Prolonged fasting: What happens to your body after 72 hours?

Food is readily available and eating patterns have become more frequent than ever; however, prolonged fasting has become a topic of intrigue and exploration. 

Beyond the common understanding of skipping meals, the practice of prolonged fasting—abstaining from food for a span of 72 hours or more—has garnered significant attention due to its potential physiological and psychological effects on the human body. 

Delving into the depths of our metabolic intricacies, prolonged fasting unveils a cascade of changes that unfold as the body adapts to a state of nutrient deprivation.

Is there any science behind fasting?

Fasting, in its various forms, has been practiced by humans for centuries, often intertwined with cultural and religious traditions. 

In recent years, scientific research has begun to uncover the intricate mechanisms behind fasting’s effects on the body.

At the heart of this exploration lies a deep understanding of how fasting influences our metabolism, cellular responses, and hormonal regulation. The foundation of fasting’s effects lies in its profound impact on metabolism [1].

When we abstain from food, our body shifts from utilizing glucose as its primary energy source to tapping into stored fat reserves.

This transition triggers a metabolic state known as ketosis, characterized by the increased production and utilization of ketone bodies. These molecules serve as an alternative energy source, particularly for the brain and other organs, in the absence of glucose.

Is there any science behind fasting?
Photograph: Neirfy007/Envato

Cellular responses to nutrient deprivation

At a cellular level, fasting initiates a process called autophagy. This remarkable mechanism involves the body’s recycling of damaged or dysfunctional cellular components. 

By breaking down these elements and using their building blocks for energy, autophagy contributes to cellular detoxification and repair. This process not only aids in removing harmful substances but also supports the longevity and functionality of cells.

Role of insulin and glucose

Insulin, a hormone produced by the pancreas, plays a central role in regulating our blood sugar levels. During fasting, insulin levels drop, allowing the body to tap into its glucose reserves stored in the liver and muscles. 

As reserves are depleted, blood sugar levels decrease, triggering the use of alternative energy sources like fat. This insulin sensitivity improvement can have positive implications for individuals at risk of insulin resistance and type 2 diabetes.

Featured product offer
ProLon® 5-Day Meal Program
  • The only nutrition program designed and tested by 14 renowned universities worldwide.
  • Clinically shown to reduce fine lines and wrinkles after 3 cycles (15 days total).
  • Each kit contains various soup flavors, snacks, teas, and supplements.

What are the stages of prolonged fasting?

Prolonged fasting is not a monolithic experience; rather, it unfolds in distinct stages, each marked by its own set of physiological adaptations and effects on the body. 

As we embark on a journey of abstaining from food for 72 hours or more, our body navigates through these stages, each with unique significance.

Initial phase (0-24 hours)

  • Glycogen depletion and transition

The first phase of prolonged fasting begins as soon as you cease intake of food. Your body immediately begins tapping into its readily available glucose stores, stored in the form of glycogen in the liver and muscles. 

As these glycogen reserves are depleted within the first 24 hours, glucose becomes scarcer, and your body starts shifting its energy source from glucose to stored fats.

Adaptation phase (24-48 hours)

  • Entering ketosis

As your body realizes that glucose is in short supply, it starts producing ketone bodies from the breakdown of fats in the liver—a process called ketogenesis. 

Ketones become a primary source of energy, especially for the brain, which can use them in the absence of glucose [2]. This shift into a state of ketosis is a hallmark of prolonged fasting and brings about unique metabolic benefits.

Deep ketosis (48-72 hours)

  • Enhanced cognitive clarity and autophagy

Deeper into the fast, usually after about 48 hours, you enter a phase of deep ketosis. This stage often comes with increased mental clarity and focus. 

The elevation of ketone bodies, in addition to their energy-providing function, has been linked to improved cognitive function and reduced brain inflammation.

How does fasting affect body composition?

Prolonged fasting doesn’t just affect how our body generates energy and repairs itself; it also has a significant impact on our body composition—the proportion of fat, muscle, water, and other tissues that make up our physique. 

Understanding these effects is crucial for those interested in optimizing their weight and overall body composition.

Fat breakdown and utilization

During a prolonged fast, as your body exhausts its glycogen reserves and transitions to ketosis, it turns to stored fat as its primary energy source.

This process of breaking down and utilizing fat reserves can lead to gradual fat loss over the fasting period. 

As fatty acids are released from adipose tissue and used for energy, you may notice a reduction in body fat percentage.

Preservation of lean muscle mass

One common concern with fasting is the potential loss of muscle mass. However, during prolonged fasting, the body employs several mechanisms to prioritize the preservation of lean muscle tissue. 

The surge in growth hormone, observed particularly during deep ketosis, plays a key role in sparing muscle while promoting fat breakdown. 

Additionally, the metabolic shift to ketones as an energy source helps conserve amino acids, the building blocks of proteins.

Featured product offer
ProLon® Reset – 1 Day Fasting Kit
  • 1-day nutrition program uses plant-based nutrients for essential nutrition and energy without triggering food sensing.
  • A scientific breakthrough with 20 years of research and awarded 40+ patents—including the first-ever nutrition program designed to improve longevity and healthspan.
  • 1 day kit contains soup mixes, snacks, energy bars, and tea.

Prolonged fasting vs. calorie restriction

Unlike traditional calorie restriction diets, which often lead to a combination of muscle and fat loss, prolonged fasting appears to have a more targeted impact on fat stores.

This can be attributed to the body’s preferential use of fat for energy, sparing lean muscle mass as long as growth hormone levels remain elevated. 

Calorie-restriction diets, on the other hand, often result in a higher proportion of muscle loss due to the consistent reduction in energy intake.

What are the metabolic effects of prolonged fasting?

The process of prolonged fasting triggers a cascade of metabolic changes within the body that extend far beyond simply switching energy sources.

These changes influence various aspects of our health, from insulin sensitivity to the regulation of hormones, all of which play pivotal roles in maintaining overall well-being.

Improved insulin sensitivity

Insulin sensitivity, the ability of cells to respond effectively to insulin and regulate blood sugar levels, is a key factor in metabolic health.

Prolonged fasting contributes to improved insulin sensitivity by reducing the body’s reliance on glucose for energy. 

As blood sugar levels decrease and the body switches to utilizing ketone bodies, cells become more efficient at responding to insulin, potentially reducing the risk of insulin resistance and type 2 diabetes.

improved insulin sensitivity

Impact on growth hormone levels

One of the remarkable effects of prolonged fasting is the surge in growth hormone production. Growth hormone plays a crucial role in tissue repair, maintenance of lean muscle mass, and overall growth. 

The elevation of growth hormone levels during fasting supports the preservation of muscle tissue while promoting the breakdown of fats for energy. 

Role of the liver in fatty acid metabolism

The liver acts as a central hub for many metabolic processes, including the breakdown of fatty acids during prolonged fasting. 

As fat reserves are mobilized and released into the bloodstream [3], the liver plays a vital role in converting these fatty acids into ketone bodies, which become the primary energy source for various tissues, including the brain. This process of ketogenesis in the liver is central to the metabolic adaptations that occur during fasting.

How does prolonged fasting affect the brain?

Beyond its influence on physical health and metabolism, prolonged fasting holds fascinating implications for cognitive function and mental clarity. 

As the body adapts to a state of ketosis and undergoes cellular repair processes, the brain also experiences a unique set of benefits that can enhance focus, clarity, and overall cognitive performance.

Enhanced mental clarity and focus

During prolonged fasting, the brain shifts its primary energy source from glucose to ketone bodies. This metabolic adaptation has been linked to heightened mental clarity and improved focus. 

Many individuals report experiencing a sense of heightened alertness and concentration during fasting periods.

Neurological reasons behind improved focus

Ketones are not only an efficient energy source but also have potential neuroprotective effects [4]. They may help reduce oxidative stress and inflammation in the brain, contributing to improved neural function. 

Additionally, the activation of pathways associated with autophagy—cellular cleanup and repair—can lead to the removal of dysfunctional cellular components within brain cells, potentially promoting optimal neuronal function.

Neuroplasticity enhancement and brain health

Neuroplasticity, the brain’s ability to reorganize and adapt by forming new neural connections, plays a vital role in learning, memory, and cognitive resilience. 

Prolonged fasting may support neuroplasticity by fostering an environment conducive to neural growth and repair. 

The reduction in oxidative stress and inflammation, coupled with the production of brain-derived neurotrophic factor (BDNF) triggered by fasting, contributes to the brain’s adaptability and long-term health.

Closing thoughts

The practice of prolonged fasting emerges as a compelling avenue for exploration, offering insights into the remarkable adaptability of the human body. 

We’ve embarked on a comprehensive journey, dissecting the stages of fasting, unraveling its effects on metabolism, body composition, cognitive function, and beyond. 

From the initiation of ketosis to the activation of autophagy, each facet of prolonged fasting contributes to a holistic tapestry of benefits that extend far beyond the absence of food.


Is prolonged fasting safe for everyone?

Prolonged fasting can be safe for many individuals, but it’s essential to consider your medical history, current health conditions, and individual needs. Consulting a healthcare professional before embarking on a prolonged fast is strongly recommended, especially if you have pre-existing health issues.

Can I drink water or other liquids during a prolonged fast?

Yes, staying hydrated is crucial during prolonged fasting. Water, herbal teas, and non-caloric beverages can be consumed to maintain hydration and support the fasting process. However, beverages containing calories or sugars should be avoided, as they can break the fast.

Will fasting lead to muscle loss?

While fasting can result in some muscle breakdown, the effects are usually minimal, especially if the fast is conducted properly and for a reasonable duration. The surge in growth hormone and the body’s preferential use of fat for energy help mitigate muscle loss.

Featured product offer
ProLon® Intermittent Fasting Bars
  • The recipe is a unique blend of macronutrients and micronutrients that has been researched for over 20 years at USC.
  • Created with proprietary formula made up of high-quality ingredients that don't activate the body's nutrient-sensing system.
  • Each box contains 12 bars--all plant-based, non-GMO, soy-free, dairy free, and gluten-free.


Photograph: melis82/Envato
The information included in this article is for informational purposes only. The purpose of this webpage is to promote broad consumer understanding and knowledge of various health topics. It is not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of your physician or other qualified health care provider with any questions you may have regarding a medical condition or treatment and before undertaking a new health care regimen, and never disregard professional medical advice or delay in seeking it because of something you have read on this website.