The Health Benefits of Fasting – Is It Sustainable And Is It Safe?

The Health Benefits of Fasting - Is It Sustainable And Is It Safe?
Jaymes Gleeson


Dec 01, 2022

“The philosophy of fasting calls upon us to know ourselves, to master ourselves, and to discipline ourselves the better to free ourselves. To fast is to identify our dependencies, and free ourselves from them.” Tariq Ramadan

The long association between human beings and food has a common theme: scarcity. Our metabolic machinery has been moulded over thousands of generations by this overarching reality. Fast forward to the modern Western world; it also has a common theme: abundance. With bodily mechanisms programmed to expect (and thrive on) periods without food, times of cheap and omnipresent abundance can monkey-wrench our system. And so it has.

Fasting is one of the smartest strategies you could adopt. But does it work? Yes. Study after study shows that fasting protocols work very well for fat loss. Here are a few recent ones.

In non-obese patients, alternate day fasting increased fat oxidation and weight loss.

In obese patients, alternate day fasting was an effective way to lose weight; dietary adherence remained high throughout.

In young overweight women, alternate day fasting was just as effective as caloric restriction at causing weight loss, and adherence to the former was easier than to the latter.

This and plenty of other controlled clinical trial studies (take a look at PubMed for the latest) have proven it works for fat loss – but is it sustainable and is it safe?

I was a skeptic – an essential character trait for any researcher – and so, having been satisfied with its efficacy, set about an attempt to allay my fears or confirm my suspicions regarding its safety.

Reviewing the work of many authors and researchers of repute, Dr. Brad Pilon and Dr. Jason Fung being two important leaders in the field, I found some very illuminating information.


“Fasting also stimulates autophagy and mitophagy, the process of culling the old, dysfunctional mitochondria. So the ancient wellness practice of intermittent fasting essentially gets rid of the old mitochondria and at the same time stimulates new growth. This process of renewing your mitochondria may play a huge role in the prevention of many of the diseases we currently have no acceptable treatment – diseases of excess growth.” ~ Dr. Jason Fung

Concern #1: How Can Starving Yourself be Healthy?

Fasting and starvation differ in a crucial way: control.

Starvation is the involuntary absence of food: there is no control.

Fasting is the voluntary withholding of food: you have full control. Food is available but you choose not to eat it. You can start and stop at any time. There is no ‘correct’ duration. It could be 12hrs, 14hr, 24 hrs, or weeks on end.

Fasting has been interwoven throughout every culture and all major religions from time immemorial. It’s as much a part of being human as sleep.

Think about the term ‘break-fast’. Every night you are in a fasted state. You break your fast upon awakening or delay it until whatever time you choose. There is nothing strange or unusual about fasting. It’s an ancient and powerful dietary intervention yet modern people have forgotten its power and therapeutic potential.

Concern #2: Fasting and your metabolism

Dr. Brad Pilon writes that he was “startled to find that being in a fasted state for short periods of time does not decrease your metabolism.”

Our metabolism is based on the energetic costs of keeping the cells in our bodies alive. Your basal metabolic rate is the number of calories it takes to keep your body functioning at rest. Almost all of the calories you burn in a day result from your basal metabolic rate. Beyond that, the only significant way to increase the number of calories you burn in a day is to exercise.

The research on metabolism and calorie intake, identified by Dr. Pilon, is remarkably conclusive. Note the following studies that measured metabolic rate in people that were either fasting or on very low-calorie diets:

STUDY: Webber J, Macdonald IA, The cardiovascular, metabolic and hormone changes accompanying acute starvation in men and women. British Journal of Nutrition 1994; 71: 437-447

Researchers found that when they made people fast for three days, their metabolic rate did not change. Curiously, the ‘bro-science’ that tell us that eating numerous small meals throughout the day will increase your metabolic rate turn out to be untrue.

STUDY: Cameron JD, Cyr MJ, Doucet E. Increased meal frequency does not promote greater weight loss in subjects who were prescribed an 8-week equi-energetic energy- restricted diet. Br J Nutr. 2010 Apr;103(8):1098-101. doi: 10.1017/S0007114509992984. Epub 2009 Nov 30

Researcher Mark Sisson explains that “there is no metabolic advantage to eating multiple meals. Your body expends metabolic energy to process and digest food, but it doesn’t matter when or how it’s eaten. So, assuming macronutrient ratios and caloric content are identical, eating more frequently doesn’t make your metabolism “burn” brighter. If it did the study above would have ruled in favor of increased meal frequency as an effective tool in weight loss for obese patients.”

STUDY: Leidy HJ, Armstrong CL, Tang M, Mattes RD, Campbell WW. The influence of higher protein intake and greater eating frequency on appetite control in overweight and obese men. Obesity (Silver Spring). 2010 Sep;18(9):1725-32. doi: 10.1038/oby.2010.45. Epub 2010 Mar 25.

This recent study actually suggests that eating more frequently reduces measures of satiety and fullness in overweight and obese men while eating less frequent, higher-protein meals increase satiety and reduce hunger.

STUDY: Heilbronn LK, et al. Alternate-day fasting in non-obese subjects: effects on body weight, body composition, and energy metabolism. American Journal of Clinical Nutrition 2005; 81:69-73

Researchers here found that people who fasted every other day for a period of 22 days also had no decrease in their resulting metabolic rate.

STUDY: Keim NL, Horn WF. Restrained eating behaviour and the metabolic response to dietary energy restriction in women. Obesity Research 2004; 12:141-149

Researchers in this study found that women who ate half the amount of food that they normally eat for 3 days saw no change in their metabolism.

STUDY: Vorboeket-Van De Venne WPHG, et al. Effect of the pattern of food intake on human energy metabolism. British Journal of Nutrition 1993; 70:103-115

STUDY: Bellisle F, et al. Meal frequency and energy balance. British Journal of Nutrition 1997;77: (Suppl. 1) s57-s70

Both sets of researchers in these two studies found no change in the metabolic rate of people who skipped breakfast, or people who ate two meals per day compared to seven meals per day.

Dr. Pilon concludes that “food has virtually nothing to do with your metabolism. In fact, metabolism is much more closely tied to your body weight. If your weight goes up or down, so does your metabolism. The only thing that can affect your metabolism (in both the short and longer term) is exercise and weight loss. Even in the complete absence of food for three days your metabolism remains unchanged.”

Dr. Jason Fung, the author of the Complete Guide To Fasting, said that in his research, 4 days of fasting actually increased the body’s resting metabolic rate by 10%!

So, if food intake has no effect on the metabolic rate, what other myths have we been led to believe in ‘scientific facts’?

Concern #3: Fasting and Exercise

The Secret To Instant Self-Confidence1

During a period of fasting, the systems of your body are relying on fat and the glycogen that is stored in your liver for energy. Your muscles still have their own glycogen that they need for quick high-intensity exercises like weightlifting or sprinting.

STUDY: Knapik JJ, Jones BH, Meredith C, Evans WJ. Influence of a 3.5 day fast on physical performance. European Journal of Applied Physiology and Occupational Physiology 1987; 56 (4): 428-32

Researchers in this study found that a three and a half day fast caused minimal impairments in physical performance measures such as isometric strength, anaerobic capacity or aerobic endurance. In other words, they found that a three-day fast had no negative effects on how strongly your muscles can contract, your ability to do short-term high-intensity exercises or your ability to exercises at moderate intensity for a long duration.

STUDY: Knapik JJ, Meredith CN, Jones LS, Young VR, Evans WJ. Influence of fasting on carbohydrate and fat metabolism during rest and exercise in men. Journal of Applied Physiology 1998; 64(5): 1923-1929

Surprisingly, researchers found no change in soldiers’ metabolism who were exercising until exhaustion either right after a meal or after fasting for three and a half days.

From this research, we can see that you should be able to work out while fasted and not see any change in your performance.

The only situation where there may be a negative effect of fasting is during endurance sports like marathons or Ironman style triathlons, where you are exercising continuously for well over an hour. These types of competitions require the athletes to eat during the actual event in order to maintain performance.

However, Dr. Pilon cautions, “it should be noted that the “negative effect” that occurs from fasting before long endurance activity only affects that athlete’s time until exhaustion. So the amount of time an athlete can exercise while fasted before becoming exhausted is less than the amount of time it takes for a fed athlete to become exhausted. While the amount of time it takes before a fasted athlete becomes exhausted is decreased, it actually has positive effects on these athletes fat burning.”

Athletes performing long endurance activity while fasted actually burn more fat than athletes who are fed. So depending on your goals, fasting before endurance exercise may be beneficial.

Concern #4: Fasting and Muscle Mass

Boat Pose
Strengthening the abdominal and hip muscles as well as bringing focus to body awareness and breath.

The other great myth about dieting or fasting is that you lose your muscle mass when you diet. Fasting does not result in a loss of your hard earned muscle (as long as you are involved in some type of resistance training).

STUDY: Bryner RW. Effects of resistance training vs. Aerobic training combined with an 800 calorie liquid diet on lean body mass and resting metabolic rate. Journal of the American College of Nutrition 1999; 18(1): 115-121

Researchers found that even with a twelve-week long diet consisting of only 800 calories and only 80 grams of protein per day, the people in the study were able to maintain their muscle mass as long as they were exercising with weights three times per week.

STUDY: Rice B, Janssen I, Hudson, R, Ross R. Effects of aerobic or resistance exercise and/or diet on glucose tolerance and plasma insulin levels in obese men. Diabetes Care 1999; 22:684-691

Researchers found that men who restricted their caloric intake by eating 1000 Calories less per day than they normally ate for 16 weeks while undergoing a training program 3 days a week were able to maintain all their muscle mass while losing over 20 pounds of body fat. As long as you’re using your muscles, they will not waste away during short periods of fasting.

Concern #5: Fasting and Hunger

Most people get noticeably hungry if they have gone more than 2 to 3 hours without eating, but during this time, metabolically speaking, they are still in the fed state. This means that their bodies are still processing the food they ate at their last meal. In other words, there is still unused energy from their last meal in their system, and they are already feeling hungry enough to eat again. How can this be?

“Most likely”, says Dr. Pilon, “this is a learned response to a combination of metabolic, social and environmental cues to eat. It turns out the 10 billion dollars the food industry spends per year on advertising is very effective.” According to Brian Wansink, author of Mindless Eating, we make as many as 200 food-related decisions every day and are subjected to countless food advertisements.

Counterintuitively, when blood sugar levels are stable, periods of fasting have been associated with being more alert, ambitious, competitive and creative.

Fasting is a skill that improves with practice. It becomes easier to manage as your body gets used to the feeling of having a truly empty stomach. Hunger is like the tide: it comes in, and when you ignore it, it recedes.

I always ensure my clients have stabilised their blood sugar levels before commencing any type of fasting protocol. Stable blood sugar levels allow for much greater fasting control.

Not everything is for everyone. Fasting is not advisable for anyone who has suffered from an eating disorder, or children and pregnant women. Please seek professional advice before commencing if in doubt.

J.A Gleeson is a personal fitness instructor 


By Jaymes Gleeson


J. A Gleeson is a health writer for The Carousel and Personal Trainer at Tribe Social Fitness, in the Sutherland Shire, Sydney. He has over 25 years experience as an athlete, athletics coach, consultant, personal trainer, educator and independent researcher. Jaymes won an Athletics Scholarship and studied in the United States in 1991. - San Francisco State University (Psychology, Nutrition, Athletics) - American Collage of Sports Medicine (Personal Training) Throughout the 90s he worked as athletics coach and personal trainer in the US. In the early 2000s, he worked in Snow Sports throughout Japan and returned to Australia in 2008 to continue wellness research and personal training in high end health clubs in Sydney.



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