Expert Feedback On The Science of Fasting
Fasting can be a controversial topic. The science is often disputed among leading health care experts. Some would argue that fasting deprives the body of essential nutrients, including adequate muscle-building proteins.However, other experts contend that fasting activates the body’s natural healing processes, such as autophagy, where the body recycles old and depleted cells; paving the way for improved metabolic health.
Despite the controversy, this much is clear: a little fasting can be good for your health. Several experts have weighed in on the benefits of fasting. This post will include snippets from popular health care experts (plus a bio-hacker, life coach, and mathematician) on the science of fasting. These experts will include:
Dr. Casey Means, Dr. Andrew Huberman, Nassim Taleb, Tim Ferriss, Dr. Pradip Jamnada, Dr. David Sinclair, Dr. Peter Attia, Dr. Jason Fung and Tony Robbins/Dr. Peter Diamandis/Dr. Robert Hariri.
Dr. Casey Means
Casey Means is a Stanford trained physician. She is also the co-founder of Level Health, a company focused on metabolic health. Level Health aims to boost metabolic function to prevent chronic diseases, like diabetes, heart disease, and obesity. Recently, Casey’s dedication to health optimization reached the NYT Best Sellers list with her book, Good Energy.
When it comes to fasting, here’s what she had to say:
“Erratic daily food patterns and ultra-frequent food intake predispose us to metabolic dysfunction. In contrast, choosing to eat at consistent times, and cutting off the last food intake earlier in the evening, is an example of time-restricted feeding (TRF) – [a form of fasting]” … “Research conducted on overweight individuals without diabetes revealed that practicing TRF for just four days can considerable reduce fasting glucose, fasting insulin, and mean glucose levels.” …
“Fasting lets your body practice – and over time, improve – its process of switching between burning available carbohydrates and glucose (when you eat) and burning fat for energy (when you’re not eating/fasting). Insulin normally promotes fat storage and restricts fat breakdown, so when we fast, we allow insulin levels to fall and let fat get mobilized for energy. Fasting is also a stressor on the body, so it should be used with intention and thoughtfulness.” …
“No matter which fasting style you follow, you should try to reduce your daily eating window to avoid food in the late evening, and try to eat your last food before dark as often as possible. This tweak alone will be life-changing.”
Advice: “Aim to eat within a ten-hour window each day and fast for at least fourteen hours. Choose what window you want to adhere to, like 10:00AM to 8:00PM, or 8:00Am to 6:00PM.
Dr. Peter Attia
Peter Attia, a Stanford-trained doctor with surgical training at Johns Hopkins, is dedicated to the science of longevity. He has dedicated his life’s work to “lifespan” – which emphasizes the time a person is health, not merely just alive. Like Dr. Means, Attia has also recently entered the NYT Best Sellers 
list with, Outlive: The Science and Art of Longevity.
When it comes to fasting, here’s what Peter had to say:
“Fasting, or time-restricted (TR) eating, presents us with a tactical conundrum. On the one hand, it is a powerful tool for accomplishing some of our goals, large and small. On the other, fasting has some potentially serious downsides that limit it’s usefulness. While intermittent fasting and eating “windows” have become popular and even trendy in recent years, I’ve grown skeptical of their effectiveness.”
“… There is no denying that some good things happen when you are not eating. Insulin drops dramatically because there is no incoming calories to trigger an insulin response. The liver is emptied of fat in fairly short order. Over time, within three days or so, the body enters a state called “starvation ketosis,” where fat stores are mobilized to fulfill the need for energy…”
“…Fasting over long periods also turn down mTOR, the pro-growth and pro-aging pathways. This would also be desirable, one might think, at least for some tissues. At the same time, lack of nutrients accelerate autophagy, the cellular “recycling” process that helps our cells become more resilient, and it activates FOXO, the cellular repair genes that may help centenarians live so long. In short, fasting triggers many of the physiological and cellular mechanisms that we want to see. So why don’t I recommend it to all my patients?
“… A 2020 clinical trial by Ethan Weiss and colleagues found no weight loss or cardiometabolic benefits in a group of 116 volunteers on a 16/8 (hours) eating pattern. Two similar studies also found minimal benefits. “…in my view sixteen hours without food simply isn’t long enough to activate autophagy or inhibit chronic mTOR elevation, or engage any of the other longer-term benefits of fasting that we would want to obtain.
“…[A]nother drawback is that you are virtually guaranteed to miss your protein target with this approach… one small but revealing study found that subjects on an alternate- day fasting diet did lose weight – but they also lost more lean mass (i.e., muscle) than subjects who simply ate 25 percent fewer calories every day.”
Advice: Dr. Attia concludes that fasting can be helpful for some patients. Those often include patients that have failed other dietary interventions. In such instances, Attia may prescribe what he calls “hypocaloric fasting.” Hypocaloric fasting is a less aggressive form of fasting that takes place just one week each month. During that week, patients are limited to around 700 calories per day, primarily fat, with minimal protein and carbohydrates. This approach is also combined with a starch-and-sugar restricted diet during the remainder of the month. Based on Attia’s observations, this method can lead to significant improvements in blood biomarkers within a week.
Dr. David Sinclair
David Sinclair, an Australian biologist and Harvard Medical School Professor, is renowned for his research on aging and longevity. He co-Directs Harvard’s Paul Glenn Center for the Biology of Aging Research. Sinclair also authored the best-selling book, Lifespan: Why Age – and Why We Don’t Have To.
Here is what Sinclair says about fasting:
“After twenty-five years of researching aging and having read thousands of scientific papers, if there is one piece of advice I can offer, one surefire way to say healthy longer, one thing you can do to maximize your lifespan right now, it’s this: eat less.”
“… Not malnutrition. Not starvation. These are not pathways to more years, let alone better years. But fasting – allowing our bodies to exist in a state of want, more often than most of us allow in our privileged world of plenty – is unquestionably good for our health and longevity.”
“…That doesn’t make a calorie restrictive diet a good plan for everyone… It’s nonetheless a hard sell for many people. It takes strong will power to avoid the fridge at home or snacks at work. There’s an adage in my field: if calorie restriction doesn’t make you live longer, it will certainly make you feel that way. … but it turns out that’s okay, because research is increasingly demonstrating that many of the benefits of a life of strict and uncompromising calorie restriction can be obtained in another way. In fact, that way might even be better:
Intermittent fasting (or IF) – eating normal portions of food but with periodic episodes without meals. In fact, human studies are confirming that once-in-a-while calorie restriction can have tremendous health results, even if the times of fasting are quite transient.”
Advice: “Over time, some of these ways of limiting food will prove to be more effective than others. A popular method is to skip breakfast and have a late lunch (the 16:8 diet). Another is to eat 75 percent fewer calories for two days a week (the 5:2 diet). If you’re a bit more adventurous, you can try skipping food a couple of days a week (eat, stop, eat) or go hungry for an entire week every quarter.“
Dr. Andrew Huberman
Andrew Huberman is a neuroscientist and professor at Stanford University School of Medicine, where he researches neurobiology and ophthalmology. His work primarily focuses on brain function and neuroplasticity. He is also widely known for his Huberman Lab Podcast. Andrew uses the podcast to translate complex scientific concepts into accessible, practical advice on optimizing physical and mental performance.
Here is what Huberman says about fasting:
Andrew Huberman often discusses time-restricted feeding (TRF). TRF differs from transitional fasting. Instead, it focuses on a controlled eating window, not a lack of food. A popular method he references in his podcasts is the 16:8 schedule, where one fasts for 16 hours and eats within an 8-hour window. He also touches on more extended fasting options, like alternate-day fasting and 24-hour fasts. Huberman stresses that the timing of meals can have a significant impact on metabolic health.
According to Huberman, fasting offers several health benefits. He highlights improvements in gut health, noting its potential to improve conditions like irritable bowel syndrome (IBS). While fasting may enhance metabolic function and benefit liver health, Huberman is more cautious about claims of its ability to reverse non-alcoholic fatty liver disease (NAFLD).
Advice: Huberman advocates for aligning fasting schedules with circadian rhythms to optimize health benefits. He recommends avoiding food for the first 60 minutes after waking and stopping eating 2-3 hours before bedtime. This approach enhances metabolic health, supports digestion, and improves sleep quality by allowing the body to focus on natural repair processes during rest.
Tim Ferriss
Tim Ferriss is an American entrepreneur, author and podcaster. He is best known for his best-selling books, like the 4-Hour Workweek, 4-Hour Body, 4-Hour Chef, Tools for Titans, and Tribe of Mentors. He also has a popular podcast called, The Tim Ferriss Show. Tim is often recognized for his focus on lifestyle design, productivity, and health optimization. Due to his relentless self-experimentation and life-hacking methods, he is often regarded as a “human guinea pig.”
Here is what Ferriss says about fasting:
“In the last 2 years, I’ve done a lot of fasting experiments, focusing on real science instead of old wives’ tales (e.g. you must break you fast with shredded cabbage and beets). I now aim for a 3-day fast once per month and a 5-to7-day fast once per quarter. I would like to do one 14-to-30 day fast per year, but the logistics have proven too inconvenient.”
“…The longest fast I’ve done to date was 10 days. During that fast, I added vitamin C IVs and hyperbaric oxygen (2.4 ATA x 60 minutes) 3 times per week. I did DEXA body scans every 2 to 3 days for tracking and also consumed roughly 1.5 g of BCAAs upon waking and roughly 3g of BCAAs intra-workout. After a 10-day fast, I had lost zero muscle mass. In contract, I lost nearly 12 pounds of muscle [in his first 7 day fast].”
“How and why the difference? …
“…First, I allowed trace amounts of BCAAs and 300 to 500 calories of pure fat per day on my “fast.”
“…Second, I got into ketosis as quickly as possible to skip muscle wasting. I can now do this in under 24 hours instead of 3 to 4 days. The more often you enter keto, the fast the transition takes place. There appears to be a biological “muscle memory” related to monocarboxylate transporters and other things beyond my pay grade…”
Advice: The following is Tim’s protocol for a 3-day fast (From Thursday dinner – Sunday dinner), as quoted:
- On Wednesday and Thursday, plan phone calls for Friday. Determine how you can be productive via cell phone for 4 hours. This will make sense shortly.
- Have a low-card dinner around 6pm on Thursday. On Friday, Saturday, and Sunday mornings, sleep as late as possible. This point is to let sleep do some of the work for you.
- Consume exogenous ketones or MCT oil upon waking and 2 more times throughout the day at 3-to-4 hour intervals. I primarily use KentoCaNa and caprylic acid (C8), like Brain Octane. The exogenous ketones help “fill the gap” for the 1 to 3 days that you might suffer carb withdrawal. Once you’re in deep ketosis and using body fat, they can be omitted.
- On Friday (and Saturday if needed), drink some caffeine and prepare to WALK. Be out the door no later than 30 minutes after waking. I grab a cold water or smartwater out of my fridge, add a dash of pure, unsweetened lemon juice to attenuate boredom, add a few pinches of salt to prevent misery/headaches/cramping, and head out. I sip this as I walk and make phone calls. Podcasts also work. Once you finish your water, fill it up or buy another. Add a little salt, keep walking, and keep drinking. It’s brisk walking – NOT intense exercise – and constant hydration that are key…
- Each day of fasting, feel free to consumer exogenous ketones or fat (e.g. coconut oil in tea or coffee) as you like, up to 4 tablespoons. I will often reward myself at the end of each fasting afternoon with an iced coffee with a bit of coconut cream in it. Truth be told, I will sometimes allow myself a SeaSnax packet of nori sheets. Oooh, the decadence.
- Break your fast on Sunday night. Enjoy it. For a 14 days or longer fast, you need to think about refeeding carefully. But for a 3-day fast, I don’t think what you eat matters much. I’ve done steak, I’ve don salads, I’ve done greasy burritos. Evolutionarily, it makes no sense that a starving hominid would need to find shredded cabbage or some such nonsense to save himself from death. Eat what you find to eat.
Nassim Taleb
Nassim Taleb is a famous scholar and statistician. He is also a former trader and author known for his work on risk, probability, and uncertainty. Taleb is particularly famous for his books, collectively known as “The Incerto,” which includes titles such as, The Black Swan, Antifragile, Fooled by Randomness, and Skin in the Game.
Here is what Taleb says about fasting:
“Caloric restriction activates healthy reactions and switches that, among other benefits, lengthens life expectancy in laboratory animals. we humans live too long for researchers to test if such restriction increases our life expectancy (if the hypothesis is true, then the subject would outlive the researchers). But it looks like such restriction makes humans healthier (and may also improve their sense of humor). But since abundance would bring the opposite effect, this episodic caloric restriction can be also interpreted as follows: too much regular food is bad for you, and depriving humans of the stressors of hunger may make them live less than their full potential; so all hormesis seems to be doing is reestablishing the natural dosage for food and hunger in humans. In other words, hormesis is the norm, and its absence is what hurts us”
Dr. Pradip Jamnadas
Pradip is a respected cardiologist and the founder of Cardiovascular Interventions in Orlando, Florida. He has over 30 years of experience in cardiology, with a focus on interventional cardiology and preventative care. He is also the founder and Chairman of the Galen Foundation, a non-profit organization. Galen is dedicated to raising awareness and funds for cardiovascular research.
Watch what Pradip says about fasting:
Advice: Key takeaways from Pradip’s lecture –
1. Fasting Throughout History: Dr. Jamnadas highlights fasting’s long history, from Socrates using it for mental clarity to Paleolithic humans fasting due to food scarcity.
2. Mental Clarity from Fasting: Contrary to popular belief, fasting enhances mental clarity. When we consume sugar or carbohydrates, the brain reacts with a dopamine release, leading to short-term satisfaction. Over time, fasting breaks this “sugar addiction” and reduces cravings.
3. Biochemistry of Fasting: Within 12 hours of fasting, the body depletes glycogen stores from the liver and muscles. After 12 hours, gluconeogenesis (production of glucose from proteins) kicks in, and ketosis (fat breakdown for energy) begins after 18 hours. By 24 hours, the body begins producing ketones, which can be utilized as energy.
4. Insulin and Fat Metabolism: Pradip stresses that fasting lowers insulin levels, which is crucial for unlocking fat stores. Insulin resistance, a hallmark of metabolic syndrome, prevents fat from being burned and fasting is a natural way to lower insulin levels – enabling fat loss and improved metabolic health.
5. Autophagy: Starting around 18 hours into fasting, autophagy begins, where the body recycles damaged cells and regenerates new ones. This process peaks around the third day of fasting, promoting cellular rejuvenation and longevity.
6. Stem Cell Activation: After prolonged fasting, particularly after three days, the body stimulates stem cell production, which helps in the regeneration of tissues. This has potential implications for boosting the immune system and aiding in recovery from illness.
7. Growth Hormone Surge: Jamnadas mentions that fasting for 48 hours or more can increase growth hormone levels by up to 2,000% in men and 1,300% in women. Growth hormone is crucial for maintaining muscle mass, skin health, and overall youthfulness.
8. Application of Fasting: Fasting shows promise in reducing cancer risk, improving outcomes in autoimmune diseases, and even aiding in Alzheimer’s and diabetes treatment. He also notes that fasting has been show to improve chemotherapy tolerance and reduce cancer cell survival.
9. Myths Around Eating: The idea that humans must eat three or more times a day, or that breakfast is the most important meal, is debunked. Pradip emphasizes that our Paleolithic ancestors did not eat frequently, and the body is designed to function optimally on less frequent meals.
10. Refeeding After Fasting: After an extended fast, Jamnadas warns about the “refeeding syndrome,” where improper refeeding can lead to electrolyte imbalances. He suggests using bone broth for its nutrient density when breaking a fast.
Dr. Jason Fung
Dr. Fund is a Canadian nephrologist. He is a top expert on intermittent fasting and low-carb diets for treating type 2 diabetes and obesity. He’s best known for his books such as, The Obesity Code and The Diabetes Code, where he explains how fasting and diet can help people manage weight and reverse metabolic disease.
Among his many books on fasting, Jason outlines his thoughts in his other book, The Obesity Code Cookbook. In that book he writes:
“As a healing tradition, fasting has long met with success. For example, among the treatments prescribed and championed by Hippocrates of Kos (c. 460 – c. 370 BC), who is widely considered the father of modern medicine, were the practice of fasting and the consumption of apple cider vinegar. He wrote, “To eat when you are sick, is to feed your illness.”
“…The truth is that fasting is just as effective as treating our modern illnesses – obesity, diabetes, the entire constellation of aliments resulting from metabolic syndrome – as those of our ancestors.”
“The three most common fasting periods I recommend are sixteen hours, twenty-four hours, and thirsty-six hours.”
- A daily sixteen-hour fast means you eat your meals within an eight-hour window. So, if you begin your fast at 7:00pm, for example, you don’t eat anything until 11:00AM the following day. You consume two or three meals from that point on and resume your fast at 7:00PM that evening.
- For a twenty-four-hour fast, you fast from, for example dinner at 7:00PM on the first day until dinner at 7:00PM the next day.
- For a thirty-six-hour fast, you fast from, for example, dinner at 7:00PM on the first day until breakfast at 7:00AM two days later.
Advice:
Tips for Success…
- Drink water: Start each morning with a glass of water.
- Stay busy: It’ll keep your mind off food. It helps to choose a busy day at work when you’re planning to fast.
- Drink Coffee: Coffee is a mild appetite suppressant. Also try green tea, black tea, and bone broth.
- Ride the waves: Hunger comes in waves; it is not continuous. Be patient and distract yourself.
- Don’t tell everybody your fasting: people may try to discourage you if they don’t understand the benefits.
- Give yourself thirty days: It takes time for your body to get used to fasting. Don’t be discouraged if you experience s setback.
Tony Robbins, Dr. Peter Diamandis, & Dr. Robert Hariri (w/ Dr. Valter Longo) – Life Force
Tony Robbins is a well-known life coach, motivational speaker, and author. He specializes in personal development, leadership, and financial freedom. Robbins is famous for his self-help books such as, Awaken the Giant Within and Unlimited Power.
Dr. Peter Diamandis is an entrepreneur, author, and futurist known for his work in advancing technology and space exploration. He is the founder of the XPRIZE Foundation, which incentives breakthroughs in science and technology. Diamandis has co-authored several books such as, Abundance, Bold, and the Future is Faster Than You Think.
Robert Hariri is a scientist, surgeon, and entrepreneur in the fields of aerospace and biotechnology. He is the founder of Celularity, a company focused on regenerative medicine and cell therapies using placental cells.
Valter Longo is a biogerontologist known for his research on aging and nutrition. He is the Director of the USC Longevity Institute and author of The Longevity Diet. Longo is also famous for developing the “Fasting-Mimicking Diet,” which mimics the benefits of fasting without full food deprivation.
Life Force is a book co-authored by Tony Robbins, Peter Diamandis, and Robert Hariri. The book aggregates advice from world-leading health experts. It explores cutting-edge advancements in health, wellness, and longevity; combining science and practical strategies for improving and extending life.
The following are excerpts from Dr. Longo included in Life Force:
“Back in the 1980s, Longo observed in laboratory experiments that when you starve yeast and bacteria, “they live longer.”…
“…Since then, he and many other scientists have conducted a number of studies in animals and humans suggesting that fasting can be a powerful weapon against illnesses like obesity, diabetes, hypertension, cancer, asthma, arthritis, multiple sclerosis, cardiovascular disease, Parkinson’s disease, and Alzheimer’s?”
“…One popular approach that he [Longo] recommends is a strategy called “time-restricted feeding.” … “He suggests that you eat an early dinner (ideally, you finish your meal three hours before sleep) and then eat nothing at all for at least the next twelve hours.”
“…There are many other variations of intermittent fasting. For example, fans of the 5:2 diet cut out around 75 percent of their calories on two nonconsecutive days each week and eat normally the other five days. Another common approach, which many people find reasonably easy and sustainable after a week or two of adjustments, is to skip breakfast and fast for sixteen hours every day. Peter Diamandis practices a more intense version of time-restricted eating: He fasts for nineteen hours a day, typically eating in a five-hour window between 1PM and 6PM…”
“…Dr. Longo developed a five-day “fasting-mimicking diet” that’s less grueling than a water-only fast. On the first day, it consists of 1,100 calories. For the next four days, that drops to 800 calories a day, largely in in the form of vegetable soups. As purists will tell you, this isn’t truly a fast. But it’s designed to have the same benefits without as much hardship. So far, says Longo, more than 200,000 people have tried his fasting-mimicking diet, which is marketed by a company called L-Nutra as a rapid way to lose fat and “enhance cellular renewal.”
“…Dr. Longo explains that this cycle of “starvation and refeeding” triggers a “regenerative and self-healing process” that can reduce the “biological age” of your cells and organs.”
Conclusion: As you can see, many experts support some form of fasting. However, each doctor seems to advocate different methods, with their science reaching slightly different conclusions. While one doctor sees significant benefits after 24 hours, another finds results only after several days. This highlights the need for more research on fasting. In the meantime, cautious experimentation with fasting seems prudent. One thing is clear among these experts: the pros of fasting appear to outweigh the cons.