The hidden cost of being constantly on alert
The body is designed to adapt to danger, pressure and change. But when those systems are activated repeatedly, scientists are investigating how the accumulated physiological burden may affect the brain, metabolism, immunity, sleep and long-term health.
FOSTER! NEWS | HEALTH & MEDICINE
17 SEPTEMBER 2026
THE BODY DOES NOT SIMPLY "HANDLE" STRESS
When the human body detects a threat, it does not wait for a person to consciously decide what to do.
The brain can rapidly activate systems involving the autonomic nervous system and the hypothalamic-pituitary-adrenal axis. Heart rate, blood pressure, metabolism, attention and hormone activity can all change as the organism prepares to respond.
For short periods, this is not a malfunction. It is an essential survival mechanism.
The problem begins with repetition.
Scientists use the term allostasis to describe the way the body maintains stability by changing its physiological state. When those adaptations are repeatedly activated or fail to switch off efficiently, the accumulated burden is known as allostatic load.
It is a concept that is changing the way researchers think about the relationship between stress and disease.
THE STRESS RESPONSE IS NOT JUST ABOUT CORTISOL
Stress is often reduced to one hormone: cortisol.
The biology is considerably more complicated.
The stress response involves interacting systems that include the autonomic nervous system, neuroendocrine pathways, cardiovascular regulation, metabolism and immune function.
Research has shown that chronic stress can influence several of these systems simultaneously rather than affecting one isolated part of the body.
That matters because the consequences of prolonged physiological activation may not appear as one obvious symptom.
They can emerge across different systems.
Sleep can change.
Concentration can suffer.
Metabolic regulation can be affected.
The immune system can be altered.
Cardiovascular responses can remain elevated.
And the body may gradually operate from a different physiological baseline.
THE NEW QUESTION: HOW MUCH DOES ADAPTATION COST?
One of the most interesting areas of research concerns something surprisingly fundamental:
energy.
Every physiological response requires resources.
Researchers studying the energetic cost of allostasis have proposed that chronic activation of stress-related systems may increase the amount of energy the organism needs simply to maintain its adapted state.
This is known as the energetic model of allostatic load.
The hypothesis is that when the energy required to maintain repeated stress responses becomes excessive, fewer resources may be available for processes associated with maintenance and repair.
This does not mean that ordinary tiredness proves someone has accumulated allostatic load.
It means researchers are investigating whether chronic physiological demand itself can become part of the biological problem.
CELLS MAY PAY A PRICE TOO
The question becomes even more intriguing at cellular level.
A study published in Nature examined human fibroblast cells exposed to chronic glucocorticoid signalling. Researchers observed approximately a 60% increase in cellular energy expenditure, alongside metabolic changes and markers associated with accelerated cellular ageing.
The finding does not demonstrate that a stressed person ages 60% faster.
It was a cellular experimental model.
But it provides an important clue: prolonged exposure to stress-related biological signals can alter how cells use energy.
That has pushed researchers towards a broader question:
Could chronic physiological adaptation itself contribute to biological wear?
THE BODY'S SYSTEMS ARE CONNECTED
The significance of allostatic load is that it does not belong to one medical speciality.
The research framework can involve the brain, endocrine system, cardiovascular system, metabolism and immune system.
A large multi-cohort analysis involving 67,126 people examined 40 biomarkers across 12 physiological systems while researchers worked towards a more consistent definition of allostatic load.
The systems examined included:
→ stress-response pathways
→ autonomic nervous system function
→ inflammation and immunity
→ cardiovascular function
→ glucose metabolism
→ lipids
→ kidney function
→ liver function
→ respiratory function
→ body composition
The significance is not that one blood test can reveal a person's "stress level".
It cannot.
The significance is that researchers are increasingly looking at patterns across multiple biological systems.
FATIGUE IS NOT A DIAGNOSIS
This distinction is crucial.
Feeling exhausted does not automatically mean that the body has accumulated allostatic load.
Persistent fatigue can have many possible causes, including sleep disorders, infections, medication effects, nutritional problems, endocrine disorders, mental-health conditions and other medical conditions.
Even in research on chronic stress and burnout, there is currently no single biomarker or questionnaire capable of reliably diagnosing clinical burnout.
That uncertainty is important.
The science is developing.
And the concept of allostatic load should not be turned into a new internet diagnosis.
WHEN THE ALERT SYSTEM DOES NOT FULLY SWITCH OFF
The more important discovery may be conceptual.
The human body does not have a simple ON/OFF stress switch.
Its response is dynamic.
It anticipates.
It adjusts.
It compensates.
And it continually tries to maintain internal stability while conditions around it change.
Researchers describe prolonged or inappropriate activation of these systems as potentially producing physiological "wear and tear" across multiple systems.
This helps explain why chronic stress cannot be understood simply as a feeling.
It is also a biological process.
MODERN LIFE CREATES A DIFFERENT KIND OF PRESSURE
The human stress response evolved to deal with changing conditions.
But modern sources of pressure can remain present for weeks, months or years.
Financial uncertainty.
Continuous work demands.
Sleep disruption.
Social pressure.
Information overload.
Caregiving.
Chronic illness.
Persistent insecurity.
The biological system may respond to very different circumstances through overlapping physiological pathways.
The body does not necessarily distinguish between the modern concept of "being overwhelmed" and the biological requirement to adapt to a persistent challenge.
But scientists are still working to determine exactly how different stressors translate into long-term biological effects.
THE BRAIN AND BODY ARE IN CONSTANT CONVERSATION
The relationship also works in both directions.
The brain interprets the environment and helps organise physiological responses.
At the same time, signals from the body continuously return to the brain.
Hormones, inflammation, metabolism, cardiovascular activity and autonomic signals all contribute to that conversation.
Research on chronic stress describes this interaction as a complex network rather than a single pathway.
That may be why the traditional division between "mental" and "physical" stress is increasingly difficult to maintain scientifically.
SCIENTISTS ARE NOW LOOKING FOR BETTER MEASUREMENTS
One of the biggest problems is measurement.
There is still no universally accepted definition of allostatic load or one standard set of biomarkers that can be used in every study.
The large international meta-analysis of 67,126 participants was partly an attempt to address precisely this problem.
New technologies could push the field further.
Recent research is combining the allostasis framework with multi-omics, induced pluripotent stem cells and organoid models to investigate how prolonged physiological adaptation may interact with complex diseases.
The goal is moving beyond simply asking:
"Are you stressed?"
Researchers increasingly want to know:
"What is happening across the biological systems responding to that stress?"
THE MOST IMPORTANT PART MAY BE WHAT SCIENCE DOES NOT YET KNOW
Allostatic load is a powerful research framework, but it is not a universal explanation for fatigue, ageing or disease.
The field still faces major questions about measurement, causality and individual differences.
A recent review of chronic stress and burnout found substantial variation in how chronic stress is defined and measured, while also identifying consistent associations with disturbances involving the HPA axis, immune function, autonomic regulation and sleep.
That distinction matters.
Association is not proof of causation.
And a scientific hypothesis should not become a medical diagnosis simply because it sounds plausible.
FOSTER! ANALYSES
For decades, stress was often discussed as something that happened to the mind.
Modern physiology tells a more complicated story.
The brain and body are continuously communicating. Hormones, nerves, metabolism, immunity and cardiovascular function form an interconnected system capable of adapting to extraordinary circumstances.
That ability is one of the reasons humans survive.
But adaptation has a cost.
The emerging science of allostatic load is investigating what happens when the biological machinery designed to protect us is repeatedly called upon without sufficient opportunity to return to equilibrium.
The most important discovery may not be that stress makes us tired.
It may be that the body keeps adapting long after we have stopped noticing that it is doing so.
And scientists are only beginning to understand what that adaptation costs.
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