rapid weight loss.

In his book, Dr. Fung describes how he consciously induces autophagy by restricting calories during a 36-hour fast, and as a result, he claims that he is able to burn up to 500 calories a day without even exercising. So, does this work?. Read more about how to prevent muscle loss when fasting and let us know what you think.


Since the dawn of time, involuntary periodic fasting and its intentional counterpart, fasting, have been a part of human nature. Food was not always accessible until lately. To make it through difficult times, early humans had to store energy in the form of fat. We would have died a long time ago if we didn’t have an effective method to store and get food.

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Most human societies and faiths recommended deliberate fasting once the supply of food became more dependable. For example, it is claimed that Jesus fasted for 40 days and 40 nights, and many of his subsequent followers did the same without experiencing any ill effects. Many Muslims fast twice a week throughout the holy month of Ramadan and throughout the rest of the year.

Fasting was seen as a cleansing process with no fear of damaging muscular burning. Muscle mass was not harmed by repeated eating and fasting cycles. Traditional civilizations, such as Native American, Inuit, and African tribes, are described as vibrant and active, rather than hungry and feeble.

There are no pictures of listlessness or frailty in the accounts of contemporary Greek Orthodox Church members who fast for many days. Humans were probably certainly not designed to retain dietary energy as fat, yet when food was scarce, we burnt muscle. This includes everyone under the age of 20. The century that followed the Pir-Famine cycle was nearly entirely fat, whether due to intermittent fasting or fasting. Instead, they were svelte and powerful.


Repeated fasting does not result in muscle atrophy, according to new clinical evidence. Patients were able to shed substantial body fat with no change in lean mass in a 2010 trial of daily alternating fasts. The participants eat normally on food days and fast on fasting days under this regimen. Furthermore, numerous metabolic advantages, such as reduced cholesterol and triglyceride levels and a smaller waistline, have been discovered in addition to weight reduction.

An intermittent fasting approach was compared to daily calorie restriction, the conventional weight reduction method advised by most health professionals, in a 2016 research. Despite the fact that both groups dropped the same amount of weight, the intermittent fasting group lost just 1.2 kg of lean mass vs 1.6 kg in the calorie restriction group.

A comparison of the percentage increase in fat-free mass revealed that fasting raised it by 2.2 percent vs 0.5 percent in the calorie restriction group, suggesting that fasting may be 4 times better at preserving fat-free mass. It’s worth noting that the people who fasted shed more than twice as much harmful visceral fat.

Other significant advantages were also discovered in the same research. Intermittent fasting did not decrease the basal metabolic rate, while chronic calorie restriction did. Because hunger causes the body to produce counter-regulatory hormones, while prolonged calorie restriction does not, the body shifts to other fuel sources rather than shutting down.

Furthermore, prolonged calorie restriction, unlike starvation, raises the amount of the hunger hormone ghrelin. You are more likely to adhere to your diet if you feel less hungry while fasting than when on the CR. These two variables have obvious weight-loss advantages.


Despite concerns that fasting would result in muscle loss, years of human experience and clinical research indicate the exact reverse. Intermittent fasting seems to be more successful than conventional weight reduction techniques in maintaining lean tissue. When considering gluconeogenesis, this seems to be paradoxical at first. How is it better for muscle maintenance if intermittent fasting promotes gluconeogenesis (the conversion of protein to glucose)?

The fact that gluconeogenesis does not begin until 24 hours after the last meal is part of the explanation. Another factor is hormonal adaptation to hunger, which acts as a counter-regulatory wave.

Hormones that are anti-regulatory

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Insulin levels fall during fasting, while other hormones known as counter-regulatory hormones increase in response. These hormones are named from the fact that they operate in opposition to or against insulin. These counter-regulatory hormones decrease when insulin levels rise. When insulin levels fall, these hormones rise.

On the other hand, the effects on glucose metabolism are diametrically opposed. While insulin reduces blood sugar by promoting the storage of dietary energy, counter-regulatory hormones raise blood sugar by stimulating the utilization of stored food energy. Insulin causes glucose and fat to be stored in the body, whereas counter-regulatory hormones induce glucose and fat to be consumed.

The sympathetic nervous system, adrenaline and noradrenaline, cortisol, and growth hormone are the major counter-regulatory hormones. The fight-or-flight response is controlled by the sympathetic nervous system. If you are suddenly faced by a hungry lion, for example, your sympathetic nervous system is activated to ready you to fight or escape.

Your pupils dilate, your heart rate rises, and glucose is injected into your circulation to provide energy. Although this is an extreme example, a lesser type of sympathetic nervous system activation occurs during early fasting. As part of the body’s general activity, the hormones cortisol, adrenaline, and norepinephrine are released into the circulation.

Fasting, contrary to popular belief, does not paralyze the body, but rather pushes it to activate and prepare for action, even when done for extended periods of time. This is because these counter-regulatory hormones have a stimulating impact. Fasting for even four days increases resting energy use (or basal metabolic rate).

This is the heat-producing energy used by the body, brain, heart, liver, kidneys, and other organs. Studies indicate that after four days of fasting, the body needs 10% more energy for metabolism than it did at the start of the fast. Most people believe that fasting causes the body to shut down, but this is not the case. Fasting does not deplete your energy; on the contrary, it increases it.

The body simply switches its fuel source from food to stored food energy, commonly known as fat, while fasting. Consider us to be cavemen and cavewomen. It’s the dead of winter, and food is in short supply. We haven’t had anything to eat in four days. Finding and obtaining food becomes increasingly more challenging when our bodies begin to shut down. We’d be stuck in a never-ending cycle. We have a considerably tougher difficulty collecting the energy to hunt or gather every day we don’t eat. Our odds of surviving are dwindling by the day.

The human race would not have survived if this had happened. Our bodies aren’t that naive. Instead, our bodies switch fuel sources and provide energy so that we can eat enough. To hunt, we boost sympathetic tonus and norepinephrine, which raises basic metabolism. As a result, VO2, a measure of resting metabolic rate, rises.

Growth hormone is another noteworthy counter-regulatory hormone that rises substantially during hunger. Fasting for one day boosts growth hormone by a ratio of two to three, according to studies. Even after a 5-day fast, growth hormone production continues to rise.

This seems to be paradoxical at first sight. Why would we want to supplement our bodies if we aren’t eating? After all, human growth hormone (HGH) accomplishes precisely what its name implies. It instructs the body’s tissues to expand and grow taller. Why grow if there are no nutrients?

Allowing our bodies to go through the entire cycle of eating and fasting provides the solution. Glucose and amino acids are absorbed and delivered to the liver when we eat. Insulin is secreted, telling the body to save the energy (calories) it receives from meals. This is government property. All of the body’s tissues consume glucose, while the remainder is stored as glycogen in the liver.

Blood sugar levels begin to decline a few hours after a meal, leading insulin production to diminish and a condition of hunger to develop. As previously stated, the body adapts to a condition of hunger or malnutrition in a predictable manner. To convert glucose to energy, liver glycogen is recruited and broken down. Gluconeogenesis is the process of turning some proteins into glucose.

The body starts to shift its metabolism from glucose to fat. Growth hormone levels rise during this time, but little protein is generated due to low insulin and mTOR levels. As a result, despite the high amount of GH, no actual growth occurs.

Your body returns to a state of satiety after you have eaten or broken your fast. Growth hormone levels are high after extended fasting, and since amino acids are plentiful after a meal, our systems create all the proteins required to replace the proteins that have been damaged. Insulin promotes the production of proteins.

The body now possesses high amounts of insulin, growth hormones, amino acids, and glucose for energy, all of which are required to make or repair proteins in their refined form. This is a regeneration process, similar to autophagy, in which the body mainly eliminates superfluous proteins while rebuilding the most essential. Fasting rejuvenates the lean tissue in this way.


When you’re hungry, your body prioritizes a number of things. Maintaining sufficient glucose levels for optimal brain function is the primary concern. As the liver and muscles convert to fatty acids and the brain to ketones, the demand for glucose is substantially decreased. The quantity of glycerol from fatty acids that can be converted to glucose is restricted. The remainder must come through gluconeogenesis, which implies a tiny quantity of protein must still be broken down.

This protein, however, is not unique to muscle cells. The proteins that are regenerated the quickest, on the other hand, are broken down first to produce glucose. These include the skin and the mucosa of the intestine. Despite the fact that my rigorous diet program with therapeutic fasting for weight loss has been operating for more than five years, I have not had to send a single patient for skin removal surgery, despite the fact that they have dropped more than a hundred pounds.

This helps to explain why there was an anti-inflammatory impact in the clinic. Muscle cells that do not replenish themselves often are mostly unaffected. Protein catabolism is decreased from 75 grams per day to only 10-20 grams per day on average. When you’re fixed for a long period, this enables you to conserve protein.

Is this protein deficit, however, really serious? It isn’t required. When a thin individual is compared to an obese person, the fat person is believed to contain 50% more protein. All superfluous skin, connective tissue that retains fat cells, blood arteries that add volume, and so forth. Take a look at this photo of a World War II survivor of a Japanese POW camp. Is this body covered with extra skin? No, all that excess protein was either burnt for energy or used to support other, more essential tasks.


Perhaps it’s the power of autophagy, a cellular recycling mechanism with significant health implications. The nutritional sensor mTOR is reduced during famine, which always involves protein shortage, encouraging the body to eliminate old, non-functional subcellular components. After a meal, the body creates a new protein to replace the old one in a complete circulatory cycle.

The old components are re-synthesised instead of being kept. Anti-aging is the process of replacing old components with new ones. Furthermore, many illnesses of old age are characterized by excessive protein as well as fat development. Alzheimer’s disease, for example, is characterized by an overabundance of proteins in the brain that prevent normal signal transmission. Cancer is the result of the growth of numerous things, including a variety of proteins.

Between lean and obese individuals, there is a substantial variation in protein metabolism. Overweight individuals burn 2 to 3 times less protein than thin persons during a prolonged fast. That’s logical. People’s bodies utilize more fat when they have more fat to burn. People must rely on protein if there is less fat.

This is true not just for people, but also for animals. It was discovered more than a century ago that animals with a high body fat content (mammals, geese) had a lower percentage of energy from protein than lean animals (rodents, dogs). You will utilize more fat if you have more. Obese individuals lose protein more slowly than thin ones, despite having a higher overall protein intake.

Protein will provide almost 40% of the energy needed during a lengthy fast for someone with a BMI of 20 (borderline underweight). When compared to someone with a BMI of 50 (obesity), who can only obtain 5% of their energy from protein reserves. This demonstrates our bodies’ innate capacity to survive. We utilize fat reserves if we have them in our bodies. We don’t have them if we don’t have these businesses.


The quantity of protein required when fasting is determined on the underlying illness. Fasting is particularly helpful if you are overweight, since you burn much more fat than protein. If you’re skinny enough, starvation may be counterproductive since you’ll burn more protein. It may seem self-evident, yet our bodies are much more intelligent than we give them credit for. He can utilize food and fasting to his advantage. It is presently unclear how the body can adapt.

Fat oxidation contributes for approximately 94 percent of energy consumption during extended fasting in obese individuals, compared to 78 percent in lean people. Because there are little carbohydrate reserves in the body beyond the first 24 hours or so, the remainder of the energy comes from protein oxidation.

Other distinctions exist between slim and obese individuals. Ketone production increases considerably more quickly in lean individuals than it does in fat ones. It’s simple to comprehend. Thin individuals burn proportionately more protein than obese ones, thus they transition to fat metabolism much sooner, conserving protein.

– Jason Fung, M.D.

Also available on idmprogram.com.

From the standpoint of a clinical endocrinologist, there are two very simple ways to lose weight: caloric restriction and exercise. Both of these strategies are based on the fact that the body is a fat-storing, fuel-burning machine, that will use fat as a source of fuel during times of energy shortage (e.g. fasting) or when it is burning more calories than it takes in. In fact, the basis of the caloric restriction diet is that the body will use its stored fat as a fuel source to keep the metabolic rate elevated until it once again gets sufficient calories from food.. Read more about does intermittent fasting cause muscle loss and let us know what you think.

Frequently Asked Questions

Does fasting destroy your muscle?

No, fasting does not destroy your muscle.

Does dry fasting burn muscle?

Dry fasting does not burn muscle, it just reduces the amount of water in your body.

Does fasting in Ramadan make you lose muscle?

Yes, fasting in Ramadan can make you lose muscle.