This post was updated on 5/19/22
Intermittent fasting isn’t just a weight loss strategy– It is at its best a healthy lifestyle informed by human evolution and the study of metabolism. It asks the human body to be much more efficient and self-protective than it is accustomed to in modern times.
What you’ll find in this article:
What is intermittent fasting | Intermittent fasting schedules | How to do intermittent fasting |What happens to your body during a fast? | The 5 stages of intermittent fasting | How to break a fast
What is intermittent fasting
Fasting is an ancient practice that involves avoiding calories for a period of time. It is a simple way to control your weight that doesn’t require you to follow a specific diet, count calories or measure food.
Intermittent fasting schedules
There are many ways to practice intermittent fasting for weight loss. The following are some of the most popular schedules:
Time-restricted eating
This involves consuming all of your meals within a period of 4-12 hours in a day. For example, you could choose to have your meals and calories between 8 am and 6 pm. Limiting your calorie intake to daylight hours is called “eating with the sun”. Time-restricted eating is particularly beneficial if you eat your meals before sunset. Not only does this help you eliminate bad habits like late-night snacking, but it improves your sleep and blood sugar control. Over time, time-restricted eating can lower your blood sugar levels, make you more sensitive to insulin, and reduce your blood pressure.
One-meal-a-day (OMAD)
This involves packing all of your day’s calories in a single meal that you consume within 1-2 hours. A small study showed that fasting for 24 hours 3 times a week and eating only dinner on fasting days, eliminated the need for insulin in type-2 diabetic patients. This fasting approach also resulted in improved HbA1C, lower body mass index, and reduced waist circumference.
Alternate-day Fasting
This involves alternating between days in which you consume no calories and days in which you eat normally. Another way of looking at ADF is as three 36-hour fasts per week. For example, eat normally on Monday until 6pm. Don’t eat until breakfast on Wednesday morning. Finish dinner by around 6pm Wednesday night. Now don’t eat again until breakfast on Friday. Eat normally the rest of the day and finish dinner by 6pm or so Friday night. Now don’t eat again until breakfast on Sunday. This way of fasting increases life span in rodents by 80%, possibly by decreasing glucose levels and regulating insulin. It also reduces the incidence of cancer in lab animals genetically predisposed to it. Alternate-day fasting is the most studied method of intermittent fasting in humans, and it’s been shown to lead to weight loss and to improve heart health, as it reduces circulating levels of LDL or ‘bad’ cholesterol.
The 5:2 “diet”
The 5:2 plan involves eating normally for five days each week and then fasting for the last two. For example, you might decide to fast Monday and Tuesday and then eat normally the rest of the week. You can also look at it as one 60-hour fast per week. Finish dinner at 6pm on Sunday night and don’t eat again until breakfast on Wednesday morning, for example.
How to do intermittent fasting
If you are just getting started with intermittent fasting, we recommend slowly working up to your target fasting schedule over the course of several weeks or months. Start by reflecting on your eating habits. Do you snack in between meals or after dinner? If so, try a 12-hour fast overnight. This means you finish dinner by 7 pm, for example, and don’t eat anything else until 7 am the next day. If you usually don’t feel the need to eat after dinner or between meals, try a 14-16 hour fast (For example, finish dinner by 6 pm and don’t eat again until 10 am the next day). If you’d like to fast longer, add 1-hour increments every 5 days or so. Drink water to keep you both hydrated and distracted from hunger. If you need caffeine, drink unsweetened black coffee and tea.
What happens to your body during intermittent fasting?
There are many things that happen during intermittent fasting that either don’t happen or that happen very slowly when we are always eating.
When you eat, your cells enter growth mode
In a well-fed state, the individual cell in your body is in “growth” mode. Its insulin signaling and mTOR pathways that tell the cell to grow, divide and synthesize proteins are active. By the way, these pathways, when overactive, have implications in cancer growth.
The “mammalian target of rapamycin” or mTOR loves having plentiful nutrients around, especially carbohydrates and proteins. When active, mTOR tells the cell not to bother with autophagy (literally cellular “self-eating”), a recycling and cleanup process that rids your body of damaged and misfolded proteins, for example. The well-fed cell isn’t worried about being efficient and recycling its components – it’s too busy growing and dividing.
In a well-fed state, your cells and their components are also highly acetylated. This means that various molecules in your cells, including the “packaging” proteins called histones that wrap your DNA up nicely within the core of your cells, are “decorated” with acetyl groups on their lysine (amino acid) residues. Don’t worry if you don’t understand the jargon in that last sentence. What you really need to know is that the well-fed cell has many genes, including those associated with cellular survival and proliferation, turned on. This is because acetylation tends to loosen the packaging proteins that normally keep your DNA wrapped up, and lets your DNA be read for protein production.
While your cells turn on cellular growth and proliferation genes when you aren’t fasting, they also turn other genes off. These include genes related to fat metabolism, stress resistance and damage repair. Actually, with intermittent fasting some of your fat gets turned into ketone bodies that appear to reactivate these genes, leading to lowered inflammation and stress resistance in the brain, for example.
But during starvation, things are very different
When you practice intermittent fasting, your body reacts to what it sees as an environmental stress (low food availability) by changing the expression of genes that are important in protecting you from, well, stress.
We have a well-preserved starvation “program” that kicks our cell into a completely different state when food, particularly glucose or sugar, isn’t around. With intermittent fasting and exercise, you activate the AMPK signaling pathway. AMPK or 5′ AMP-activated protein kinase is the brake to mTOR’s gas pedal. AMPK signals the cell to go into self-protective mode, activating autophagy and fat breakdown. It inhibits mTOR. At the same time, while you are fasting the levels of a molecule called NAD+ begin to rise because you don’t have the dietary proteins and sugars around that normally convert NAD+ to NADH through the Krebs cycle. NAD+, a molecule whose precursor is Vitamin B3, activates the sirtuins, SIRT1 and SIRT3. (Have you heard of the “longevity” molecule in wine called resveratrol? Yep, it became famous as being a potential activator of the sirtuins). These sirtuins are proteins that remove the acetyl groups we talked about above from histones and other proteins. In this process, the sirtuins silence genes related to cell proliferation and activate proteins involved in creating new mitochondria (the power-generating factories of your cells) and cleaning up reactive oxygen species.
Ketones, also produced during fasting, work as deacetylase inhibitors (in other words, keeping acetyl groups in place). This turns on genes related to antioxidant processes and damage repair.
Whew, that’s a lot happening while your body isn’t taking in any calories. But when exactly do these things happen? We’ve helped you visualize the timeline below and in the LIFE Fasting Tracker app, with a series of icons on the LIFE Fasting arc that represent the five stages of intermittent fasting!
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The Five Stages of Intermittent (and Prolonged) Fasting
By 12 hours, you’ve entered the metabolic state called ketosis (Anton et al., Obesity 2018). In this state, your body starts to break down and burn fat.
Some of this fat is used by the liver to produce ketone bodies (ketones). The two main ketones, acetoacetate and β-hydroxybutyrate (BHB), serve as an alternative energy source for the cells of your heart, skeletal muscle, and brain, when glucose isn’t readily available. Did you know that your brain uses up some 60% of your glucose when your body is in the resting state? During intermittent fasting, ketone bodies generated by your liver partly replace glucose as fuel for your brain as well as other organs. This ketone usage by your brain is one of the reasons that intermittent fasting is often claimed to promote mental clarity and positive mood – ketones produce less inflammatory products as they are being metabolized than does glucose, and they can even kick-start production of the brain growth factor BDNF! Ketones have also been shown to reduce cellular damage and cell death in neurons and can also reduce inflammation in other cell types.
By 18 hours, you’ve switched to fat-burning mode and are generating significant ketones (Anton et al., Obesity 2018). You can now begin to measure blood ketone levels above your baseline values. Under normal conditions, the concentration of ketones in your plasma ranges between 0.05 and 0.1 mM. When you fast or restrict the carbohydrates in your diet, this concentration can reach 5-7 mM. You can help accelerate ketone production with some heart-pumping exercise! For example, intermittent fasting combined with running cause rewiring of nerve cells in the brain which leads to improved learning and memory in lab animals
As their level in your bloodstream rises, ketones can act as signaling molecules, similar to hormones, to tell your body to ramp up stress-busting pathways that reduce inflammation and repair damaged DNA for example.
Within 24 hours, your cells are increasingly recycling old components and breaking down misfolded proteins linked to Alzheimer’s and other diseases (Alirezaei et al., Autophagy 2010). This is a process called autophagy.
Autophagy is an important process for cellular and tissue rejuvenation – it removes damaged cellular components including misfolded proteins. When your cells can’t or don’t initiate autophagy, bad things happen, including neurodegenerative diseases, which seem to come about as a result of the reduced autophagy that occurs during aging. Intermittent fasting activates the AMPK signaling pathway and inhibits mTOR activity, which in turn activates autophagy. This only begins to happen, however, when you substantially deplete your glucose stores and your insulin levels begin to drop.
Intermittent fasting is one way in which you can increase autophagy in your cells and possibly reduce the effects of aging. A 2019 study with 11 overweight adults who only ate between 8 am and 2 pm showed increased markers of autophagy in their blood after fasting for around 18 hours, compared to control participants who only fasted for 12 hours. A second study detected autophagy in human neutrophils starting at 24 hours of fasting. In a third study, skeletal muscle biopsies of healthy male volunteers who fasted for 72 hours showed reduced mTOR and increased autophagy. In mice deprived of food, autophagy increases after 24 hours and this effect is magnified in cells of the liver and brain after 48 hours.
But intermittent fasting is not the only way to enhance the ability of your cells to recycle old components. Some of the known benefits of exercise for overall health have to do with increased autophagy. For example, autophagy induced by exercise delays the progression of heart disease by giving the heart better quality cell parts and reducing oxidative damage.
Exercise, just like intermittent fasting, inactivates mTOR, which increases autophagy in many tissues. Exercise mimics the effects of going without food for an extended period: It activates AMPK as well as autophagy-related genes and proteins.
In mice, endurance exercise increases autophagy in the heart, liver, pancreas, fat tissue, and brain. In humans, autophagy increases during high intensity exercise, including marathon running and cycling.
By 48 hours without calories or with very few calories, carbs or protein, your growth hormone level is up to five times as high as when you started your fast (Hartman et al.,1992).
Part of the reason for this is that ketone bodies produced during fasting promote growth hormone secretion, for example in the brain. Ghrelin, the hunger hormone, also promotes growth hormone secretion. Growth hormone helps preserve lean muscle mass and reduces fat tissue accumulation, particularly as we age. It also appears to play a role in mammalian longevity and can promote wound healing and cardiovascular health.
By 54 hours, your insulin has dropped to its lowest level point since you started fasting and your body is becoming increasingly insulin-sensitive (Klein et al., 1993).
Lowering your insulin levels through Intermittent fasting has a range of health benefits both short term and long term. Lowered insulin levels put a brake on the insulin and mTOR signaling pathways, activating autophagy. Lowered insulin levels can reduce inflammation, make you more insulin sensitive (and/or less insulin resistant, which is especially a good thing if you have a high risk of developing diabetes) and protect you from chronic diseases of aging including cancer.
By 72 hours, your body is breaking down old immune cells and generating new ones (Cheng et al., 2014).
Prolonged fasting reduces circulating IGF-1 levels and PKA activity in various cell populations. IGF-1, or insulin-like growth factor 1, looks a lot like insulin and has growth-promoting effects on almost every cell in the body. IGF-1 activates signaling pathways including the PI3K-Akt pathway that promotes cell survival and growth. PKA can also activate the mTOR pathway (and, of interest, too much caffeine during a fast may promote activation of PKA).
You might see where this is leading – pressing the brakes on IGF-1 and PKA through nutrient restriction and fasting can turn down cellular survival pathways and lead to breakdown and recycling of old cells and proteins. Studies in mice have shown that prolonged fasting (greater than 48 hours), by reducing IGF-1 and PKA, leads to stress resistance, self-renewal and regeneration of hematopoietic or blood cell stem cells. Through this same mechanism, prolonged fasting for 72 hours has been shown to preserve healthy white blood cell or lymphocyte counts in patients undergoing chemotherapy.
How to break a fast
We almost forgot about the last and perhaps most important stage of intermittent fasting – the refeeding stage! It’s important to break your fast with a nutritious, balanced meal that will further improve the function of cells and tissues that went through cleanup while you were fasting. From Mark Mattson and colleagues at the National Institute on Aging:
“Upon refeeding, ingested carbohydrates* and glucose stimulate release into the blood of the incretin hormone glucagon-like peptide 1 (GLP1) from enteroendocrine cells in the gut. GLP1 enhances clearance of glucose from the blood by stimulating insulin release from the pancreas and increases the insulin sensitivity of cells. GLP1 crosses the blood–brain barrier and can act directly on neurons to promote synaptic plasticity, enhance cognition and bolster cellular stress resistance.”
*Update: This isn’t a recommendation to break your fast with lots of carbs and sugars, which may in fact lead to problematic blood sugar spikes. A few carbs can go a long way. It’s best to break your fast with a balanced meal including plenty of vegetables, plant fibers and plant fats, with healthy proteins and some whole grains or legumes if you choose. Avoid simple sugars and processed/packaged foods. Learn what works best for your body, and what you feel best eating following your fasts.
So what are you waiting for? Try fasting for 12, 16, or even 48 hours or more with the LIFE Fasting Tracker app! You’ll be rewarded with badges on your fasting arc that give you updates on your progress toward autophagy and cellular renewal! Talk to your physician before attempting fasts of 48 hours and longer.
Hi,
I’m dealing with uterine fibroids and was wondering if fasting could help shrink the tumors by chance?
I know it doesn’t get rid of it but just curious if I can see shrinkage?
What are your thoughts on fasting and people with a history of gout attacks? Any ways to avoid a gout flareup that I’ve read can happen?
Fasting can increase uric acid levels, although this seems to be temporary and may not be a problem (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6579333/). Dehydration is a known trigger for gout, so drink plenty of water if you are going to fast.
Hi there, is there any information on fasting and breast cancer? I have just been diagnosed with oestrogen receptive invasive ductal carcinoma. Will fasting help or hinder at this stage? Am awaiting surgery for lumpectomy. I’ve read fasting can help either chemo and radio but not sure if fasting can help before then? Any advice would be much appreciated. Thank you. 😊
I’m not sure about your specific cancer type, but metabolic dysfunction ( high blood sugar and high insulin) is associated with higher breast cancer risk and higher breast cancer recurrence. Fasting is very effective at lowering both blood sugar and insulin, so it could help you even before you need chemo. I hope your lumpectomy is successful and that you fully recover!
Great Article!
I have had Covid 4 weeks ago and since then it has really been up and down. Learning about Autopathy and the effect fasting can have on inflammation I decided to give it a bash. I started after dinner last night and am about 19 hours into the 72 hour fast.
My hope is that with autopathy and the decrease in inflammation (as COVID increase inflammation of the lungs) I can reset my immune systems.
I listen to a very good podcast with Dr Hsu who heads research initiative on intermitted fasting, and it just made sense.
My biggest question however. I am a ultra runner, running 8 – 12 hours per week, and doing 2-4 hour runs on Saturdays. With intermitted fasting and that load/energy expenditure there must be a point where you are not replenishing enough protein and nutrients to maintain the running load. So what is a good strategy here, 1 x 72 fast per 3 months, and 1 x 24h per week on suitable running days (e.g. not rest days as you need to feed the body to recover)?
What does the science say about such conundrums?
Our recommendation is up to 4 extended fasts per year (one per season). You may or may not need to adjust your running times during those days depending on how you feel. Keep in mind that exercise also increases autophagy so you might not need as many extended fasts. More on autophagy here: https://lifeapps.io/fasting/autophagy-the-recycling-mechanism-that-delays-aging-and-prevents-disease/
Thank you for this article! I just started a 16:8 daily fast cycle and my primary goal is to mitigate risk of Alzheimer’s, of which I have a strong family history. Is this “enough” to reap the benefits or should I be incorporating longer fasting periods? Thank you!
After 16 hours of fasting, neurons switch from using glucose to using ketones as their primary energy source. Ketones have been shown to promote autophagy in neurons. Therefore, it’s possible neurons ramp up autophagy 16 hours into a fast (unfortunately, you can’t measure it to know for sure). You can also try exercise while doing your 16:8 fast to maximize autophagy. Some people like doing longer fasts of 24 hours and above. If you want to try that, make sure you increase your hours gradually– add 1 hour of fasting every week or so. Practice the other 4 pillars of health to help you in your efforts to avoid Alzheimer’s: 5 servings of fruits and veggies per day, 7-9 hours of sleep per night, a few minutes of mindfulness per day, and 30 min of daily exercise.
I am on 66 hours and feeling better than yesterday. Can I just keep going? I am doing this as my sugars were elevating, and don’t want medication. I take really good supplements. I still need some more weight off.
You can keep going as long as you are feeling well! Make sure you are supplementing with electrolytes and staying hydrated. We don’t recommend fasts over 7 days.
I fasted for 3 1/2 weeks. Usually around 15 to 18 hours a day and one 24 hour fast per week. I lost 12 pounds and was feeling great!! Then I added exercise. I work with a trainer 2x a week. I’m still fasting and logging all of my meals. By the 3rd workout I’ve gained 3 pounds back. Now I’m very discouraged and wish I would have waited before I started exercising. Do you have any tips for me?
Don’t get discouraged! Weight loss is never a straight line and regression is normal. Don’t just keep track of your weight. Focus also on improvements on your body measurements (waist, thighs, chin, etc), blood glucose, blood pressure, sleep, energy levels, etc. The scale is only one measure but there are so many other things that also are indicators of health!
Don’t get discouraged! You gain weight. But it’s muscle weight. That’s a good thing!
Keep in mind that after you workout your muscles store blood and water to heal. This could be the added weight gain you’re experiencing.
What is the best fasting schedule for someone who has 25 pounds to lose and is insulin resistant? Thanks
That’s where I am now. Doing ADFs 48 hours and longer ones too to break my stall and insulin resistance.
You can get started with overnight fasts of 12-14 hours if you’ve never fasted before and add hours gradually. You could work up to fasts of 18-20 hours. If you are insulin resistant, eliminate simple carbs and focus on protein, healthy fats, and non-starchy vegetables. More info on fasting schedules here: https://lifeapps.io/fasting/what-fasting-style-is-right-for-you/
Great article, one of the best that I have read. Congratulations, Paige, and excellent feedback on comments and questions, Luisa!
One question: it seemed that autophagy began at 24 hours fasting in your article, that is when the concept was introduced, but it happens before, right? Can you give us some sense as to the magnitude of autophagy at 12, 16, 18 and 24 hours?
Thanks
Yes, autophagy increases throughout your fast and it’s been observed starting at 18 hours (according to published data). More info here: https://lifeapps.io/fasting/autophagy-the-recycling-mechanism-that-delays-aging-and-prevents-disease/
I am interested in starting alternate day fasting (3 fasts, 4 eating days, per week), but have a question about exercising. I enjoy light running (35min-45min) most days of the week. Can I continue to resume my run days? I will of course listen to my body.
yes! it’s perfectly safe to exercise while fasting, although you might have to modify your workout if you feel dizzy. Once you get used to it you won’t feel any discomfort.
Hello,
I’m new to this and have 1 question please.
If I’m fasting and I lets say that I’m 6,8,10 etc. hours into the fast and say eat 1 small boiled egg (just an example) will this then stop my fast and restart from beginning again or would 1 egg be too small to interupt the fast?
I hope this makes sense, I thknk what I’m trying to ask is would any small amount of food stop the cycle, so does fasting actually mean NO FOOD WHATSOEVER however small the food is, IE 1 biscuit or 1 boiled sweet or 1 slice of apple
Jason
Fasting means no calories. You can drink water, black coffee or tea with no sugar. Fasting generally means no solid food. However, for fasts of 3 days and longer, some people like to eat a few calories (under 500) of low-carb, low-protein foods.
I’m on a 336 hour fast that’s 2 weeks, I’ve done a 20 day haven’t lost any muscle it 14 days seem to be the sweet spot! This is an extended water fast with of course the essential hydration techniques I’ve used in the past! Stay safe stay healthy stay hungry!
How is your fast going ? I just started a 720 hour fast Sunday. Going to see if I can do it for a month .
How do you do this and why so long?
You don’t have to fast long to see the benefits of fasting. Even 12 overnight fasts can be beneficial. We always recommend starting slow (with 12-16 hour overnight fasts), and to add hours gradually if you’d like to fast longer. You can drink water, black coffee and/or tea (no sugar) during your fast. More info here:https://lifeapps.io/fasting/what-fasting-style-is-right-for-you/
I just don’t know how you an any person for this amount of time have the will power to do this.
Three days is such a mind battle for me .
It’s definitely not for everyone, and you certainly don’t have to fast for that long to see benefits.
Thank you very much for your article above, very interesting and although I have been practicing intermittent and prolonged fasting for many years I am still learning a lot from your extensive research and studies.
Right now I am on prolonged fast and so far I am feeling okay, might keep going until tomorrow, if I can make it…
Great! how did your fast go?
When I had GI bleeds and ended up on an IV drip for five days I learned that five days without food was possible as long as I was hydrated.
Knowing the psychological challenges spelled out in Jason Fungs book on Intermittent fasting made the journey like a tour guide to success.
Currently I have finished the five days and looking at 1/2 day more today. I take my blood pressure, ease up on the exercise. Especially watch for light headedness or getting crabby. I take B vitamins and sometimes SAM for emotional support.
I’ll loose five pounds but always game it back within a week. My question is a discussion on how to keep at least some of those weight loss pounds off for good?
It’s normal to gain weight back after a 5-day fast. You could try losing those pounds slowly through shorter fasts of 18-24 hours 3-4 days a week. if you’d like to keep doing 5-day fasts, we recommend doing them no more than 3-4 times a year.
Question. Is 72 or longer recommend over the 60 hr fast I was told has little benefit after 60 hrs. And if I do 60-72 hr fast how many times a year should I do it? And why shouldn’t I fast more than that? Thank you
It depends on your reasons for doing a long fast. Some people like to do long fasts (72 hours and longer) to break through a plateau or for autophagy. If you decide to do them, we recommend up to 4 long fasts per year. More info here: https://lifeapps.io/fasting/7-safety-tips-for-practicing-extended-fasting/
Interesting take on intermittent fasting. We all know that different eating patterns, when pushed to the extreme, can have repercussions on health. This I think explains why concepts such as food cycling can be a sensible way out of our fasting conundrum.
Great aticle right there. I will finished my 7 days fast with Water and herbs tommorrow. I Just want to know if i’m doing it alright. I fast one whole week with out eating cooked Food But i eat fruits in between the day Like dates fruits, Guava, or plum. I want to know if eating those fruits are wrong or right as I do my fast ?
Fruit is not allowed during fasting, as it contains sugar and calories.
What is your scientific reason for specifically recommending plant fats instead of any fat?
Our recommendation is to avoid seed oils because they are highly processed. Plant-based oils like olive oil, avocado oil, etc don’t require industrial processing. Other plant fats like avocados and nuts are great. Fat from animal protein is great too.
I have been fasting 16 hours 4 days a week. I started intermittent fasting form Labor Day 2021. I am also not eating any kind of carbohydrates and sugar. I have not eating starchy vegetables and fruits which has high sugar. After 2 weeks I noticed that I don’t need to wear glasses for readings. Let me clarify this, I am wearing contact lenses because I am nearsighted. Top of that I was wearing glasses. I still wear contacts lenses but do not have to wear glasses. When I started intermittent fasting my weights was around 160 labs within 3 weeks I was 154labs. I also start to feel
good overall. Now I am trying to do 18 hours fasting , continue not eating sugar , carbohydrates and can food as possibly I can. I don’t feel hungry these day. Before I would be hungry must of the times! I am very positive about intermittent fasting!! I also do wim hof breathing exercise and cold showers yoga kind of exercise!
That’s fantastic! lowering blood sugar can improve your vision. Keep going!
So, how do I determine which fast is best? I started about two weeks ago. My Dr. recommend 16:8 but said I can play with it to see what is best for me. I started this week to increase my fast to 18:6 and feel amazing. I’ve lost 4 pounds so far and am definitely in it for weight loss but I also want the full benefits of a fast including autophagy. Am I missing out by a sticking to the 18:6 plan?
I use the BodyFast App to help guide me through my Fasting schedule. I also use My Fitness Pal to track my food. It has really helped me a lot, you should try it!!
Good luck!
Autophagy increases throughout your fast and you are still getting the benefits of that at 18 hours.
More on fasting styles here:https://lifeapps.io/fasting/what-fasting-style-is-right-for-you/
and on autophagy here: https://lifeapps.io/fasting/autophagy-the-recycling-mechanism-that-delays-aging-and-prevents-disease/