The Stages of a 72-Hour Fast: Understanding the Changes Inside and Outside Your Body

Numerous studies on intermittent fasting have reported positive effects on various aspects of health, including blood sugar control, cellular repair, weight loss, inflammation, and more. While you may experience similar effects from a 72-hour fast, it’s crucial to understand how fasting impacts your body, and how your efforts before and after contribute to the overall process. Discover how Fastic can assist you through your fasting experience, making it smooth and effortless.

Continue reading to learn what happens during the different stages of fasting by the hour, helping you better understand how your exertions factor into your overall health.

Pre-Fast Preparation: Setting Yourself Up for Success

Proper preparation can make it easier for your body to adapt, minimize discomfort, and maximize fasting effects.

Mindset and motivation

Defining clear goals is necessary to sustain the fast for its entirety. Set clear objectives, such as improving mental clarity, promoting longevity, and encouraging weight loss. Most importantly, your goals must be realistic and attainable.

Nutritional preparation

Before beginning the fast, ideally days in advance rather than the day before, eat nutrient-dense, low-carb meals. Light, whole-food foods such as fruits and vegetables, provide essential nutrients to help you start the fast on a strong note. Avoid processed foods, and focus on staying well-hydrated before, during, and after the fast.

Mental and emotional awareness

Be ready to experience and manage hunger. Understand how your body best deals with these feelings. It could be through meditation, drinking water, reading, or exercising. Try to adjust your schedule to reduce your workload and prevent mental overwhelm.

Overview of the Stages and Effects of a 72-Hour Fast

To provide a clear summary of what occurs during a 72-hour fast, the following table outlines the key stages of fasting, their respective timeframes, and the associated impacts on the body.

Stage	Time Frame	Body Processes	Impacts on the Body
Stage 1	0 - 4 hours	- Digestion and nutrient absorption (fed state) - The body utilizes nutrients from the last meal	- No feelings of hunger - Normal physiological function
Stage 2	4 - 12 hours	- Glycogenolysis begins (breakdown of glycogen for energy) - Energy needs dictate glycogen expenditure	- Possible onset of hunger - Energy levels may begin to fluctuate
Stage 3	12 - 48 hours	- Transition on fat metabolism (metabolic switch) - Autophagy activation - Ketogenesis (production of ketone bodies)	- Side effects may occur - Increased BDNF production - Weight loss may begin due to fat utilization
Stage 4	48 - 72 hours	- The body enters survival mode (burning fat exclusively) - Further amplification of autophagy, ketogenesis, and other processes	- Slowed metabolism - Conservation of energy - Focus on vital processes for basic functioning

Stages of a 72-Hour Fast: A Comprehensive Breakdown

Pinpointing the precise stages of fasting hour by hour can be challenging, as individual physiological responses vary widely; some people may activate these processes sooner or later than others. That said, numerous studies and research provide valuable insights into what generally occurs in the body during fasting, allowing us to outline the relevant stages of a 72-hour fast with reasonable accuracy.

Stage 1 (0 - 4 Hours)

At the beginning of the fast, your body will work as it usually does; using all the nutrients from your last meal to perform all essential functions. The body is digesting and absorbing nutrients, which occurs in a condition known as the fed state or postprandial state. You will most likely not feel hungry at this point.

Stage 2 (4 - 12 Hours)

During this time, especially when energy is needed, glycogen stores begin to be broken down for energy through a process known as glycogenolysis. The timing of this process varies based on factors such as activity levels, the nutritional characteristics of the last meal consumed, and individual metabolism. For instance, meals high in carbohydrates lead to increased glycogen stores that can be quickly mobilized during periods of high energy demand. You may start feeling hungry due to the depletion of these glycogen stores. This sets the stage for the next phase, where the body’s energy utilization continues to evolve.

Stage 3 (12 - 48 Hours)

As fasting continues, you may start experiencing side effects such as headaches, nausea, and fatigue. This stage also marks the metabolic switch, which can occur within 12 to 36 hours after food intake ceases, signifying the body’s transition from glycogen utilization to fat metabolism. This transition is influenced by factors such as initial liver glycogen content and physical activity levels.

During this stage, liver glycogen stores are largely depleted (though some small amounts may remain), and fatty acids from fat tissue start to mobilize, meaning the body is now running on the burning of stored fat for energy, which can lead to weight loss. When glycogen stores are low, the liver converts fatty acids into ketone bodies through ketogenesis, and insulin levels are reduced. Ketone bodies serve as an alternative energy source for various tissues, primarily for the brain.

Autophagic processes are activated, allowing cells to degrade and recycle damaged components to maintain cellular health. A study investigating the effects of fasting on animals suggests that this cellular activation may occur within 24 to 48 hours of fasting, although further research is needed to confirm these findings. Autophagy reduces inflammation by degrading damaged cellular components and pathogens, thereby preventing the accumulation of harmful substances that can trigger inflammatory responses.

As the body adapts to the shifts in energy sources, the brain is undergoing shifts of its own. BDNF is a critical neurotrophic factor that supports neuronal health, promotes neuroplasticity, and enhances cognitive function. BDNF production is a key neuronal adaptation to fasting. During this period, the production of brain-derived neurotrophic factor (BDNF) may rise, with some studies indicating significant increases around the 24-hour mark, although individual responses can vary. Higher levels of BDNF are linked to improved mood and enhanced cognitive performance.

Stage 4 (48 - 72 Hours)

During this stage, your body is undeniably in survival mode, relying entirely on the burning of fat for energy. It conserves energy by slowing down metabolism and reducing non-essential physiological functions, prioritizing vital processes to ensure basic functions are fulfilled. The physiological processes activated in the previous stage, such as autophagy and ketogenesis, are further amplified.

Breaking the 72-Hour Fast: Tips for Healthy Post-Fasting

After fasting, avoid going back to your usual diet. Reintroduce foods gradually in small portions, preferably starting with smoothies. Start with light, easily digestible foods, such as chicken or bone broth or smoothies. Doing so will help prevent refeeding syndrome, which can lead to life-threatening health complications due to sudden electrolyte imbalances. Learn more about what to eat after fasting so you know exactly what to include on your shopping list.

Considerations Before and After a 72-Hour Fast

There are a few questions you should ask yourself before and after undergoing a fast of this magnitude.

Before a 72-hour Fast

Am I prepared to fast for 72 hours?

Before undergoing a 72-hour fast, trying other less intense fasting protocols is advised. Some options with shorter fasting windows are the 16:8 or the 14:10 fast. Once you’ve mastered these, you can step it up and see how your body responds to longer fasting periods.

When should I do a 72-hour fast?

Schedule a 72-hour fast when you don't have any major responsibilities, this could be during the weekend or whenever your work commitments are lighter. This enables you to concentrate fully on the fast without adding any additional stress beyond what the fasting itself may entail.

After a 72-hour Fast

How did my body adapt to the fast?

Reflect on your body’s response to the fast. It might be helpful to journal while fasting to document your journey. Evaluate how your body adapted and if it would be able to handle fasting again. If your body doesn’t respond well to fasting, don’t be discouraged! Consider exploring different intermittent fasting methods; you’re likely to find one that aligns perfectly with your body.

What modifications would I make for my next fast?

Think about adjustments you could make to improve your experience next time. You may find that a longer preparation period or selecting different types of foods before the fast could have better supported your energy levels during the initial stages. For those wishing to dive into the world of water fasting, read this article on 7-Day Water Fasting.

From Knowledge to Action: Your Next Steps in Fasting

As you’ve learned, numerous physiological processes occur during fasting, especially in stage 3 (12 to 48 hours). However, remember that not all bodies respond to fasting in the same way; consider this breakdown a general guideline and not a one-size-fits-all approach. Download the Fastic app to discover how it supports you through every stage of your fasting journey — before, during, and after. Get expert guidance on key fasting questions, including what to eat before and after fasting, your optimal hydration schedule, and effective relaxation exercises to help manage hunger.