Food Fasting: A Scientific Approach

There has never been so much food available, and there have never been so many problems related to eating. Where does fasting come in?

Airton Humber · 20/12/25 · 4 min de leitura

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Introduction

We live in a historical contradiction.
There has never been so much food available, and there have never been so many problems related to eating.

On one side, hunger.
On the other, excess.
In the middle, ultra-processed foods, continuous consumption and a human body that was not designed to eat all the time.

In this scenario, fasting usually appears wrapped in religious, moral or ideological discourse. But when we remove those filters, something very simple is left:

What happens in the human body when it goes a while without eating?

This is a scientific question.
And this text is built from it.

What fasting is, from a scientific point of view

Fasting is the voluntary suspension of caloric intake for a period longer than the usual interval between meals.

It is not punishment.
It is not virtue.
It is not a miracle method.

It is simply the human body functioning outside the constant fed state, something biologically normal throughout most of human history.

The human body was designed to alternate states

Throughout practically all of human existence, the pattern was simple:

eat when there was food
go hours or days without eating when there was none

The organism learned to:

store energy
use that energy
enter saving mode
activate natural mechanisms of adaptation and maintenance

The modern problem is not fasting.
It is the complete absence of metabolic pauses.

What happens in the body during fasting

Fed state × Fasted state

By reducing caloric intake, the body leaves the constant storage mode and starts using stored energy, activating natural mechanisms of adaptation.

Drop in insulin

Insulin is the storage hormone.
When we eat all the time, it stays elevated.

In fasting:

insulin drops
the body leaves "store" mode
and enters "use" mode

This allows:

greater fat burning
improved metabolic sensitivity
reduction of low-grade inflammation

All of this is measurable in laboratory tests.

Using fat as fuel

After liver glycogen is depleted, the organism starts using fat as its main source of energy.

This process is called metabolic flexibility, the ability to switch between glucose and fat.

People who never take breaks from eating tend to lose this ability.

Autophagy: cellular cleaning

Autophagy literally means "the cell recycling itself."

During fasting:

defective proteins are broken down
damaged cellular structures are removed
the accumulation of cellular waste decreases

This mechanism is so relevant that it was recognized with the Nobel Prize in Medicine.

It is not mysticism.
It is biological maintenance.

Short fasting versus prolonged fasting

Metabolic timeline of fasting

The body does not change state all at once. Fasting works in phases, with different benefits and risks depending on the time.

Short fast (12 to 24 hours)

It is the most studied, safe and sustainable.

Most common effects:

drop in insulin
start of autophagy
metabolic improvement
greater mental clarity
low physiological stress

It works as metabolic hygiene, similar to rest or sleep.

Intermediate fast (24 to 48 hours)

Here fasting stops being just maintenance and becomes intervention.

Possible benefits:

increased autophagy
more intense reduction of inflammation

Possible costs:

drop in blood pressure
greater need for electrolytes
increased physiological stress in some profiles

It is not a practice for routine.

Prolonged fast (over 48–72 hours)

Although it strongly activates autophagy, it involves real risks:

loss of muscle mass
electrolyte imbalance
reduction of basal metabolism
hormonal impact

It is not a population health strategy.
It only makes sense in supervised clinical contexts.

More time does not mean more benefit.

Fasting and the Nervous System

Fasting and the brain

During fasting, the brain does not lose energy. It changes the type of fuel and the neurochemical state, favoring clarity, adaptation and efficiency.

Reduction of brain metabolic noise

Smaller fluctuations in glucose and insulin reduce neural inflammation.
This is usually noticed as:

a clearer mind
less "brain fog"
greater emotional stability

Ketone bodies and brain efficiency

During fasting, the brain starts partially using ketone bodies:

more stable energy
lower oxidative stress
better mitochondrial efficiency

This explains the increase in focus reported by many people.

Increase in BDNF

Fasting stimulates BDNF, an essential factor for:

neural plasticity
memory
learning
emotional adaptation

Low levels of BDNF are associated with depression and cognitive decline.

Autonomic nervous system: alert without panic

Short fasting lightly activates the sympathetic system:

more vigilance
more attention

This is not anxiety.
Anxiety arises when stress is excessive or prolonged.

When fasting becomes neural stress

The body always warns:

mental confusion
intense weakness
persistent irritability
worse sleep

This is not adaptation.
It is neuroendocrine dysregulation.

The most common mistake: turning fasting into a rule

Fasting is not:

a moral virtue
proof of discipline
a universal solution

It is only:

a physiological tool for metabolic rest

Tools need context, limits and common sense.
Consistency beats intensity.

Conclusion

From a scientific point of view, fasting is an interval without food that allows the body to activate natural mechanisms of balance, adaptation and cleaning.

In a world marked by scarcity, excess and poor food quality, perhaps the greatest value of fasting is not in "going without eating," but in relearning to give the body pauses.

Credits: CHATGPT

Learn more: BDNF What is it?