“I gained an inch and a half on each of my thighs, lost an inch on my waist, gained an inch and a half in my chest, lost 1.5% body fat, and now spend $100 less per week on groceries.”
This was a testament of a competitive bodybuilder to a large group after learning more about the stages of fasting by hour and trying intermittent fasting for nearly 3 months.
How in the world could this happen?
How could a bodybuilder actually gain muscle while condensing his eating window? Here’s a hint….It wasn’t the class he attended that sparked him to try this; it was reading the book Fast Like a Girl (highly recommended if you haven’t already read it).
Let’s dive in and look at some of the benefits, strategies, and different stages of fasting by hour?
Some food for thought
Where does your body direct your energy when you eat?
The digestive, repackaging, and excretion processes associated with eating. Think hormones, neurotransmitters, HCL, pepsin and digestive enzymes, gut microbiota, chewing, swallowing, peristalsis, glucose, free fatty acid, amino acid, vitamins, mineral transport and storage, etc.
All of this takes energy.
Now, where does your energy and prioritization go when you don’t eat (or fast)?
As Dr. Valter Longo discovered years ago, your energy, in essence, goes toward recycling.
In other words, getting rid of the junk and bringing in the good stuff.
As of October 2024, there are currently 2596 studies that come up with keyword “intermittent fasting” on pubmed dating back to a 1946 study by Carlson and Hoelzel. Depending on several baseline health factors, food choices, and the length of the fast, here are just a few of the potential benefits researchers have found as a result of fasting:
- Inflammation down. Check.
- Mitochondrial efficiency and biogenesis. Check.
- Increased growth hormone. Check.
- Autophagy. Check.
- Improved blood sugar. Check
- Ramped up ketone production. Check
- Increased thermogenesis and Brown Adipose Tissue (BAT) activation. Check
- Stem cell activation (refeeding is essential here as well). Check
- Improved gut health. Check..
- Improved cognitive function and brain health. Check.
- Improved cholesterol and triglycerides. Check.
- Improved mood. Check
- Increased energy. Check.
- Better cardio. Check..
- May improve sleep. Check.
- Improved body composition and decreased visceral fat. Check.
- Not hitting the panic button when you don’t have food readily available. Check.
- Improved weekly grocery bill:)?
A major factor in seeing these benefits is hitting the switch, specifically the metabolic switch. Switching from using glucose as a fuel source to fatty acids and ketone bodies. This results in a better ratio of oxygen used to carbon dioxide produced (respiratory-exchange ratio) and greater metabolic efficiency and flexibility. This means your body can be trained to switch from glucose (when present) utilization to fatty acid utilization more efficiently. (More on the potential benefits of ketone bodies in a little bit).
But is there a minimum fasting duration needed for these benefits? Are there different stages of fasting by hour?
While fasting is not a one-size-fits-all, research shows that different fasting lengths impact different physiological parameters. While our genetics, age, gender, body composition, hormonal profile, food choices, etc, may affect the results, the available research on the topic may provide some insight into the potential benefits of different fasting lengths.
Let’s look at the potential impacts of the different stages of fasting by hour.
- ***Before we begin, here is a simple tip if you are interested in starting a fasting journey. I learned this from Dr. Longo a few years ago. Start slowly by shortening the goalposts. For example, if your first meal is at 7:00 AM and your last meal is at 7:00 PM, this equates to 12 hour eating window and 12 hour fasting window (12/12).
- To slowly shorten the goalposts, eating the first meal an hour later (8:00 AM) and the last meal an hour sooner (6:00 PM) will create a 10-hour eating window and a 14-hour fasting period.
- Once the body has become accustomed to this, the goalposts can be slightly adjusted again, perhaps the first meal at 9:30 AM and the last at 5:30 PM. This creates an 8-hour eating window and a 16-hour fasting period.
Photo by Gene Gallin on Unsplash
What are the stages of fasting hour by hour?
Stages of fasting by hour: 8-Hour Fast
If you are sleeping nearly 8 hours per night or sleeping six and not eating a couple of hours before bed, you are already hitting this mark. An 8-hour fast (or 8/16 time-restricted eating model) starts the process of breaking down glucose that is stored in your muscles and liver (glycogen). Depending on your metabolic/ketogenic flexibility, you can also start to break down triglycerides into fatty acids and glycerin.
Your hormones may change; insulin drops, glucagon, and insulin begin to rise, and cortisol increases. Blood pressure and resting heart rate may also slow down a bit.
Now, if I could fast a couple of hours longer….
10-14 Hour Fast
A common question regarding the different stages of fasting by hour is: how many hours into fasting does fat burning start?
A 2019 New England Journal of Medicine study found that “energy restriction for 10 to 14 hours or more results in depletion of liver glycogen stores and hydrolysis of triglycerides (TGs) to free fatty acids (FFAs) in adipocytes (2)”.
Once those free fatty acids enter the bloodstream, they are transported to cells in the liver called hepatocytes, which then form the ketones B-hydroxybutyrate and acetoacetate. These ketones can then enter the cells, where they become a part of ATP synthesis, providing you with more energy.
Something else these free fatty acids do is they trigger the release of two proteins that regulate gene expression, which then triggers the release of something called FGF21 (a growth factor (hormone) that plays a role in regulating metabolism, energy homeostasis, metabolism, and adaptation to the cold energy restriction.
Image by healthguru from Pixabay
One of the benefits of ketone bodies is that they are a very efficient fuel source. The average human uses roughly 50kg of ATP per day.
- 100 g of d-3-hydroxybutyrate yields 10.5 kg of ATP (22 ATP per molecule of d-3-hydroxybutyrate)
- whereas 100 g of glucose only yields roughly 8.7 kg of ATP.
The benefits of ketone bodies can stretch beyond a fuel source. They can regulate genes and proteins in the body that impact overall health and longevity. A few examples of these include:
- Peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α): a transcriptional coactivator (involved in gene expression) that helps to regulate energy homeostasis.
- Fibroblast growth factor: a protein that plays a significant role in development, repair, wound healing, blood vessel formation, metabolism, and more.
- Nicotinamide adenine dinucleotide (NAD+): a coenzyme that plays a major role in energy production, metabolism, DNA repair, cellular regeneration, brain health, and more.
- Sirtuins: the “what doesn’t kill you makes you stronger genes” that have been linked to hormesis. These play a significant role in cellular stress response, mitochondrial function and biogenesis, inflammation, cellular metabolism, and more.
- Brain-derived neurotrophic factor (BDNF): this has been called “fertilizer for your brain.” BDNF has been linked to memory, stress resilience, brain function, mood regulation, and more.
12-16 Hour fast
Image by huumsauna from Pixabay
A 14-week study wanted to test the weight loss effects of two different stages of fasting by hour: a 12-hour fast vs a 16-hour fast. Subjects consumed the same amount of calories and worked out the same amount, but one group had four more hours of eating autonomy.
The researchers found that the subjects with shorter eating windows burned more fat per day (15g on average/day) while maintaining lean muscle mass (14).
Around this time, thermogenesis (body heat production) can start to ramp up. A 2019 paper found that Intermittent fasting can also increase thermogenesis through its impacts on Brown Adipose tissue (BAT), which, in essence, promotes ATP production and increases energy expenditure (5) If you are working out with weights, you may not need to worry about losing muscle mass as you go further into different stages of fasting by hour. In 2020, Australian researchers found that male subjects who fasted daily for 16 hours while resistance training not only lost fat but were able to maintain muscle mass despite the shorter eating windows.
Stages of fasting by hour: 17+ Hour fast
Autophagy, from the Greek words “auto,” which means “self,” and “phagein,” which means “to eat.” According to the Cleveland Clinic,
- “Autophagy (pronounced “ah-TAH-fah-gee”) is your body’s process of reusing old and damaged cell parts. Cells are the basic building blocks of every tissue and organ in your body. Each cell contains multiple parts that keep it functioning. Over time, these parts can become defective or stop working. They become litter, or junk, inside an otherwise healthy cell.
- Autophagy is your body’s cellular recycling system. It allows a cell to disassemble its junk parts and repurpose the salvageable bits and pieces into new, usable cell parts. A cell can discard the parts it doesn’t need.
- Autophagy is also quality control for your cells. Too many junk components in a cell take up space and can slow or prevent a cell from functioning correctly. Autophagy remakes the clutter into the selected cell components you need, optimizing your cells’ performance.”
- While the different stages of fasting by hour and other factors play a role in initiating autophagy, author of Fast Like A Girl, Dr. Mindy Pelz, suggests it begins around the 17-hour mark and ramps up significantly from there (24 hours +).
24+ Hour fast (Alternate-Day Fasting)
Here is where even more of the magic seems to happen. Depending on several factors, including metabolic flexibility, diet, metabolism, age, and more, once muscle glycogen has been depleted, Autophagy may be in full force, reaching a peak at roughly 48-72 hours.
According to this Zero Longevity article written by researcher Rich Lafountain PhD, “Autophagy is not an on/off switch — instead, it is much more similar to a dimmer that gradually illuminates as your fast duration increases past 16 hours of fasting. According to clinical research, your fasting autophagy dimmer switch is not at peak “brightness” until at least 36–72 hours into a fast.”
Along with the benefits of Autophagy, researchers have also found something else remarkable happens: stem cells become re-activated.
Whether through bone marrow aspiration, PRP (platelet-rich plasma), adipose tissue extractions, or injections, stem cells have been used to treat injuries and dramatically improve the healing process.
In 2017, researcher and Director of the Columbia Stem Cell Initiative Emmanuelle Passegué and his team published a study in Nature, that found that fasting, then the resumption of eating, ramped up the regenerative potential of the stem cells.
The researchers found that the refeeding process after fasting rejuvenated the older stem cells’ youthful metabolic capacities. This makes sense as the stem cells become active during the fasting period and can then use the nutrients from refeeding to repair and regenerate tissue, etc.
Along with the benefits of blood sugar, tissue repair, and inflammation, a 24-hour fast can also have significant impacts on cardiovascular health. A 2013 study found that alternate-day fasting (normal eating followed by a 24-hour fast, etc.) for 8-12 weeks led to
- 15-30% drops in triglycerides
- 20-25% drops in LDL cholesterol
Another study, this one from 2015, looked at the impacts of alternate-day fasting over a longer period (12-24 weeks) and saw:
- Up to 50% drops in triglycerides
- And up to 20% drops in total cholesterol
Other benefits of 24-hour fasts may include better running endurance and improved balance and coordination. According to a 2024 Yale Medicine article, SIBO may be something worth investigating if you get bloating after eating certain foods. In this video, Dr. Mindy Pelz discusses the potential benefits of a 24-hour fast on SIBO.
Stages of fasting by hour: 36 Hour Fast
Are you interested in losing belly fat?
A 2019 study titled Alternate Day Fasting Gets A Safe Bill of Health looked at the benefits of 36-hour fasts followed by 12-hour eating windows. The researchers saw a moderate to significant decrease in belly fat, and surprisingly, they found that ketone production continued, even throughout the eating window.
What about ramping up the “fountain of youth hormone” growth hormone?
Image by Rudy and Peter Skitterians from Pixabay
Did you know that fasting can also increase GH levels?
Research has shown that it is during these stages of fasting by hour that growth hormone levels begin to spike significantly.
A 2021 paper from Endocrinology and Metabolism stated, “In humans, fasting for 37.5 hours elevates basal GH concentrations by 10-fold and reduces metabolic clearance rate of GH ” (6).”
An earlier paper, this one from 1988, looked at the impacts longer fasts had on Growth Hormone secretion. The researchers found that “While the increase in integrated GH concentrations had occurred by the first fasted day, there was a further increase in the nonpulsatile component of GH release from fourfold on the first fasted day to 10-fold on the fifth fasted day (3).”
One of the earliest studies on the hormonal impacts of different stages of fasting by hour came from the University of California at San Francisco. While this study only used a single 41-year-old male subject, it still revealed some interesting insight into fasting, blood sugar, insulin, and growth hormone.
The subject fasted for 40 days (religious purposes), while the researchers continually monitored levels. Aside from a 46% drop in blood sugar and a 79% drop in insulin, the researchers saw a 300% increase in growth hormone after just 5 days of fasting. By day 12, growth hormone spiked 462%, and by day 26, it peaked at a 1251% increase. While this article is not advocating for an extended fast like this, the research findings did make for an interesting discussion.
Stages of fasting by hour: 48 Hour Fast
In 2016, a study on the impacts of 48 hours of fasting in Amateur Weightlifters wanted to examine if extended fasting would impact brain health, mood, and nervous system functioning.
Three of the significant findings from the study include:
- The extended fasting period led to significant improvements in parasympathetic nervous system activity. The parasympathetics were balancing out the sympathetics, potentially increasing stress resilience.
- 48-hour fasting decreased resting frontal brain activity and “improved prefrontal-mediated cognitive functions such as mental flexibilty and set shifting (9).”
- No effect on hippocampus (memory) related cognitive performance after fasting.
This is significant because fasting can not only provide the recycling benefits of Autophagy, the restoration and repair benefits of rejuvenated stem cells, and the energy boost from increased mitochondrial efficiency, but it can also positively impact the brain’s memory and information processing circuitry.
72 Hour+ Fast: Taking stages of fasting by hour to another level
Recently made more popular by UFC President Dana White and health guru Gary Brecka, extended fasting periods have existed for thousands of years. A 2022 paper in the scientific journal Nutrients traces the historical importance of fasting back to Hippocrates, stating that:
“Hippocrates, who is considered the father of modern medicine, once wrote, “To eat when you are sick, is to feed your illness”. The ancient Greeks believed that fasting improves cognitive abilities. Plutarch, an ancient Greek writer wrote, “Instead of using medicine, better fast today”. Other intellectual figures were also strong proponents of fasting. Philip Paracelsus, the founder of toxicology and one of three fathers of modern Western medicine (along with Hippocrates and Galen) wrote, “Fasting is the greatest remedy—the physician within”. Benjamin Franklin (1706–1790), a founding father of the United States, wrote of fasting, “The best of all medicines is resting and fasting (13)”.”
Aside from all the scientific evidence we’ve already seen regarding fasting benefits, the abundance of anecdotal evidence surrounding a three-day fast includes:
- Significant weight and fat loss
- Healthier looking skin
- Improved cognitive function decreased brain cloud
- Injury and pain management
- Increased energy
For more information, check out this Mens Journal article on Dana White’s experience with a 3-day fast (or check out his social media video on it).
Now, what if you don’t want to fast completely, is there a way you can mimic a fast and still eat?
Well, I’m glad you asked.
Let’s delve into the Fast Mimicking Diet program.
The Fast Mimicking Diet Program
After researching survival factors in yeast, USC biochemist and researcher Dr. Valter Longo began discovering the impacts fasting and nutrition had on aging. He published his groundbreaking paper on longevity and stress resilience in the April 2001 edition of Science Magazine.
After years of diving deeper into research on the benefits of fasting and the stages of fasting by hour, he concluded that certain diets and caloric restriction/fasting strategies could rejuvenate stem cells, enhance Autophagy, and promote longevity.
Dr. Longo created the five-day Fast-Mimicking Diet, a five-day caloric-restrictive diet that mimics the effects of a water fast while providing necessary vitamins, minerals, and nutrients. Essentially, the diet tricks the body into thinking it’s fasting while providing these valuable nutrients.
Due to his breakthroughs, Dr. Longo was recognized as one of Time Magazine’s most influential medical professionals of 2018. He has since become the Director of the Longevity Institute at the University of Southern California and one of the global leaders in the science of aging and age-related disease.
The Fast Mimicking Diet has been the subject of several studies, ranging from adjunctive cancer therapy and type 2 diabetes care to neurodegeneration, anti-aging, and more. Let’s look at a few of the studies.
Aging
A 2024 study examined the impacts of 3 separate 5-day Fast-Mimicking Diet cycles. The researchers found that.” based on a validated measure of biological age predictive of morbidity and mortality, 3 FMD cycles were associated with a decrease of 2.5 years in median biological age, independent of weight loss. Nearly identical findings resulted from a second clinical study (1).”
Brain Health and Neurodegeneration
A 20022 study looked at how the Fast Mimicking Diet can potentially improve neuroinflammation and long-term brain health. Through its positive impacts on neuronal stem cell regeneration and decreased inflammation and amyloid protein buildup, the researchers found that cycles of the Fast Mimicking Diet may counteract or delay Alzheimer’s progression (8).
Blood Sugar and Type 2 Diabetes
A 2024 randomized, controlled, assessor-blinded study found significant improvements in HbA1c and glucose management in 53% of the subjects using the Fast Mimicking Diet versus just 8% of the control (11).
By offering similar stages of fasting by hour benefits as traditional fasting, the Fast Mimicking Diet may also be a promising strategy for those seeking to optimize their wellness.
Embracing the benefits of the different stages of fasting by hour
Now, you may think “Can I really stay committed to something like this?”
This article has examined the benefits of the different stages of fasting by hour, from increased energy, brain function, and fat burning to autophagy, stem cell rejuvenation, and longevity.
By understanding the different stages of fasting by hour (and consulting with your doctor), you’re equipped to make informed choices that resonate with your personal health goals.
Just remember, it’s a journey, and each stage you navigate can be a step toward your own personal health and well-being goals.
Disclaimer: For educational and informational purposes only. Before starting any new diet and exercise program, please check with your doctor and clear any exercise and/or diet changes with them Any recommendations made about weight training, nutrition, supplements or lifestyle, or information provided to you in person or on this website should be discussed between you and your doctor. We expressly disclaim responsibility to any person or entity for any liability, loss, or damage caused or alleged to be caused directly or indirectly as a result of the use, application or interpretation of any material provided to you as the client.
References
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- De Cabo R and Mattson M. Effects of intermittent fasting on health, aging, and disease. The New England Journal of Medicine . 2019;381(26). 2019.
- Ho K et al. Fasting Enhances Growth Hormone Secretion and Amplifies the Complex Rhythms of Growth Hormone Secretion in Men. J. Clin. Invest. 81; Pp 968-975. 1988
- Kerndt P et al. Fasting: The History, Pathophysiology and Complications. West J Med. 137(5); Pp 379–399. 1982.
- Kim Y. H., Lee J. H., Yeung L. H., et al. Thermogenesis-independent metabolic benefits conferred by isocaloric intermittent fasting in ob / ob mice. Scientific Reports;9(1); Pp 2479. 2019.
- Kim B et al. Effects of Intermittent Fasting on the Circulating Levels and Circadian Rhythms of Hormones. Endocrinology and Metabolism. 36; Pp 745-756. 2021
- Puchalska P and Crawford P. Multi-dimensional Roles of Ketone Bodies in Fuel Metabolism, Signaling, and Therapeutics. Cell Metabolism . 2017;25(2); Pp 262–284. 2017.
- Rangan P et al. Fasting-mimicking diet cycles reduce neuroinflammation to attenuate cognitive decline in Alzheimer’s models. Cell Rep. .40(13):111417. 2022.
- Solianik R et al. Effect of 48 h Fasting on Autonomic Function, Brain Activity, Cognition, and Mood in Amateur Weight Lifters. Biomed Res Int. 2016:1503956. 2016.
- Tinsley G. and La Bounty P. Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition Reviews . . 73(10); Pp 661-674. 2015
- Van den Burg et al. Integration of a fasting-mimicking diet programme in primary care for type 2 diabetes reduces the need for medication and improves glycaemic control: a 12-month randomised controlled trial. Randomized Controlled Trial Diabetologia. 67(7): Pp 1245-1259. 2024.
- Varady K. A. et al. Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutrition Journal 12(1); 2013.
- Vasim I et al. Intermittent Fasting and Metabolic Health. Nutrients.;14(3):631. 2022.
- Zubrzycki A et al. The role of low-calorie diets and intermittent fasting in the treatment of obesity and type-2 diabetes. Journal of Physiology and Pharmacology. 69(5). 2018.
