Written by: THE BALANCE
Medically reviewed by Dr. Sarah Boss, MD
Last updated: February 5, 2026
Fact checked
Quick Summary
  • When it comes to notions of maintaining a healthy lifestyle, appropriate eating and sufficient exercise are frequently regarded as cornerstones.
  • While this is absolutely true, there are more aspects of our connection to food and a healthy lifestyle to consider.
  • Fasting is the voluntary restriction or elimination of food, drink, or both for a time period.
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When it comes to notions of maintaining a healthy lifestyle, appropriate eating and sufficient exercise are frequently regarded as cornerstones. While this is absolutely true, there are more aspects of our connection to food and a healthy lifestyle to consider.

Fasting is the voluntary restriction or elimination of food, drink, or both for a time period. Although short-term fasting is commonly perceived as unpleasant, restricting, or limited for religious purposes, it can provide excellent health advantages. As research in the field of health expands, fasting is gaining legitimacy as a technique of weight management and illness prevention. Nonetheless, it is essential that fasting be conducted in a proper and healthy manner.

Fasting cleanses the body of impurities and causes cells to undergo procedures that are not normally stimulated when a constant supply of fuel from food is available. During a fast, the body is deprived of its normal supply of glucose, requiring cells to manufacture energy from alternative sources.

Although fasting is not a new concept, it is witnessing a comeback in popularity as more people become aware of its possible health benefits. The body goes through various fasting stages when we fast. If you are preparing for your first fast or searching for methods to enhance your next fast, there are several things you need to do to prepare. Understanding the various stages of fasting is the first step. This information helps you physically and mentally prepare for the effects of fasting on the body.

The stages of fasting described here are based on water fast, a typical fast in which one abstains from eating and drinks only water for 12-48 or more. Individual experiences can vary based on the type of fast, the individual’s age, or health, but this should give you a basic notion of what to anticipate while you fast: (source: CDC, 2024)

Battery save mode

Stage one will last for the first few days of the fast, or approximately 12-48 hours after the last meal. During this first stage, hunger and weariness are triggered by a number of biological processes. When you consume food on a regular basis, your body processes glucose to obtain the energy it requires to operate normally.

 During a fast, your body must create sugar for energy, so it initiates a process known as gluconeogenesis. Gluconeogenesis is the liver’s conversion of non-carbohydrate substances such as lactate, amino acids, and lipids into glucose. Your basal metabolic rate becomes more productive and consumes less energy as your body enters “battery-saving mode.” 

This method of conserving energy involves reducing your blood pressure and heart rate. At this point, you can feel exhausted. However, if you persist for a while longer, a portion of that energy expended will return. [2]

Fat burning mode

The second stage begins at the conclusion of the second day and lasts until the seventh day. As the body through the process of ketosis, you may feel less hungry and less exhausted. When a typical diet of carbohydrate-rich meals is consumed, the body converts starches and sugars into glucose. 

The body’s major source of energy is glucose. Nonetheless, when you fast or enter ketosis, glucose becomes hard to obtain and your body must rely on fat storage for energy. The breakdown of fat into fatty acids and glycerol. The liver produces ketones from glycerol. The glycerol is fragmented down by the liver to produce more glucose, which is then utilized by the brain when glucose becomes scarce.

Healing mode

Stage three usually occurs between days eight and fifteen. In the third stage, your body begins to adopt a “healing mode.” This healing process commences as your process of digestion rests from the daily stressors and pollutants it is exposed to.  (source: NIH (NLM), 2020)

Consequently, your body receives fewer free radicals, and oxidative stress lowers. On the other hand, fasting induces stress which offers another advantage. This stress is similar to the stress induced by physical activity, which eventually strengthens you and your immune system more resistant.

Extended healing mode

The fourth stage is the continuation and completion of the cleansing and healing processes initiated in earlier phases. The longer one fasts, the more time and opportunities the body has to mend and purify. 

Fasting, especially after the initial seven days, requires unwavering commitment. In these later stages, the benefits of the fast may be a combination of all the previous stages or the achievement of an individual health objective. For some, it is weight loss, whilst for others, it is a technique to treat a specific health issue. [4]

Easing out of fasting mode

How you decide to break your fast is essential. Depending on the duration of your fast, you might have to ease back into solid foods. Fruit juices, cooked vegetables, and broths can assist your body and digestive system adjust to eating as internal functions return to normal.

The practice of refraining from food and drinks for a length of time constitutes fasting. Essentially, fasting increases our metabolism, decreases inflammation, enhances brain abilities, and improves our general health.

Not every fast provides the same benefits. There are various stages of fasting, and the benefits of fasting differ according to how long you can fast. The five principal phases of fasting are as follows:

  • Stage one – Ketosis
  • Stage two – Autophagy
  • Stage three – Growth hormone
  • Stage four – Insulin suppression
  • Stage five – Immune cell regeneration

Ketosis [5]

After 16–18 hours of fasting, the metabolic state of ketosis develops as the body starts to dissolve and burn calories. As an outcome of this condition, ketone bodies are produced. Brain, heart, and skeletal muscle cells utilize ketone bodies as an alternate energy source when glucose is unavailable.

Ketone bodies are molecules created by the liver throughout gluconeogenesis, the process by which glucose is generated during fasting. It is an alternate energy source. During fasting times, when glucose is unavailable, the brain uses ketones as its primary energy source.

As ketones suppress appetite, your interest in food will begin to diminish during the fasting phase. Additionally, this stage is an excellent alternative for those who wish to lose weight. Your body enters a fat-burning phase, and since you haven’t eaten in a while, you begin to swiftly shed body fat.

Autophagy (source: NIDDK (NIH), n.d.)

After a full day of fasting, the body enters its repair phase. It begins regenerating damaged and aged cells and reduces inflammation. This phase is optimal for anti-aging and anti-inflammatory actions. When the body experiences moderate stress, the cells become more functional.

One of their functions is to induce autophagy. During autophagy, cells evaluate all of their individual components and replace everything that is old, damaged, or malfunctioning with brand-new versions. The aged components will be either recycled or thrown away. When talking about how long to fast for autophagy, just twice per month is recommended.

Growth hormone

Any sort of fasting is not particularly advantageous because it induces a strong biological push, causing the hunger center and appetite hormones to go into excess. This is a response the body has when it is deprived of nutrients.

Humans have a propensity to always reward themselves for their hard effort. On non-fasting days, people typically desire to indulge in a variety of unhealthy meals and beverages, for example, following the end of the fasting period or after strenuous exercise. (source: NIDDK (NIH), n.d. (Metabolism))

Insulin decrease

Although fasting reduces insulin levels, this does not indicate that it is entirely advantageous. For instance, insulin levels within the body reach their lowest level 54 hours after fasting commences.

It is viewed favorably because it can prevent irritation, increase insulin sensitivity, and decrease insulin resistance. However, reducing blood sugar levels might result in undesirable side effects such as increased thirst and hunger.

Immune cell regeneration (source: MedlinePlus, n.d. (Gluconeogenesis))

After 72 hours of fasting, the body eliminates old immune cells and regrows new ones. In numerous cell groups, extended fasting decreases the levels of insulin-like development factor and protein kinase in the circulation.

Several days of fasting probably won’t harm the majority of healthy people, assuming they don’t become dehydrated. To maintain health, your body requires minerals, nutrients, and other vitamins from your diet. Insufficiency can cause symptoms such as weariness, drowsiness, constipation, dehydration, and inability to tolerate cold temperatures.

Pregnant or breastfeeding women, those with a chronic illnesses, the elderly, and toddlers should avoid fasting. These three methods of calorie restriction or fasting have shown positive impacts on lifespan in laboratory studies:

Time-restricted consumption (source: StatPearls, 2024 (Ketosis))

This is the technique of restricting calorie consumption to a specific time period that corresponds to our internal clock. Circadian rhythm, the natural cycle that informs our bodies when to rest, awake, eat, and more, is commonly referred to as our “body clock.”  

In accordance with our circadian cycle, we only consume food during a daily eight to twelve-hour window while fasting, such as between 10 a.m. and 6 p.m. When body systems are in sync, they function more efficiently, eating around midnight when our body would normally be sleeping throws off our natural repair system. In addition, it is essential for our health to allow our systems more time for restoration.

Intermittent reduction of calories

Dieting is the practice of consuming fewer calories per day. The focus of research has been on a two-day diet in which calories are halved and carbohydrates are restricted for two straight days per week. The body undergoes a brief, rigorous course of therapy using this method. 

The intermittent calorie restriction method also serves as a reminder that we do not need to consume continuously. When we do consume, we can make intelligent decisions and continue routine activity and exercise while consuming less fuel. [10]

Periodic fasting using diets resembling fasting

This involves restricting caloric intake for between three and five days to induce glycogen depletion and ketosis in the cells. Although it is possible to accomplish this without eating, it is not the safest option. 

A five-day calorie-restricted diet is appropriate to simulate fasting without nutritional depletion. It is believed that this strategy is preferable to the two-day fast because it allows the body to achieve ketosis and commence a genuine detox.

Fasting can be difficult and often unpleasant, the psychological and physical benefits have been demonstrated to have numerous positive effects on overall health, including increased weight loss and improved cognitive function. (source: NHS, n.d. (Low BP))

Cognitive ability and heart health

The first few days of fasting might be challenging, but there are emotional and physical benefits. Fasting is a fantastic approach to developing your willpower on a mental level. Similar to how a runner might feel after forcing his or her body to sprint an extra mile, individuals who choose to fast can feel the energy as they resist their natural appetite.

 Physically, there are also tremendous detoxifying and heart health benefits. As BMR decreases, blood fat begins to vanish as it is digested for energy. This mechanism supports heart health and improves cholesterol levels in some individuals by increasing HDL levels.

Weight loss and detoxification

There are numerous health benefits associated with fat burning, the first being weight loss. Ketosis is a foreseeable method for targeting fat deposits that would otherwise be unaffected by a healthy lifestyle. Moreover, losing this excess fat has a cleansing impact on the body.  (source: PubMed, 2010 (Autophagy))

To prevent them from wreaking havoc on your system, your body’s natural defenses store harmful elements and other toxins in fat cells. During ketosis, these harmful metals and chemicals are safely eliminated from the body as fat resources are diminished.  This cleansing impact may temporarily alter the complexion or trigger other symptoms of a healing crisis in some individuals.

Optimal aging

When the benefits of this stage are added together, they can lead to major health gains. When you restrict free radicals and oxidative stress, you promote healthy aging and put yourself in a position to experience fewer health difficulties.  Despite little research, this treatment method appears to improve the health of some individuals.

Despite little research, this treatment method appears to improve the health of some individuals. Mental and physical advantages can: (source: PubMed, 2018 (Oxidative stress))

  • Enhance mental performance
  • Protect against obesity and related chronic illnesses
  • Reduce inflammation and enhance general fitness
  • Facilitate weight loss
  • Reduce the likelihood of metabolic disorders
  • Benefit cancer patients — A recent study on mice with cancer demonstrated that fasting throughout chemotherapy stimulates immune function and reveals cancer cells. Eliminating old, harmful cells from the body and replacing them with new, healthy cells may be the solution.

Personal growth and goals

Upon reaching this point, the advantages become personal. Fasting, especially after the initial seven days, requires unwavering commitment. In these later stages, the benefits of the fast may be a combination of all the previous stages or the achievement of an individual health objective. For some, it is weight loss, whilst for others, it is a technique to treat a specific health issue.

Commence something new

With precise consideration and evaluation, fasting can be a fantastic stepping stone to a healthy lifestyle. One option is to make preparations before beginning the fast. Write out what you hope to gain from it and whatever you wish to achieve. If done properly, the conclusion of a fast is the ideal opportunity to begin a much-improved diet and lifestyle.

FAQs

What Is The Fat-Burning Stage Of Fasting?
Whilst in the Partial Ketosis stage, your body begins to burn fat. After fasting for approximately 12 hours, the liver starts breaking down fats and producing ketones. The duration of fat burning rises from sixteen to twenty-four hours. Many individuals who fast frequently claim that sixteen-hour fast yields the finest effects. The benefits of 16-hour fast include benefits in weight loss, inflammation, and overall health. Many individuals also notice enhanced mental clarity and concentration.

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