Ancient man’s lifestyle of walking long distances daily in search of food and shelter, typified by opportunistic eating and either fasting or feasting, also meant that they had to go for long periods of time without food when none was readily available.  It makes you wonder if the human body was originally designed to have three square meals a day, with snacking on demand. The bodies of our ancestors must have been able to function without food for irregular and extended periods of time. 

The body fortunately still has biological survival mechanisms that gets activated when deprived of food for extended periods.

Modern man practices fasting for health reasons or for cultural or religious beliefs, such as the Ramadan fast in Islamic culture, where fasting takes place between sunrise and sunset for a period of a month.  Fasting for health reasons has gained popularity in recent years for weight management and improvements in metabolism, as studies have shown that fasting have many health benefits.

Types of fasting:

Fasting involves a total or partial restriction of food and sometimes drink for any period.  Variations of fasting have been studied for their effects on the body and their ability to improve health related factors such as inflammation, weight loss, body fat, insulin sensitivity, blood pressure, and functionality of the nervous system and brain.

The broad categories of fasting are intermittent fasting and periodic prolonged fasting.

Intermittent fasting is characterized by an eating pattern that varies between periods of regular eating and cycles of fasting.

  • The 16/8 method – eat your daily meals (usually two meals) within an 8-hour period and fast for 16 hours, which means skipping either breakfast or dinner, whichever works the best for you. Eat your dinner at least three hours before going to bed. (Your body uses the least number of calories while sleeping, so prevent having extra fuel to burn while you sleep.)
  • Eat-stop-eat – this refers to fasting for 24 hours, once or twice per week.
  • The 5:2 Diet – for two non-consecutive days per week, skip either breakfast or supper. Normally eat the other days of the week.
  • Any of these methods work well, depending on which you find the easiest to follow. Intermittent fasting presupposes that you do not binge when fasting ends but focus on eat healthy meals during the eating period. You may have coffee or tea, unsweetened and preferably black, and water, while fasting.

Periodic prolonged fasting refers to fasting without any food for three days and longer.  These fasting methods are also referred to as a “water fast”, as you should be drinking plenty of water to stay hydrated.

Effects of fasting on the body:

Individual differences in genetics, lifestyle, and health means that people respond differently to fasting.  For the average healthy person, there are general timelines of responses in metabolism that occurs as the fast continues from hours into days, says the organization Zero Fasting.


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For the first three hours after a meal the digestive system breaks down carbohydrates into glucose, fat into fatty acids, and protein into amino acids, which are then metabolized for energy or stored to be used later.  An abundance of carbohydrates in the meal means high levels of glucose in the blood stream, which triggers the pancreas to release insulin to enable glucose to be taken up in cells for energy production, or to be stored short term as glycogen or stored long-term as fat tissue.  After about three hours, insulin and glucose return to the usual pre-meal levels.

Two hormones are also involved during this time. Ghrelin is the so called “hunger” hormone that turns on the appetite, which levels drop after a meal, while the levels of leptin, the “satiety” hormone, rises to reduce appetite.


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Stored nutrients are utilized during the 4 to 24-hour phase.  The body’s metabolism will initially break down stored glucose (glycogen) to keep blood glucose at normal levels.  Towards the end of this phase, as glycogen levels are facing depletion, fat stores and ketone bodies are roped in to make up the shortfall in fuel for the cells.


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From day one to day three the glycogen stores are virtually depleted, which means the body must switch to fat-burning mode.  Ketone bodies are now produced by the liver.  By breaking down fat for fuel, fat cells release free fatty acids while a regulator of fat metabolism in the liver called PPAR-alpha is activated to ensure the fatty acids are transformed into ketone bodies, to be used for fuel.  Ketone levels are measured as beta-hydroxybutyrate (BHB) molecule levels in the blood stream. 

Ketone bodies are now the primary fuel for cells in the body, however, the brain still needs some glucose to function.  With glycogen stores depleted, the body activates a process called gluconeogenesis in the liver, a process whereby the body synthesizes glucose from non-carbohydrate sources, such as glycerol and amino acids.  From initially being highly activated, levels of the hunger hormone ghrelin start to decline, which explains why hunger levels seem to decline from the third day of fasting and beyond.


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This prolonged fasting phase occurs from day three to five, when the body is in a steady state of nutritional ketosis, producing enough ketone bodies to fuel the body.  With normal nutrients restricted, the liver decreases the production of a hormone that is involved in growth and development, called insulin like growth factor (IGF-1).  Decreases in IGF-1 activity are associated with less oxidative stress, with positive health effects on the body.  Oxidative stress is harmful to the body and occurs when free radicals outnumber antioxidants in the cells, which can lead to cell- and tissue damage, while contributing to chronic inflammation, diseases such as cancer and diabetes, and neurogenerative diseases such as Alzheimer’s. 

Decreased levels of growth hormone, glucose, and insulin in the blood stream are associated with benefits related to inflammation, the immune system, neurogenesis (forming new brain cells), and improved metabolic health from diseases such as diabetes, high blood pressure, and obesity.  Prolonged fasting has also been linked to cellular resistance to toxins and stress.


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Any fast longer than five days leads to a steady state of ketosis, and it is best to work with a medical practitioner to monitor ketone levels and other health indicators.  When ketones build up in the body, they make it more acidic.  Overly high levels of ketone bodies can result in a condition called ketoacidosis, with dangerously acidic blood, which can be a life-threatening condition, resulting in a coma or even death. 

Staying hydrated means drinking more water than usual, as you normally get 20% to 30% of your water intake from food.

Health benefits of fasting:

Main health benefits of intermittent fasting:

  • It is an effective way to lose weight and body (belly) fat. Most people try intermittent fasting to lose weight, as it leads to a lower intake of calories and enhances metabolism.
  • It can change the function of cells, hormones, and genes, including the way genes are expressed.
  • Fasting has been found to stimulate the growth of new brain cells, as it boosts the production of a protein called brain-derived neurotrophic factor (BDNF), which amongst other benefits, improves memory, mood, and learning.
  • During fasting the insulin levels drop and the body becomes more insulin sensitive, which means the body is more efficient at reducing blood sugar levels.
  • Levels of the human growth hormone increases, which have powerful anti-aging and longevity benefits, also for brain health.
  • It stimulates cellular repair processes through a process called autophagy, which helps the body break down and recycle old parts of the cells in the body.  Autophagy may help to extend life span.
  • Research has shown that intermittent fasting can reduce inflammation in the body.  Inflammation drives many chronic diseases and can impair brain health, as chronic inflammation is one of the many factors that may play a role in dementia and Alzheimer’s.
  • It may reduce risk factors for heart disease, such as reducing LDL cholesterol levels and a reduction in inflammation markers, insulin resistance levels and blood sugar levels.
  • Intermittent fasting has been found to stimulate the abundance of healthy gut bacteria.

Main health benefits of periodic prolonged fasting:

A one-year observational study with 1422 participants into the health effects of periodic fasting for periods of 4 to 21 days has found improvements in physical and emotional well-being.  The participants fasted with a daily caloric intake of 200-250 kcal, drinking 2-3 liters of water daily, accompanied by a moderate-intensity lifestyle program.  Participants were grouped in fasting period lengths of 5, 10, 15, and around 20 days.  The study found several benefits:

  • Marked reductions in weight were observed.
  • Abdominal circumference reduced significantly.
  • Blood pressure reduced significantly.
  • 84,4% of the participants with pre-existing health conditions reported an improvement.
  • Adverse effects were experienced by less than 1% of the participants.
  • Improvements in several cardiovascular and general health risk factors were found.

Staying hydrated is crucial during fasting, as deeper into a fast the body runs out of stores of carbs and water to fall back on.  The body also needs substances called electrolytes to manage and maintain important bodily functions and low levels of electrolytes may have many different symptoms.

  • Sodium (salt) depletion can lead to headaches and muscle spasms.
  • Potassium regulates fluid in muscles and helps the nerves to send signals.  Depletion can lead to painful leg spasms, cramps, and high blood pressure.
  • Magnesium plays a role in the functioning of muscles and nerves, while aiding sleep.  Early symptoms of depletion are nausea, vomiting, fatigue, and weakness.
  • Zinc supports nerve function and the production of testosterone.  Signs of depletion are loss of appetite, abnormal hair loss, and an impaired immune system, which for example may manifest in the slow healing of wounds.
  • Calcium not only plays an important role in bone health, it helps muscles contract, heartbeat, and helps the blood to clot.  Depletion manifests in muscle aches, cramps, as well as numbness and tingling in the hands, arms, legs, and feet.


Rodent studies have consistently showed various health benefits of intermittent fasting, such as weight loss, increased insulin sensitivity, as well as improvements in the metabolic and cardiovascular profiles.  Human studies have shown less consistent results, as rodent studies are easier to control in a laboratory environment.  Human studies are done in clinical trials with less stringent controls possible.  Individual differences also play a role, such as genetic predisposition, metabolic status, sex, age, the quantity of the energy intake, and even the thermal (hot or cold) environment.

Fasting has undisputable health benefits and intermittent fasting has become popular in recent years.  Intermittent fasting is an easy to implement therapeutic intervention for people who wish to improve their metabolic, cardiovascular, and cognitive health.  People who practice intermittent fasting swear by it, as they claim to feel so much healthier.

It may take time to get used to the eating patterns associated with intermittent fasting, as our metabolism undergoes changes and adaptations.  We get better at it as we practice intermittent fasting and it helps to practice low intensity activities which do not use too much energy, such as walking and meditating, or keeping your mind busy by reading or watching a movie, to distract you from feeling hungry.  Overeating, snacking, and eating junk food between fasting periods will cancel out the benefits of fasting.


Physiology, fasting.  Published online and updated 26 July 2021.  National Center for Biotechnology Information.  National Library of Medicine.  National Institutes of Health.  USA.  (

Intermittent fasting: Physiological implications on outcomes in mice and men.  Published 15 April 2020.  Physiology.  Journal of the American Physiological Society.  (

Physiological changes during fasting in Ramdan.  Published May 2015.  Pubmed.   National Center for Biotechnology Information.  National Library of Medicine.  National Institutes of Health.  USA.  (

The physiology of fasting.  Published 29 September 2019.  Zero Fasting.  (

How to stay hydrated while fasting.  Published 26 July 2019.  Zero Fasting.  (

Can you drink water while fasting?  Published 22 March 2021.  Healthline.  (

Metabolism.   Published online and reviewed July 2019.  KidsHealth.  (

Safety, health improvements and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects.  Published 2 January 2019.  National Center for Biotechnology Information.  National Library of Medicine.  National Institutes of Health.  USA.  (


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