There are five stages of intermittent fasting: the fed state, the post-absorptive phase, the early fasting phase, the fasting or ketosis phase, and long-term fasting. Each stage is characterized by distinct metabolic changes as the body transitions from burning recently consumed food to drawing on stored energy.
Intermittent fasting has attracted considerable research interest for various potential metabolic associations. Some research has explored various physiological changes associated with this eating pattern. The body does not instantly switch into a fasting state. It gradually moves through intermittent fasting stages over time. Each phase involves unique adjustments in how your body uses energy. Understanding these stages of intermittent fasting helps explain how fasting affects metabolism and energy use over a period of hours or days.
Disclaimer: Every fasting experience is personal. How your body progresses through these stages depends on individual factors, including starting glycogen levels, metabolic rate, and overall diet. Speak with a healthcare provider before starting any fasting protocol.
| Stage | Time Since Last Meal | What Happens | Key Process |
|---|---|---|---|
| Stage 1 (Fed State) | 0–4 hours | Digestion and absorption of nutrients. | Anabolic state |
| Stage 2 (Post-Absorptive) | 4–8 hours | Shifting from food energy to stored energy. | Metabolic transition |
| Stage 3 (Early Fasting) | 8–12 hours | Glycogen depletes; fat utilization begins. | Energy transition |
| Stage 4 (Ketosis) | 18+ hours | Fat becomes the primary fuel source. | Ketone production |
| Stage 5 (Long-term) | 48+ hours | Deep adaptation to fat utilization. | Protein moderation |
Intermittent fasting, a dietary regimen that alternates between periods of eating and fasting, has five distinct stages. Let’s explore the fasting timeline by hour:
The first of the 5 stages of intermittent fasting is the Feasting phase. This stage begins as soon as you start eating. It continues until your body has fully digested and absorbed all the nutrients from your meal.
During this stage, insulin levels rise to help with nutrient absorption. The body enters an anabolic state, which promotes the building and storing of energy reserves (1). This is the stage where you decide what to eat during intermittent fasting, with an emphasis on nutrient-rich and balanced meals.
Once your body has finished digesting and absorbing your last meal, you transition into the Post-Absorptive phase. This typically happens about 4–8 hours after eating.
This is one of the key intermittent fasting times where your body begins to switch from burning recently consumed calories to burning stored energy. It is important to understand this transition because it sets the foundation for later stages.
The Early Fasting phase kicks in around 8–12 hours into your fasting window. The exact timing depends on your metabolism and what you ate during your last meal.
During this stage, your body is using its glycogen reserves for energy. As glycogen stores start to become depleted, the body increasingly draws on fat reserves. It may also convert amino acids to glucose to maintain energy levels (2).
As the Early Fasting phase ends, you enter the Fasting Phase. This might begin around 18 hours into a fast (3), though it varies for everyone.
This stage is characterized by the transition to ketosis as your glycogen stores are likely to be fully depleted. The body increasingly draws on stored fat as a fuel source, which results in the production of ketone bodies (4). Some preliminary research has explored potential associations between autophagy and cellular health markers during this time (5).
The final stage is the Long-term Fasting phase, which begins around 48 hours into a fast. This phase is typically reached in extended types of intermittent fasting.
In this stage, your body has adapted to running on stored fat. Hormonal changes are thought to help moderate muscle protein breakdown, though this stage involves significant physiological stress (6). While long-term fasts that reach this stage can be conducted (7), they are not recommended for most people without professional guidance.
When looking at intermittent fasting for weight loss, your body goes through the same fasting stages. Understanding how each stage functions can help you see how long you should fast for.
This stage does not directly involve drawing on stored energy, but it sets the foundation. Eating nutritious, balanced meals helps support your metabolism and keeps your body nourished.
During this stage, your body is shifting from using the food just eaten to stored energy sources. It is at this point that your body starts to prepare for deeper energy shifts.
Around 12 hours into fasting, the body increasingly draws on fat stores as glycogen depletes (8). This is a metabolic shift that some research associates with changes in body composition over time (9).
By now, you have likely depleted your glycogen stores. The body draws increasingly on fat as fuel, which some research associates with changes in body composition (9).
This stage usually occurs in extended types of fasting. Long-term fasts that reach this stage are not recommended for most people and should only be performed under medical supervision (7).
The metabolic changes that begin during the Early Fasting phase are often cited as the mechanism by which intermittent fasting may support body weight changes over time. In one paper, researchers described how IF may shift the body’s metabolic approach to stored fat over time (10). That said, the rate of body composition change can considerably vary from person to person. Factors such as your metabolic rate, diet quality, and activity level all play a part (11).
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For many people, 16 intermittent fasting hours strike a balance between attainability and practicality. It’s long enough for the body to use up a significant portion of its glycogen reserves and potentially transition into a fat-burning state, yet short enough to be manageable in everyday life.
Here are a few reasons why 16 hours is a commonly-used intermittent fasting time:
After about 12 hours of fasting, the body increasingly draws on stored fat (a process called lipolysis) as glycogen becomes depleted (2). Some research has explored this shift in relation to body composition changes (12). Findings vary from one person to the next. With a 16-hour fast, you allow your body time to transition into this state.
A 16-hour fast fits seamlessly into most people’s routines, making it a realistic and sustainable choice. By shifting your first or last meal, you can easily achieve the necessary fasting window without disrupting your daily routine. This ease of integration makes it more user-friendly for many people.
Autophagy is a cellular cleansing process. A 16-hour fast lands in a timeframe that allows for the potential cellular maintenance associations of autophagy, which some research continues to explore in human populations (13).
| Step # | Step | Key Action |
|---|---|---|
| 1 | Decide on windows | Choose your 16-hour fasting and 8-hour eating blocks. |
| 2 | Transition gradually | Slowly push your meals to fit the new schedule. |
| 3 | Plan your meals | Focus on nutrient-dense whole foods. |
| 4 | Hydrate consistently | Drink water and unsweetened tea throughout the day. |
| 5 | Monitor how you feel | Adjust your schedule if you experience discomfort. |
The 16:8 model involves fasting for 16 hours each day and restricting your eating to an 8-hour window. This is a popular intermittent fasting schedule for beginners. Here is how to properly fast using this method:
The first step is to choose your eating and fasting windows. Many people find it easiest to fast overnight, skipping an early breakfast and beginning their eating window at noon. This typically means their fasting window is from 8 PM to 12 PM the next day. The timing can be customized to suit your lifestyle.
Jumping straight into a 16-hour fast can be an adjustment, especially if you are used to eating at regular intervals. It can be helpful to make the transition gradually. Consider starting with a shorter 12-hour or 14-hour fast, and slowly increase the time.
Meal planning plays an important role in how you feel during your fasting hours. During your 8-hour eating window, consume nutrient-dense foods. Focus on balanced meals that include a good mix of protein, healthy fats, and complex carbohydrates.
During the fasting hours, it is important to stay hydrated. Water, black coffee, and tea without milk or sugar are generally great options. When you break your fast, you can rehydrate with nutritious fluids like bone broth.
If a 16-hour fast feels too challenging at first, it is okay to adjust your windows. The goal is to find a sustainable pattern that works for you. If you are feeling unwell, consider breaking your fast early. Always prioritize your comfort and consult a professional if you have concerns.
Read more: Is Vegan Intermittent Fasting the Right Choice for You?
There are several types of intermittent fasting you can consider. Finding the right intermittent diet schedule depends on your lifestyle, preferences, and individual needs.
The 14:10 Method
This involves a 14-hour fast and a 10-hour eating window. It is often considered a gentle starting point. This schedule allows for a longer eating period, which can be easier to manage (14).
The 5:2 Diet
In this approach, you eat a balanced diet for five days of the week. On the remaining two days, you consume a very low number of calories (often around 500–600) (15). This requires careful meal planning to ensure you get enough nutrients on those two days.
Alternate-Day Fasting
This involves fasting every other day. On fasting days, some people consume no calories, while others consume a small amount (16). This is a more intense practice and may not be suitable for everyone.
When practicing intermittent fasting, what you eat during your eating window is just as important as the fast itself. Your body needs proper nourishment to support your daily activities.
Focus on incorporating whole, nutrient-dense foods. Lean proteins, such as chicken, tofu, beans, and lentils, can help support your muscles and keep you satisfied. Healthy fats, including avocados, nuts, seeds, and olive oil, offer sustained energy (17).
Complex carbohydrates are also a great addition. Foods like sweet potatoes, quinoa, brown rice, and oats provide a steady release of energy. Incorporate plenty of vegetables and fruits to ensure you are getting essential vitamins, minerals, and fiber (18).
Try to limit your intake of ultra-processed foods, sugary snacks, and refined grains. These items can cause rapid spikes and drops in blood sugar (19), which may leave you feeling tired or hungry during your fasting window.
While many people find intermittent fasting helpful, it is not designed for everyone. Certain groups of people should avoid fasting or speak with a qualified professional before trying it.
Pregnant or breastfeeding women need consistent nourishment to support themselves and their babies. Fasting can make it difficult to meet these increased nutritional needs (20).
Individuals with a history of disordered eating should also avoid fasting. Restrictive eating windows can sometimes trigger negative relationships with food (21).
People who take medications that need to be consumed with food at specific times may find fasting challenging (22). Additionally, children and teenagers, who are still growing and developing, should not practice intermittent fasting.
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When you first start an intermittent fasting schedule, your body needs time to adjust. During this transition period, you may experience some common side effects.
Many people report feeling hungry or experiencing cravings, especially in the first few days. This is a natural response as your body adapts to a new eating pattern (23).
You might also notice changes in your energy levels. Some individuals feel fatigued or sluggish as their body shifts from using readily available glucose to drawing on stored energy.
Mild headaches, irritability, or difficulty concentrating can also occur (23). Staying hydrated and ensuring you eat balanced, nutrient-dense meals during your eating window can often help support your body through these adjustments. If these feelings persist or become overwhelming, it may be a sign to adjust your fasting window or consult a professional.
Intermittent fasting can be approached slightly differently by women. Female bodies can be particularly sensitive to changes in calorie intake and energy availability.
For some women, prolonged fasting may influence their menstrual cycle or overall energy levels. It is often suggested that women start with shorter fasting windows, such as 12–14 hours, rather than jumping straight into a 16-hour fast (24).
A gentle, gradual approach allows the body to adapt without feeling overly stressed. Paying close attention to how you feel, resting when needed, and ensuring adequate nutrition during the eating window are all important practices.
If you notice ongoing discomfort, changes in your cycle, or persistent fatigue, consider shortening your fasting window or taking a break from the practice.
Building a lasting intermittent fasting routine requires patience and consistency. Here are a few tips to help you along the way:
Fasting may support changes in abdominal fat over time through calorie restriction and metabolic changes (25), though individual outcomes vary considerably. When the body draws on stored fat for energy during a fasting window, this may include visceral (belly) fat, but fat loss location cannot be targeted (26). Overall calorie balance, diet quality, and activity level all influence outcomes.
Individual timelines vary considerably. Some people notice changes in hunger patterns or energy levels within the first 1–2 weeks. Visible body composition changes typically take longer and depend on overall diet quality, activity, and individual metabolic rate.
A 16-hour fast may not be long enough to enter ketosis for most individuals. Typically, it takes around 18 or more hours of fasting for the body to transition deeply into ketosis, where ketone production significantly increases (3).
A 16-hour fast allows more time in the fat-adapted metabolic state, which some research associates with greater changes in body composition markers (27). However, 12-hour intermittent fasting for beginners is great as a starting point and carries its own metabolic associations, making it a great option for many people.
Disclaimer: Every fasting experience is personal. How your body progresses through these stages depends on individual factors, including starting glycogen levels, metabolic rate, and overall diet. Speak with a healthcare provider before starting any fasting protocol.
Intermittent fasting has five distinct stages. In the beginning, your body uses up its glycogen reserves for energy. After this, it starts using stored fat for fuel, contributing to potential body composition changes. This process might also trigger autophagy, with some research suggesting potential long-term metabolic associations. By selecting a sustainable fasting and eating window, incorporating balanced meals, and staying hydrated, you give yourself the best opportunity to experience the potential associations explored in IF research. Understanding the stages of intermittent fasting can help you make informed choices that fit your lifestyle.
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