Sheldon Cohen and her colleagues Denise Janicki-Deverts of Carnegie Mellon and Gregory E. Miller' review study looked into behavioural and biological mechanisms through which stress contributes to disease and weigh the results of studies that have examined whether stress plays a role in depression, cardiovascular disease, HIV/AIDS and cancer.
The studies revealed that stress plays a role in triggering or worsening depression and cardiovascular disease and in speeding the progression of HIV/AIDS.
"The majority of people confronted with even traumatic events remain disease-free. Stress increases your risk of developing disease, but it doesn't mean that just because you are exposed to stressful events, you are going to get sick," said Cohen, the Robert E. Doherty Professor of Psychology at Carnegie Mellon.
According to the authors, the strongest evidence that stress contributes to disease comes from research on depression, which shows that stress is associated with the onset of depression as well as relapse in people who have recovered from it.
Cohen said that particular types of stress are the biggest culprits in depression, namely "social stressors" such as divorce and the death of a loved one. Depression also is common among people who have been diagnosed with a serious illness, suggesting that physical disease itself is a stressful event that can lead to depression. On the other hand, chronic stress -- such as stress experienced daily in the workplace -- contributes to cardiovascular illnesses such as coronary heart disease, a relationship that medical studies have clearly demonstrated, Cohen said.
"Individuals differ with regard to rate of progression through the successive phases of HIV infection. Some remain asymptomatic for extended periods and respond well to medical treatment, whereas others progress rapidly to AIDS onset, and suffer numerous complications and opportunistic infections. Stress may account for some of this variability in HIV progression," the authors write.
Exactly how stress causes and contributes to disease is a question of particular interest to researchers. Cohen said there are two likely pathways. One is behavioural -- people under stress sleep poorly and are less likely to exercise; they adopt poor eating habits, smoke more and don't comply with medical treatment. Stress also triggers a response by the body's endocrine systems, which release hormones that influence multiple other biological systems, including the immune system.
"Effects of stress on regulation of immune and inflammatory processes have the potential to influence depression, infectious, autoimmune, and coronary artery disease, and at least some (e.g., viral) cancers," the authors write.
Studies on the role of stress in cancer have not been consistent in their results. Researchers who study the influence of stress on the progression of cancer face many hurdles, according to Cohen and his colleagues. Cancer can go undiagnosed for a long time, and its progression is difficult to measure with much precision. There are many types of cancers, and it is possible that stress only influences those facilitated by sustained hormonal response and impairments in immunity.
"We will need additional studies across a broader range of cancers before we can fairly evaluate the role of stress in cancer," Cohen said.
The study is published in the Oct. 10 issue of the Journal of the American Medical Association (JAMA).