What Happens in the Body When Food Stops Coming In

Fasting is not simply the absence of food.

When nutrients stop arriving from outside, the body begins a coordinated metabolic transition.

Hormonal signals change, stored energy is mobilised, different fuels progressively become more important and resources are redistributed according to new priorities.

This transition does not occur at exactly the same speed in everyone. The amount of glycogen stored before fasting, previous diet, physical activity, body composition, metabolic health and previous experience of fasting can all influence the timing.

The stages described below should therefore be understood as a continuum rather than a timetable.

This article refers primarily to fasting in which water remains available. Dry fasting introduces additional physiological mechanisms related to water and electrolyte balance and deserves separate consideration.

From the Fed State to the Fasted State

After a meal, nutrients absorbed from the intestine are readily available.

Blood glucose rises, insulin secretion increases and the body is temporarily in a predominantly anabolic state: energy is being used, stored and incorporated into tissues. Glucose can be stored as glycogen in the liver and muscles, while excess energy can also be stored as fat.

As digestion and absorption come to an end, this situation gradually reverses.

Insulin levels fall and the ratio between glucagon and insulin increases. The body begins to rely less on recently absorbed nutrients and more on its own stored resources.

This transition is sometimes described as the metabolic switch: the gradual movement from predominantly glucose-based metabolism towards increasing use of fatty acids and ketone bodies.

It is not an abrupt event. Fat mobilisation, glucose production and ketone production begin to change progressively and overlap for many hours.

The First Reserve: Glycogen

The first readily accessible source of glucose is glycogen.

The liver stores glycogen partly to help maintain blood glucose between meals. Muscle also contains substantial glycogen, but muscle glycogen is mainly reserved for use by the muscle itself rather than being released directly into the bloodstream.

During the early fast, the liver breaks down its glycogen through glycogenolysis, releasing glucose into the circulation.

These stores are limited.

By approximately 24 hours of fasting, most hepatic glycogen has usually been mobilised, although the exact timing varies considerably with activity and previous nutrition. At the same time, fatty-acid oxidation is already increasing.

The body does not suddenly run out of energy when glycogen declines.

It changes where that energy comes from.

Making Glucose Without Eating

Some tissues continue to require glucose even when no carbohydrate is being eaten.

The body therefore manufactures it through gluconeogenesis.

The liver is initially the major organ involved, while the kidneys become increasingly important during longer fasting.

The raw materials include lactate, glycerol released from adipose tissue and carbon skeletons derived from certain amino acids.

Gluconeogenesis is already active after an ordinary overnight fast and becomes responsible for an increasing proportion of glucose production as liver glycogen falls. After approximately 48 hours, it can account for the great majority of endogenous glucose production.

This is one reason why blood glucose does not simply fall towards zero when food disappears.

The organism is capable of producing glucose internally.

Fat Takes Over

As insulin falls, stored triglycerides in adipose tissue are broken down.

This process, lipolysis, releases fatty acids and glycerol.

Many tissues can use fatty acids directly as fuel. The glycerol portion can travel to the liver and contribute to gluconeogenesis.

Fat therefore becomes progressively more important as fasting continues.

This transition is crucial because body fat represents a much larger reserve of energy than glycogen.

It also changes what happens in the liver.

As increasing amounts of fatty acids arrive there, they undergo beta-oxidation. Part of the resulting acetyl-CoA is converted into ketone bodies.

These are principally acetoacetate and beta-hydroxybutyrate, together with acetone as a by-product.

Ketone production begins early, becomes much more pronounced during the first few days and can eventually provide a major alternative source of energy.

Ketosis: A Different Metabolic State

The increase in ketone bodies during fasting produces what is known as physiological fasting ketosis.

This must be distinguished from diabetic ketoacidosis, a pathological condition usually involving profound insulin deficiency, uncontrolled ketone production, marked hyperglycaemia and dangerous acid-base disturbance.

In normal fasting physiology, some insulin remains present and ketone production is regulated.

Ketones are particularly important because fatty acids themselves cannot adequately replace glucose as a fuel for the brain. Ketone bodies, however, can cross the blood-brain barrier.

As fasting continues, the brain progressively increases its use of beta-hydroxybutyrate and acetoacetate.

It never becomes completely independent of glucose, but its requirement for glucose falls substantially.

This metabolic adaptation is one of the principal reasons humans can survive periods of food deprivation far longer than would be possible if the brain depended entirely upon glucose derived from glycogen and body protein. Classic human fasting studies were fundamental in establishing this role of ketone bodies.

Protein Sparing: A Change in Priorities

Protein is also used during fasting.

This point is important because fasting does not mean that the body consumes only fat while leaving protein completely untouched.

During the earlier stages, amino acids contribute significantly to gluconeogenesis. Protein breakdown therefore forms part of the adaptation.

As ketone concentrations rise and the brain begins using them more extensively, however, less glucose needs to be produced exclusively for the nervous system. Dependence on amino acids as substrates for gluconeogenesis can consequently decline.

Urinary nitrogen losses tend to fall during prolonged fasting.

This is often described as protein sparing.

The term should not be misunderstood: protein loss does not disappear, and negative nitrogen balance can continue. The precise extent to which protein conservation results from ketone adaptation, falling energy expenditure and other mechanisms remains physiologically complex and has been debated.

The essential point is that prolonged fasting is not equivalent to consuming muscle at a constant rate day after day.

The organism changes its strategy as fasting continues.

The Brain During Fasting

The brain provides perhaps the clearest example of metabolic adaptation.

Under ordinary fed conditions it depends heavily upon glucose.

During prolonged fasting, increasing concentrations of circulating ketones provide another usable fuel.

Studies of prolonged fasting in humans demonstrated that beta-hydroxybutyrate and acetoacetate can eventually supply a substantial proportion of cerebral energy requirements.

Some glucose remains necessary, particularly because certain cells and metabolic pathways cannot replace it completely.

But the reduction in cerebral glucose demand has an important consequence: fewer amino acids need to be converted into glucose simply to sustain the brain.

The nervous system is therefore not metabolically abandoned during fasting.

It adapts.

This is also why old descriptions claiming that nervous tissue is simply “untouched” during starvation are too crude. The meaningful observation behind them is that the organism develops mechanisms that help preserve essential functions while shifting energy increasingly towards fat-derived fuels.

Hormonal Adaptation

Fasting is also an endocrine event.

Insulin falls relatively quickly as nutrient availability decreases. The glucagon-to-insulin ratio rises, favouring glycogen breakdown, gluconeogenesis, lipolysis and ketogenesis.

Growth hormone secretion may increase during fasting, while prolonged fasting can modify thyroid signalling, particularly by reducing circulating active T3. Leptin also falls as energy availability decreases.

Catecholamines, cortisol, the renin-angiotensin-aldosterone system and several other hormonal systems respond in ways that depend on duration, individual physiology and the degree of energy deprivation.

These changes should not be viewed as isolated hormonal effects.

Together they help coordinate the transition from storage and growth towards mobilisation, conservation and adaptation.

Autophagy and Cellular Recycling

Another process frequently associated with fasting is autophagy.

Autophagy is a normal cellular recycling system through which cells degrade and reuse some of their own damaged, unnecessary or dysfunctional components.

It occurs even when we are eating.

Nutrient availability, however, strongly influences signalling pathways involved in autophagy. During fasting, falling insulin and amino-acid availability, reduced mTOR activity and changes in energy-sensing pathways can favour a cellular environment associated with increased recycling and maintenance processes.

This has generated enormous interest in fasting research.

It has also generated considerable exaggeration.

Statements such as “autophagy starts at exactly 18 hours” or “maximum autophagy occurs at 72 hours” cannot currently be applied universally to human beings.

Different tissues may respond differently, and directly measuring autophagic flux in living humans is considerably more difficult than measuring blood glucose or ketones.

The biological principle is well established.

The precise human timetable is not.

Water, Sodium and the First Loss of Weight

The rapid fall in body weight often observed during the beginning of a fast does not represent an equally rapid disappearance of body fat.

Part of it is water.

Glycogen is stored together with water, so glycogen depletion releases associated fluid.

Falling insulin also changes renal sodium handling. During the first phase of fasting, increased sodium excretion — natriuresis — can be accompanied by increased water loss.

Older human fasting studies already observed that the particularly rapid loss of body weight during the first week was largely related to negative sodium and water balance rather than fat loss alone.

With prolonged fasting, hormonal mechanisms involved in conserving sodium and maintaining circulation progressively adjust.

This movement of water and electrolytes is one of the reasons why changes on the scale cannot be interpreted simply as changes in body fat.

A Different Organisation of Metabolism

Seen as a whole, fasting is not a passive condition.

The organism does not simply wait for food to return.

It changes fuel.

It releases stored energy.

It manufactures the glucose that remains necessary.

It increases fat oxidation and ketone production.

The brain alters its choice of fuel.

Hormonal signalling changes.

Water and electrolyte handling changes.

Processes of cellular maintenance and recycling respond to altered nutrient availability.

And, as fasting becomes prolonged, the body continually adjusts how much of its different reserves it can afford to use.

Fasting is not an absence of metabolism. It is a different organisation of metabolism.

When food stops arriving from outside, the organism changes strategy.

Fasting is one of the oldest practices associated with human health, spirituality and self-discipline.

Long before the development of modern nutritional science, people in widely separated parts of the world voluntarily abstained from food for religious, philosophical, ritual or health-related reasons.

Its recurrence across cultures and across centuries suggests that fasting belongs to a very ancient part of human experience. Modern historical and medical reviews confirm that voluntary fasting has been practised for millennia and appears in numerous religious and cultural traditions.

Ancient Roots

Historical traditions describe fasting in ancient India, Persia, China, Egypt, Mesopotamia and Greece.

In the Greek world, abstinence from food appeared in religious and philosophical contexts as well as in ideas concerning health and discipline. Later biographical traditions also associate Pythagoras with periods of fasting undertaken in preparation for study or initiation. As with many accounts transmitted from antiquity, some of these stories belong as much to tradition as to verifiable biography.

What is clear is that food, abstinence and bodily regimen were already closely connected with concepts of health in Greek and Roman medicine. This relationship continued into medieval European medicine, including the tradition of the School of Salerno, where diet, moderation and the regulation of daily life occupied an important place.

Fasting in Religious Traditions

Perhaps the strongest historical continuity of fasting has been preserved by religion.

Judaism established specific fasting days, including Yom Kippur. Christianity incorporated fasting from its earliest centuries and developed many different forms of abstinence within monastic, ascetic and popular practice.

In Islam, sawm, particularly the fasting of Ramadan, became one of the Five Pillars.

Fasting also occupies an important place in Hinduism, Jainism, Buddhist traditions and other Asian religious practices. The forms vary considerably: complete abstention, restricted eating periods, avoidance of particular foods, or longer periods of voluntary renunciation.

Many of the best-known religious narratives associate fasting with a moment of transition.

The biblical tradition describes Moses fasting for forty days, Elijah undertaking a prolonged fast in the wilderness, and Jesus fasting for forty days before beginning his public ministry.

Christian history later associated fasting with hermits, monks, saints and mystics, including Benedict of Nursia, Francis of Assisi and many others.

Across these different traditions, fasting repeatedly appears alongside preparation, purification, contemplation and transformation.

From Spiritual Discipline to Natural Health

During the nineteenth century, fasting began to acquire a new place within movements concerned with natural living and health.

In the United States, what would later become known as Natural Hygiene developed through the work of figures such as physician Isaac Jennings, Sylvester Graham and physician Russell Thacker Trall.

The movement traced its beginnings to Jennings' work from around 1822, while Graham began his campaign for physiological and dietary reform during the following decade. Their approaches differed, yet they shared the idea that health depended fundamentally upon the conditions in which the organism lived rather than solely upon external remedies.

Fasting gradually became one of the practices explored within this tradition.

In the twentieth century, Herbert M. Shelton (1895–1985) became one of the most influential representatives of Natural Hygiene. He wrote extensively about fasting and supervised many fasting experiences during decades of practice. His The Hygienic System: Fasting and Sun Bathing appeared in its first edition in 1934.

For Shelton and other Hygienists, fasting was understood within a broader philosophy of physiological rest and reliance upon the organism's own regulatory processes.

That distinction would later become important: within Natural Hygiene, fasting was considered less a “treatment applied to disease” than a condition in which the organism was given an opportunity to direct its resources differently.

The European Fasting Tradition

A parallel history was developing in Europe.

One of its best-known figures was the German physician Otto Buchinger (1878–1966). After personally undergoing a nineteen-day fast following a severe rheumatic illness, Buchinger devoted much of his professional life to therapeutic fasting.

His book on therapeutic fasting, published in 1935, helped systematise an approach that combined fasting with medical observation, physical activity, nature and a strong contemplative component. His work gave rise to a European clinical tradition that continues today.

Other European physicians and practitioners developed their own approaches during the twentieth century, including Maximilian Bircher-Benner in Switzerland and later practitioners in France, Germany, Britain, Spain and Italy.

By the second half of the twentieth century, fasting had therefore developed along several parallel paths: religious practice, Natural Hygiene, naturopathic traditions and forms of medically supervised fasting.

The Russian Scientific Tradition

Russia developed a particularly important and comparatively little-known tradition of fasting research.

During the nineteenth century, Russian physiologist Viktor Vasilievich Pašutin (1845–1901) and his school carried out extensive experimental work on metabolism, energy exchange and the physiological consequences of starvation. His studies contributed to the development of pathological physiology as an experimental discipline, and his work on metabolism during fasting was considered an important part of that research programme.

During the twentieth century this line took a new direction through the work of Soviet psychiatrist Yuri Sergeyevich Nikolaev (1905–1998).

Nikolaev studied medicine in Moscow and specialised in psychiatry. Over several decades he developed what became known in Russian as RDT — Razgruzočno-Dietičeskaja Terapija, usually translated as fasting-dietary therapy or controlled fasting therapy. His doctoral work in 1960 dealt specifically with fasting in schizophrenia and its physiological basis.

His work gradually extended beyond an individual clinical practice.

In 1970, an instructional and methodological document on RDT for neuropsychiatric disorders was issued through the State Research Institute of Psychiatry of the Ministry of Health of the Russian Soviet Federative Socialist Republic, with Nikolaev identified as its compiler.

Over the following decades, fasting departments and practitioners influenced by this school appeared in different parts of the Soviet Union.

Nikolaev's book Fasting for Health (Golodanie radi zdorov'ja) became one of the principal works through which this Russian tradition reached a wider public; a second expanded edition was published in Moscow in 1988.

The Russian history is particularly significant because fasting developed there within a partially institutional medical framework rather than exclusively within alternative-health movements.

That continuity did not disappear with the Soviet Union. In 2005 a Russian Ministry-associated institute produced a physicians' manual entitled The Application of Fasting-Dietary Therapy (RDT) in Restorative Medicine.

Fasting in Italy

Italy also developed its own fasting and Natural Hygiene movement during the twentieth century.

Translations of Shelton and other Hygienist authors contributed to spreading these ideas from the 1960s onwards, followed by associations, practitioners and centres dedicated to Natural Hygiene.

An unusual political milestone occurred on 4 July 1991, when a group of members of the Italian Chamber of Deputies led by René Andreani formally presented Bill No. 5808, entitled “Regolamentazione della digiunoterapia” — Regulation of Fasting Therapy.

The parliamentary record confirms both the title and the date of presentation.

The proposal itself is historically revealing. Whatever one's view of its medical claims, it shows that by the beginning of the 1990s therapeutic fasting had achieved sufficient visibility in Italy to become the subject of a formal parliamentary proposal.

It is from this document that much of the historical material behind this article was originally drawn.

Sergei Filonov and the Development of Dry Fasting

The Russian tradition later produced another development: the systematic exploration of dry fasting, in which both food and liquids are temporarily withheld.

One of the contemporary figures to whom NHF gives particular importance is Dr Sergei Ivanovich Filonov.

Filonov grew up in a medical family and trained in medicine. In his own account, Nikolaev's work was a major influence on his early interest in fasting. After medical school he worked at the Goryachinsk resort in Buryatia, near Lake Baikal, an environment in which the Russian fasting tradition was already present.

Over years of personal experimentation, clinical observation and study of different Russian fasting approaches, he became particularly interested in dry fasting.

His book Dry Therapeutic Fasting — Myths and Reality, published in Barnaul in 2008, brought together his experience and his interpretation of the Russian fasting tradition. The work is catalogued by the Russian State Library.

Filonov subsequently developed an approach known as Fractional Dry Fasting, in which periods of dry fasting are organised into separate stages rather than undertaken as a single continuous prolonged fast. His published material places this method within the wider history of Russian RDT and earlier approaches to dry fasting.

We will examine the different forms of fasting — including water fasting and dry fasting — separately.

Here his importance is historical: Filonov represents a contemporary continuation and development of a Russian fasting tradition that stretches back through Nikolaev to the physiological research of the nineteenth century.

From Ancient Practice to Contemporary Research

At the beginning of the twenty-first century, fasting entered another phase of its long history.

Research interest increased rapidly, and fasting began appearing again in scientific literature under terms such as intermittent fasting, periodic fasting, prolonged fasting and fasting therapy. European expert panels have also published consensus guidelines for medically supervised fasting therapy.

The language has changed many times across the centuries.

Fasting has been described as purification, religious observance, ascetic discipline, physiological rest, Natural Hygiene, fasting therapy, RDT and, more recently, a subject of metabolic and cellular research.

Yet the practice itself has survived all of those changing interpretations.

For thousands of years, human beings have deliberately stopped eating for a period of time — to observe, to prepare, to heal, to seek clarity, or simply to discover what happens when the body is allowed to live without the continuous arrival of food.

Its history is still being written.