NEW YORK – Stress contributes to the onset of cardiovascular disease and depression, among other illnesses. And it is not only major stressful life events that exact a toll on our bodies; the many conflicts and demands of daily life elevate and sometimes disrupt the workings of our response systems for stress, causing wear and tear on the body and brain.
This burden of chronic stress, called “allostatic overload,” reflects not only the impact of life experiences but also our genetic constitution. Moreover, individual habits such as diet, exercise, the quality and quantity of sleep, and substance abuse also play a major role, as do early life experiences that set life-long patterns of behavior and physiological reactivity.
There are three categories of stress:
· Positive stress, for which a person feels rewarded by surmounting a challenge.
· Tolerable stress, which results from serious life events – for example, divorce, death of a loved one, loss of a job – but where the affected person has good support systems.
· Toxic stress, which involves the same types of serious events, as well as the accumulation of daily struggles, but without good support systems.
The difference between tolerable and toxic stress depends on the perceived degree of control that a person experiences. Moreover, low self-esteem exacerbates a feeling of helplessness and lack of control. Social support by friends and family is vital to ameliorating the effects of tolerable stress and keeping it from becoming toxic.
These are all functions of the brain – the key organ in our response to stress. The brain interprets what is threatening and, therefore, stressful; regulates behavioral and physiological stress responses – the latter through the autonomic, immune, and neuroendocrine systems; and is a target of stress, undergoing structural and functional remodeling of its circuits that affects its performance. This remodeling includes limited replacement of neurons in the hippocampus, a brain region important for spatial memory and memory of events in our daily lives.
The recognition of the brain’s vulnerability and plasticity under stress began with investigations of the hippocampus, and it now includes the amygdala, a brain region involved in fear, anxiety, and mood, and the prefrontal cortex, which is important in decision making, memory, and top-down control of impulsive behavior, as well as regulation of the autonomic nervous system and stress hormone axis. Repeated stress causes neurons in the hippocampus and the prefrontal cortex to shrink and lose connections with other nerve cells, while it also causes neurons in the amygdala to grow and form new connections.
Because the remodeling of neurons by stress is reversible, researchers now believe that chronic anxiety disorders and depression represent a lack of resilience, or spontaneous recovery, in susceptible individuals. Such a lack of recovery then requires medication, behavioral interventions, or both.
Hormones associated with stress protect the body and brain in the short run and promote adaptation, but the chronic activity of these same hormones brings about changes in the body that cause allostatic overload, along with its potential follow-on diseases. For example, the immune system is enhanced by acute stress but suppressed by chronic stress. By the same token, the brain shows enhanced activity during acute stress, with improvement in certain types of memory, but undergoes structural changes that increase anxiety and decrease mental flexibility and memory capacity as a result of chronic stress.
Developmental influences involving the quality of parenting and acquisition of attachment have a powerful influence on subsequent stress vulnerability during the rest of our lives – for example, abuse and neglect in childhood increase our vulnerability to physical and mental disorders, including obesity, cardiovascular disease, depression, post-traumatic stress disorder, substance abuse, and antisocial behavior.
Among the most potent causes of stress in adult life are those arising from competitive interactions between animals of the same species, leading to the formation of dominance hierarchies. Psychosocial stress of this type not only alters cognitive function in lower-ranking animals, but it can also promote disease (for example, atherosclerosis) among those vying for the dominant position.
Social ordering in human society is associated with gradients of disease, with an increasing frequency of mortality and morbidity as one descends the scale of socioeconomic status, which reflects both income and education. Although the causes of these gradients of health are very complex, they are likely to reflect, with increasing frequency at the lower end of the scale, the cumulative burden of coping with limited resources and stressors as well as differences in lifestyle, and the resulting allostatic overload that this burden places on the physiological systems involved in adaptation and coping.
The brain’s response to stress does not necessarily constitute “damage” per se and is amenable to reversal as well as prevention by treatments that include drugs, exercise, diet, and social support. And, because the social environment has powerful effects through the brain on the rest of the body, public and private-sector policies can have a positive impact on health, providing a top-down benefit to brain and body function.
This may occur through policies that improve education, provide better housing, improve commuting, regulate working conditions, increase availability of health foods, and provide tax relief for those in the lower and middle classes. Such policies might well prevent disease, thereby saving money, reducing human suffering, and promoting healthier and more meaningful lives.