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Timeline Of Fasting: A Look Into What Goes On In Your Body When You Go Without Food

Have you ever wondered what happens in your body as you fast? What about the timeline of fasting and what could be happening within the body from the time you eat, end your meal, and then get into your fasting window before getting into the eating window again?

The practice of fasting, especially short-term fasting such as intermittent fasting, continues to gain popularity and followers within and outside of the fitness and nutrition industries. Unlike many dietary practices that restrict what practitioners can and cannot eat, intermittent fasting is a less restrictive practice that has only one strict guideline, when you can and cannot eat.

This practice has been shown to have many incredible benefits and results. From helping with weight loss and body weight control to improving blood pressure, glucose levels, and heart health, as well as reducing inflammation, burning fat and improving the cell repair process (19, 3), there is no doubt that this practice is as popular as it is now.

We all know what fasting involves and the results of the practice, but what exactly happens in the body during the fasting and feeding windows to lead to said results? If you’re curious about this, read on to learn more about the intermittent fasting timeline and the fasting timeline benefits that lead to the results often seen in people who engage in this practice.

What Are The 5 Stages Of Fasting?

While we can all agree that there are several stages to fasting, the exact number is something that cannot seem to be agreed upon. While some say that there are only 4 stages of fasting, others claim that we have 5 sftages of fasting. Research, however, shows that the timeline of fasting occurs in four cycle stages (10).

A quick rundown of the timeline of fasting and the hours in which these phases occur is as follows. Each phase is marked by different physiological processes, and the timing of entry into a subsequent phase depends on several individual factors. 

  1. Phase 1 – Fed state – Anywhere between 0 to 3 hours after eating
  2. Phase 2 – Early fasting stage – Starts 3 to 4 hours after a meal and can last up to 18 hours post meal
  3. Phase 3 – The fasting state – This starts from the 18th hour of fasting and can last up to 48 hours.
  4. Phase 4 – Long term fasting state – Starts at the 48th hour mark. Anything that goes beyond the 48th hour mark is all counted under the long term fasting state.

The Timeline of Fasting: A Further Breakdown

To better understand the fasting stages by day and by the body, here is a deeper look into all the timeline of fasting phases.

The Fed State

Also known as the initial phase of the fed-fast cycle, this phase starts right after finishing a meal and lasts for a few hours (10) – the amount of time that your body spends in this state is determined by what you ate and how much of it you consumed. During this state in the timeline your body gets into the process of digesting whatever you’ve consumed and absorbing any nutrients from the food.

Once you’ve eaten, the body’s first and immediate response is to secrete insulin in order to combat the rising blood sugar levels that are triggered by eating. The amount of insulin secreted in the body depends on a range of factors. This varies largely due to how much in the way of carbohydrates and added sugars were part of your meal – this is why persons with type 2 diabetes need to be extra careful with their carbohydrate and sugar intake (16).

Aside from the insulin and blood sugar levels changing, your hormone levels are affected too. Whenever you are hungry the hunger hormone ghrelin is usually higher, as it signals to your brain that your stomach contents are low and you need to eat (8). As you start eating, the ghrelin levels gradually start falling as the levels of leptin – the satiety hormone (15) – start rising.

Once you are full, leptin sends this message to your brain, leading to feelings of satiety, and the physiological markers of the fed state begin.

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The Early Fasting State

Also known as the post-absorptive phase, it starts immediately once the feeding state is over. As stated above, how long the fed state lasts is largely dependent on how much food you ate as well as what foods were included in the meal. 

If you ate a larger meal then your fed state will last longer than that of a person who consumed a smaller meal. If your meal was made up of whole grains, complex carbohydrates, and/or protein, all of which are known to promote satiety (1, 5, 7, 9), then this state will likely also last longer.

Typically, however, this post-absorptive phase starts about three to four hours post-meal and can last up to 18 hours after eating. During this state, the blood sugar and insulin levels that had gone up due to eating start decreasing. This pushes your body into tapping into its glycogen, or stored carbohydrate, reserves and converting them into glucose, which is then used as an energy source for the body.

The longer you go without food, the more of your glycogen reserves get depleted. Once the reserves are gone, your body starts looking for other sources of energy. It gets into lipolysis – a stage right before ketosis where it starts breaking down your triacylglycerols (natural fats) and amino acids to use them as a fuel source (10, 4).

Hormones like glucagon, epinephrine, growth hormone, and cortisol, which all increase during fasting, are said to help facilitate this process (6).

Ps. When it comes to the stages of intermittent fasting with a 16/8 schedule (16 hours of fasting and 8 hours of food consumption allowed), your body will only go through the fed and the early fasting/post-absorptive state because your fasting window cuts off after 16 hours, clearly not going beyond the 18th hour mark.


The Fasting State

Once you go beyond the 18 hour mark without eating, your body enters the 3rd phase. This fasting state will continue up to 2 days – 48 hours (10) – if you do not eat before then. At this juncture, your body which had already entered lipolysis – your body will continue doing this until you eventually fall into ketosis. 

Once the body has absolutely no more glucose left, it fully gets into ketosis where it burns stored fats for energy rather than glucose. Ketones, released into the blood and urine with this process, are subsequently used by the body as the primary source of fuel. An important fact to note is that while ketosis will start during this fasting phase, it is not known exactly when this happens. 

This process is not only influenced by the lack of glucose but also other factors like your age, activity level, and metabolism (21). The debate on how long it takes the body to reach ketosis still has no conclusion – where some say just 12 hours are enough, others say that it can take anywhere between 24 to 48 hours to reach this state (22, 2).

Ps. If you are curious about the autophagy fasting timeline, chances are that this process will happen at this point of the phases. Research has shown that this process of cleaning out old damaged cells and replacing them with new healthy ones may begin anywhere between 24 and 48 hours of fasting (12).

Long-Term Fasting State

Also known as the starvation stage, this phase starts after 48 continuous hours of fasting and goes on till you eat something. Because of the lack of food and glucose for a very prolonged period of time, your insulin levels will continue to drop while your ketone levels continue to steadily rise (10). 

The lack of glucose forces the liver to initiate gluconeogenesis, a process where the body forms its own sugar in the form of glucose from non-glucose precursors. This sugar will then be turned into glucose, which will be used together with ketones as a fuel source for the brain. Over time, even this will not be enough to sustain you, and your body will have to look for more energy sources. This will force it to turn to your muscles, a potentially small energy source in earlier stages of fasting, catabolizing muscle tissue more significantly to give you energy.

The longer you stay in this prolonged fasting stage to the point where your body starts eating away at your muscle, the more life threatening your state becomes. Failure to eat will lead to organ failure and eventually, death.

If you wish to reach this prolonged, high-risk fasting state, please make sure that you have medical supervision as it is the only thing that will keep you from succumbing and dying from fasting for such an extended period. Although this dangerous extended fasting period can produce life-threatening consequences, when monitored carefully, it can potentially yield some physical and even psychological benefits (17, 13, 14).

Read more: Green Tea Intermittent Fasting: Benefits and Side Effects

Is There A Time Frame For Fasting?

Yes, there is. The time frame of fasting is different from the stages of fasting. While the latter outlines what happens to the body during a certain number of hours into the fasting window/period, the former tackles the kind of fasting you want to do -i.e., what method or schedule of intermittent fasting do you wish to follow? 

Popular intermittent fasting methods include

  • The 16/8 – Anyone looking up intermittent fasting (IF) for the first time is likely to be introduced to this method first as it is the most popular one. This practice includes fasting for 16 hours then having an 8 hour eating window to eat as desired.
  • The 14/10 – This is the 2nd most popular method. It is used by beginners of IF who feel that the 16/8 is a tad too restrictive and that they get too hungry before the 16 hours are up.
  • The 5:2 diet – Unlike the first two options that involve daily time restricted eating and do not place a huge concern on calorie or food content intake during the non-fasting window, the 5:2 diet does this. If following this method, you are expected to eat as normal on 5 days of the week and restrict your calorie intake to 500 to 600 kcal (women and men, respectively) on 2 days of the week – preferably alternating days of the week.
  • Eat-Stop-Eat – This option involves fasting for a full 24 hours once or twice a week.
  • The warrior diet – Unlike other forms of IF where you are not allowed to eat anything during the fasting window, in the warrior diet, you are allowed to eat a few servings of raw fruit and vegetables during the 20-hour fasting window. During the 4 hour eating window, however, you can eat whatever you wish.

Ps. The 16/8 and the 14/10 are also known as time restricted eating. Some other kinds of time restricted eating plans that you can come across include

  1. The 12/12 – An even easier version of the 16/8 for those who feel like the 14/10 is too much. You can start with this and then progress to the 14/10 as you hope to finally get to the 16/8 method.
  2. OMAD aka 23/1 – This is the most extreme version of the time restricted IF methods. Here you fast for 23 hours a day (without raw fruit and vegetables) and have all your daily calories in just 1 hour.

While some choose to follow the OMAD method, it is usually not recommended. It can lead to a number of side effects including but not limited to dizziness, headaches, shakiness, heart rate changes, and intolerance to cold.

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What Is The Hardest Stage Of Fasting?

The second and third days are perhaps the hardest for many people getting into fasting for the first time ever or for the first time in a long time. Psychologically, day one has all the excitement of doing something new so you may not feel the hunger as much. 

But on days 2 and 3, as your body is trying to get used to the new feeding times, and less pleasant physical symptoms of fasting can present, difficulties can increase. Remember that the mind is harder to train than the body, so if you desire to maintain the fast, hold on and distract yourself from thoughts of hunger and food. Many people become accustomed to the new schedule within a week or two.


What Does a 72-Hour Fast Do To Your Body?

As stated above, going up to 72 hours without eating pushes your body into a prolonged fasting or starvation state. Here the body burns more fat in order to sustain necessary physiological functions, and insulin levels have dropped significantly due to the lack of glucose.

The upside of such a long fast is that you can be sure that your body is now in ketosis, burning fat stores for energy (2). Aside from ensuring fat burn, which can also occur to a lesser extent in a shorter-lasting calorie deficit, there are no other benefits of a 72 hour fast once a week. While you can do it once – which we do not encourage due to the dangerous physiological consequences – just once is enough. Making this a weekly habit is not something that you should aspire to do and can put the brain and heart at a serious risk.

Fasting vs Starving: What Is The Difference?

Fasting is an intentional yet temporary absence of nutrient intake. This can be for just a few hours or a couple of days a week/month. It can include not eating at all or abstaining from certain foods for sometime for either personal, health, or religious reasons.

Starving on the other hand is going for a prolonged period of time with little to no food intake. Please note that fasting can turn into starving if you deny yourself food for a more sustained time for whichever reason.

There is no telling how much water weight one can lose on a 3-day water fast – this depends on the individual and their body. However, regardless of who you are, you will not lose any fat or fat related weight on a 3-day water fast.

Read more: Intermittent Fasting and Running: A Winning Combination or a Terrible Mistake?


  • Mediterranean Diet & Intermittent Fasting: Can You Combine The Two?

Yes, you can. Both IF and the Mediterranean diet have been shown to result in incredible benefits for weight loss and general health (20, 19, 3) so there is no problem in combining the two.

  • Does Fasting Slow Metabolism?

Not necessarily. Shorter fasts can potentially help increase your metabolism, but longer fasts will have the opposite effects, leading to a slower metabolic rate as the body looks to limit its energy expenditure to that required for essential physiological functions (11, 18).

  • Does Fasting Burn Belly Fat?

Fasting for short periods can help boost your metabolism and ketosis, both factors that will help with burning fat stores in the belly and other locations. However, fasting doesn’t work miracles – for fat loss, make sure there’s calorie deficit, food choices that can promote fullness and satiety, and exercise. 

  • Do You Lose Fat On A 5 Day Fast?

Not really. 5 days is too short of a time for you to lose significant fat stores. Plus fasts of this length lead to a more significant loss of water, such as that stored in muscles’ glycogen, rather than fat. Fat loss requires time, patience, and above all a healthy change to your diet and exercising habits.

  • Does Coffee Break A Fast?

Unsweetened coffee will not break you fast, as the caloric content is essentially zero. However, adding milk and/or sugar to the coffee will break your fast.

  • What Can I Drink While Fasting?

Unsweetened coffee, tea, and plain water can be drank while fasting without the risk of breaking your fast.

  • How Much Weight Will I Lose On A 3 Day Water Fast?

There is no telling how much water weight one can lose on a 3-day water fast – this depends on the individual and their body. However, regardless of who you are, you will not lose any fat or fat related weight on a 3-day water fast.

The Bottom Line

The timeline of fasting sheds light on the interesting processes that take place in the body anytime we choose to fast either intermittently or for a longer period. This timeline can be of use to those who want to try IF for the first time but are unsure of just how it works or for those on a ketogenic diet who wish to trigger ketosis. 

It also serves as a caution to those who wish to do longer fasts as a challenge to themselves or for other reasons. Regardless of why you choose to fast in the first place, please remember to be careful and follow the correct and healthy procedures.


This article is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional advice or help and should not be relied on to make decisions of any kind. Any action you take upon the information presented in this article is strictly at your own risk and responsibility!


  1. A dairy-based, protein-rich breakfast enhances satiety and cognitive concentration before lunch in overweight to obese young females: A randomized controlled crossover study (2024, journalofdairyscience.org)
  2. Advantages and Disadvantages of the Ketogenic Diet: A Review Article (2020, ncbi.nlm.nih.gov)
  3. Beneficial effects of intermittent fasting: a narrative review (2023, ncbi.nlm.nih.gov)
  4. Biochemistry, Lipolysis (2023, ncbi.nlm.nih.gov)
  5. Clinical Evidence and Mechanisms of High-Protein Diet-Induced Weight Loss (2020, ncbi.nlm.nih.gov)
  6. Cross Talk Between Insulin and Glucagon Receptor Signaling in the Hepatocyte (2022, pubmed.ncbi.nlm.nih.gov)
  7. Effects on satiation, satiety and food intake of wholegrain and refined grain pasta (2016, pubmed.ncbi.nlm.nih.gov)
  8. Ghrelin – Physiological Functions and Regulation (2015, ncbi.nlm.nih.gov)
  9. Health Effects of Whole Grains: A Bibliometric Analysis (2022, ncbi.nlm.nih.gov)
  10. Intermittent Fasting: Is the Wait Worth the Weight? (2019, ncbi.nlm.nih.gov)
  11. Metabolic adaptation to caloric restriction and subsequent refeeding: the Minnesota Starvation Experiment revisited (2015, pubmed.ncbi.nlm.nih.gov)
  12. Metabolic effects of fasting on human and mouse blood in vivo (2017, ncbi.nlm.nih.gov)
  13. Outcomes of a Medically Supervised Fasting Module on Healthy Females in a Controlled Residential Environment: A Brief Report (2021, ncbi.nlm.nih.gov)
  14. Physiological and Psychological Effects of Medically Supervised Fasting in Young Female Adults: An Observational Study (2023, ncbi.nlm.nih.gov)
  15. Physiology, Leptin (2023, ncbi.nlm.nih.gov)
  16. Role of Insulin in Health and Disease: An Update (2021, ncbi.nlm.nih.gov)
  17. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects (2019, ncbi.nlm.nih.gov)
  18. The cardiovascular, metabolic and hormonal changes accompanying acute starvation in men and women (1994, pubmed.ncbi.nlm.nih.gov)
  19. The Effect of Fasting on Human Metabolism and Psychological Health (2022, ncbi.nlm.nih.gov)
  20. The Mediterranean Diet: An Update of the Clinical Trials (2022, ncbi.nlm.nih.gov)
  21. The use of nutritional supplements to induce ketosis and reduce symptoms associated with keto-induction: a narrative review (2018, ncbi.nlm.nih.gov)
  22. Time to try intermittent fasting? (2023, health.harvard.edu)
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