Background
Stress is an important public health issue. One in ten Danish adults experience high levels of stress in their daily lives and stress is one of the main occupational health problems in Europe. The link between stress and health is still debated in the scientific literature and the pathways underlying these potential health effects are far from elucidated. The dissertation contributions to the literature on stress and health by empirically testing the relationship between stress and major chronic disorders and by providing new evidence on the underlying physiological, psychological and behavioral mechanisms. Stress is a complex concept and in order to better understand the relation between stress and health, the dissertation works with a clear distinction between the health consequences of different types of stress including external stressors, perceived stress, and measures of the stress response.
Objectives
The aim of the dissertation is to provide a comprehensive picture of the long-term health consequences of stress. Specifically, the dissertation aims at addressing the relationship between various measures of stress and risk of major chronic disorders including cardiovascular diseases, breast cancer, Parkinson’s disease and atopic disorders. The dissertation also aims to empirically test the relationship between measures of stress and total and cause-specific mortality, which provides a measure of the combined public health burden. Understanding the mechanisms linking stress to chronic disease is important for establishing causality and identifying potentials for prevention, and several of the papers included in the dissertation aim at illuminating such mechanisms.
Material
The dissertation is based on a comprehensive literature review combined with empirical findings on stress and health from 13 epidemiological studies. These studies are based on data from several large prospective studies including the Copenhagen City Heart Study, the Danish Nurse Study, the French GAZEL Cohort Study, a nationwide Danish register-linkage study and an American prognosis study of Parkinson’s disease patients.
Results
The combined evidence of this dissertation shows a moderately higher risk of some common chronic disorders including cardiovascular disease and atopic disorders among individuals exposed to work-related stressors and perceived stress. Most empirical studies also report higher mortality rates in the aftermath of major acute events such as natural disasters and terrorism as well as among those who experience the more enduring stress of daily life. In the dissertation, chronic stress is most strongly associated with deaths from respiratory disorders and external causes including suicide. The relationship between chronic stress and mortality is probably due to a combination of etiological pathways affecting disease incidence and factors such as treatment seeking behavior and individual vulnerability, which are likely to affect disease progression.
Several experimental studies support a connection between stress and breast cancer mediated through impaired immune functioning, deregulated estrogen metabolism and reduced DNA repair capacity of the cells. The dissertation includes a systematic review of the epidemiological evidence from prospective studies on stress and breast cancer. The combined evidence does not support the hypothesis of stress being an important risk factor for breast cancer incidence or recurrence.
Parkinson’s disease is one of the most common neurodegenerative disorders in the elderly population, but the hypothesis that stress increases the risk of Parkinson’s disease is not supported in the dissertation. The experience of major life events appears, on the other hand, to be strongly associated with onset of depression, which is a common non-motor symptom of Parkinson’s disease progression.
Mechanisms linking stress to health include a combination of pathways. The physiological stress response has the ability to directly affect vital body systems including the cardiovascular, immune, and metabolic systems. Further, stress can lead to unfavorable changes in health-related behavior, impaired sleep and poor mental health. An increasing number of well-conducted empirical studies of these pathways support a causal relation between stress and health.
Conclusions
Despite some limitations in the existing literature, the combined evidence supports a relationship between stress and major chronic disorders including cardiovascular disease, Parkinson’s disease progression, atopic disorders and premature death. Future strategies aimed at preventing chronic disorders and premature deaths may therefore benefit from including stress in standard health assessments, both at individual and population levels. The combined evidence of the dissertation also underscores the importance of developing public health initiatives to remedy the problem.
Conceptual confusion combined with a limited focus on single stressors or specific life situations may lead to misclassification of a person’s actual stress level and consequently an underestimation of the relationship between stress and health. Studies with more comprehensive stress measures, preferably over a whole life course, combined with a broader incorporation of epidemiological results with evidence from molecular, physiological and social science studies are needed to bring this area of research forward.