Beyond HRV: What the Brain-Body Connection Can Teach Us About Building Resilience.

Resilience isn't simply about tolerating more stress. It is about having the capacity to respond to a challenge, recover from it, and adapt. That distinction matters whether you are recovering from illness, navigating an inflammatory or metabolic condition, managing a demanding period in life, or trying to perform consistently at a high level.

Often times, people look to one or two measurements to help them understand their body and how resilient they are. HRV is one of the measures people look to – it measures the beat-to-beat variation in the interval between heartbeats. 

A fascinating paper published 20 years ago helps explain why.

In 2006, researchers Julian F. Thayer and Esther Sternberg published Beyond heart rate variability: vagal regulation of allostatic systems in the Annals of the New York Academy of Sciences. Rather than looking at heart rate variability (HRV) purely as a cardiovascular measure, they asked a much bigger question: could the nervous system be part of a regulatory network connecting the brain with the metabolic, hormonal, and immune systems?

Their work points toward something that has become increasingly important to how we understand health, recovery and performance: our body systems do not operate independently. Further studies have built upon Thayer and Sternberg’s work and this concept is now more widely understood in clinical literature.

Resilience is a whole-system process

The body is constantly adapting. Physical activity, infection, lack of sleep, psychological stress, intense training, sensory load, and even something as ordinary as maintaining blood glucose all require the body to adjust. 

This process is often described as allostasis, maintaining stability through change.

A healthy system isn't one that never becomes activated. Quite the opposite. We need to be able to increase heart rate when we exercise, mount an immune response when we encounter an infection, mobilize energy when it is needed, and become alert when something demands our attention.

The important capability is flexibility.

• Can the system respond appropriately when a demand appears?
• Can it turn that response down when the demand has passed?
• Can it recover and be ready for the next challenge?

That is where the 2006 paper becomes really interesting and carries helpful lessons for us to draw on. 

The vagus nerve reaches far beyond the heart

The vagus nerve is a major component of the parasympathetic nervous system and forms part of the communication network between the brain and internal organs.

Thayer and Sternberg examined evidence suggesting that vagal regulation extends beyond cardiovascular function into other systems involved in allostasis. Lower vagal function and lower HRV were associated in the reviewed evidence with higher fasting glucose and HbA1c, elevated overnight cortisol, and higher levels of pro-inflammatory cytokines and acute-phase proteins.

Those findings don't mean that low HRV causes these problems, or that raising an HRV number will resolve them. The more interesting point is that autonomic, metabolic, hormonal, and inflammatory regulation appear to be interconnected rather than separate phenomena.

That idea has continued to develop. Research on what is known as the inflammatory reflex, for example, has demonstrated neural communication between the nervous and immune systems. The vagus nerve participates in signaling pathways that help regulate inflammatory responses. 

This gives us a much richer picture of the nervous system. It isn't simply the switch that determines whether we feel “stressed” or “relaxed.” It is part of the regulatory architecture through which the brain and body continually communicate and adjust. 

The brain is part of this story too

Thayer and Sternberg also point toward the prefrontal cortex and amygdala as important components of the neural network involved in this regulation.

Later work on the neurovisceral integration model expanded this perspective, describing a hierarchy of neural systems linking brain function, autonomic regulation, and bodily physiology. This reinforces an important principle: Brain health, nervous-system regulation and physical health are not separate projects.

The brain is continually receiving information from the body, interpreting the environment and coordinating responses. The body, in turn, continually sends information back.

Thinking about this two-way communication helps explain why supporting resilience requires more than addressing one symptom, one muscle, one biomarker, or one organ system at a time.

So what can we actually do with this information?

This is where the research becomes really useful. We can't, and shouldn't, eliminate every challenge from life. Exercise is a stressor. Competition is a stressor. Learning, travel, work, and family responsibilities all place demands on our systems. Many of these are things we actively choose and value. The goal isn't to eliminate stress. It is to build the capacity to respond to it, recover from it, and adapt.

At ThriveNinety, we approach this from two directions: reducing unnecessary demands where we can, while supporting and gradually building capacity across interconnected brain-body systems. This is reflected in our PEDAL™ framework: Pacing, Exercise, Diet, Alignment of the Nervous System and Loosen. Together, these provide practical ways to support the systems involved in movement, energy, regulation, recovery, and adaptation.

The important point isn't any one tool in isolation. It is the whole-system approach. The research discussed by Thayer and Sternberg helps illustrate why this matters. Autonomic regulation doesn't exist separately from metabolic, inflammatory, or hormonal processes. The brain doesn't operate separately from the body. These systems interact continuously, and the body's ability to regulate across them contributes to how effectively we respond to changing demands.

That is also why the appropriate input isn't always more. For someone recovering from a post-infection condition, adding demand to an already heavily loaded system may be counterproductive. For an athlete, professional or otherwise healthy person, thoughtfully selected inputs may help build greater capacity for future demands. The starting points can be very different, but the underlying objective is similar: give the brain and body the conditions and inputs they need to function, recover and adapt well. That idea is central to ThriveNinety's recovery, resilience and performance approach.

Warmly,
Katie & Andrea

Reference:

Thayer JF, Sternberg E. Beyond heart rate variability: vagal regulation of allostatic systems. Ann N Y Acad Sci. 2006 Nov;1088:361-72. doi: 10.1196/annals.1366.014. PMID: 17192580. Beyond heart rate variability: vagal regulation of allostatic systems

Related Blog posts:

Recovery, Resilience, and a Scientific Story over 24 Years in the Making

How Your Nervous System and Immune System Talk - And Why It Matters for Recovery, Resilience, and High Performance

Beyond HRV: Building a More Resilient Brain and Body

The Systems Behind Energy, Performance, and Resilience