One of the most difficult challenges in the longevity and anti-aging space is identifying metrics that a) represent an accurate estimation of general biological age and b) are modifiable with lifestyle and medical interventions. In this article I will make the case that heart rate variability (HRV) is a reasonable metric that can be used as a marker for general health and a gauge for many lifestyle behaviours that correlate with better health and aging.
The autonomic nervous system or ANS which regulates the body's unconscious actions is one of the main systems involved in how well organs respond to stress. The two branches of the ANS are the sympathetic branch (activated during periods of stress) and parasympathetic branch (controls bodily functions at rest e.g activating metabolism, stimulating digestion etc.). Greater sympathetic activity reflects increased stress levels while a predominance of parasympathetic activity reflects the opposite. Dysregulation of the ANS has been shown to occur with aging. This has been linked to cognitive decline, disturbed sleeping patterns and heart disease.
HRV is a reliable, non-invasive, objective measure of autonomic nervous system (ANS) function. A low HRV generally reflects reduced parasympathetic activity and a shift towards sympathetic activation which is correlated to ageing-related decline in health. This means that long term trends in HRV can be used as a marker for decline in health and a crude estimate for biological age.
HRV has been shown to decline with age in multiple studies. However, this decline has been shown to be less severe and even absent the healthier the person is irrespective of their age. This suggests that the age-related decline in HRV is not inevitable and responds to healthy lifestyle habits.
The four pillars of healthy aging are: exercise, healthy diet, good sleep, exercise and low stress. Until we develop true anti-aging therapies, these will remain the most important levers we can pull to fight off age related diseases. HRV can be greatly influenced by all four of these pillars.
We’ve established that HRV is mainly a proximate measure for stress levels in our body. Stress can take many forms and not all of them are bad. Exercise and training are forms of physiological stress that are desirable. Short term changes in HRV after exercise can be used as a guide to reach optimal intensity and frequency. In fact, studies on athletes have shown better performance when using HRV guides training protocols.
Although more research is needed to establish the link between HRV and diet. There is evidence to suggest that certain dietary habits can positively influence HRV. This may be through anti-inflammatory effects of certain foods (and perhaps more importantly, the lack of other foods). Diets high in omega-3 fatty acids found in oily fish such as salmon, vitamin B (there are many types of B-vitamins), polyphenols and a Mediterranean diet can all contribute to maintaining a higher HRV. Any diet that helps with maintaining a healthy weight can help reduce inflammation and stress in the body which can be reflected in HRV. Foods that can have a negative effect on HRV are foods high in saturated fat and high glycemic index carbohydrates.
The autonomic nervous system (ANS) plays a key role in heart function during sleep onset and the transition between different sleep stages. The interaction between the ANS and sleep goes in both directions. The heart is affected by which sleep stage we are in. As we progress from light sleep to deep sleep a gradual decrease is observed in heart rate, blood pressure and activation of the sympathetic nervous system (the branch of the ANS which is activated with stress) with the lowest values seen in deep sleep. The opposite is then seen in REM sleep where there is an uptake in sympathetic activity and a subsequent cardiac activation. These changes can also be observed in detailed HRV monitoring. Daytime HRV has been shown to decrease in certain sleep disorders such as obstructive sleep apnea. A drop in HRV is also clearly seen after a night of bad sleep in otherwise healthy individuals. Short term and long term changes in HRV can be seen with changing sleeping habits and can be used as a useful indicator of sleep hygiene.
During chronic stress, hyperactivation of the sympathetic nervous system occurs which radiates throughout the body causing physical and mental symptoms. It has been historically difficult to quantify stress and measure it objectively. The two ways that psychological stress affects the body are through hormonal signals (called the hypothalamic-pituitary-adrenal or HPA axis) and neuronal activity of the sympathetic nervous system. HRV is a reflection of sympathetic activity through its connection with the heart. A low HRV indicates overactivation of the sympathetic nervous system and low parasympathetic activity. There is now increasingly strong evidence to support the use of HRV as an objective measure of psychological health and stress.
Hans Seyle described the stress response in three stages: the first stage is the alarm reaction stage in which the body reacts to a certain cause of stress by activating the sympathetic nervous system (also called fight-or-flight response). This is followed by the resistance stage where adaptation to the stressor occurs and the body’s outward appearance of functions are restored to normal by the parasympathetic nervous system. During this period, hormones such as cortisol, adrenaline and blood glucose are elevated to restore normal function despite the presence of the stressor. If this state continues to a point where the body’s capacity to cope is reached, the body’s resources are depleted making it at risk of disease and even death. This is the exhaustion stage. HRV can reflect an internal state of stress even when the outward appearance seems normal.
When interpreting HRV, it is important to take into consideration that while some of the causes for variation are modifiable, there are many causes that are non-modifiable such as our genetic and physiological makeup. It can be a very useful tool to guide habits and behaviours but ultimately should be interpreted within the context of other markers of health and more perhaps more importantly, how we subjectively feel.
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