Many of the studies were cross-sectional and so could not fully assess symptom duration and chronicity, latency of onset, and prognosis; this is an important limitation because many of the long-term outcomes assessed in this report, such as coronary heart disease and cancer, have latent periods of decades. Some studies of veterans of World War II, the Korean War, and the Vietnam War were longitudinal and allowed an assessment of the health of veterans over time. Finally, many of the studies looked at a variety of exposures of deployed personnel, particularly those of the Gulf War, and this makes it difficult to distinguish specific effects from any one of the multitude of exposures. The committee did not consider the effect of many co-occurring exposures of Gulf War veterans, such as exposure to oil-well fires, pyridostigmine bromide, and vaccines and, for Vietnam veterans, Agent Orange. Therefore, because this report considered the agent of interest—deployment to a war zone—as the only exposure, it may have reached conclusions different from those of other Gulf War and Health reports, particularly Volume 4: Health Effects of Serving in the Gulf War, and Veterans and Agent Orange reports.
Several large-scale, nationally representative studies of veterans have been conducted, most notably the Vietnam Experience Study (CDC 1988), which compared Vietnam theater and Vietnam-era veterans for a multitude of health and psychosocial end points; the National Vietnam Veterans Readjustment Study, which made similar comparisons (Jordan et al. 1992; Kulka et al. 1990); the National Health Survey of Gulf War Era Veterans and Their Families (Eisen et al. 2005; Kang et al. 2000); the studies of Australian Gulf War veterans (Ikin et al. 2004; Kelsall et al. 2004) and Danish Gulf War peacekeepers (Ishoy et al. 1999); the hospitalization studies of Gulf War veterans (Gray et al. 1996, 2000); and the Department of Veterans Affairs (VA) Normative Aging Study, which has followed World War II and Korean War veterans since 1961. Those and other studies that evaluated specific health effects formed the backbone of the committee’s assessment of the association between deployment-related stress and various health effects.
Many other studies also dealt with large populations of veterans, such as the study of Canadian Gulf War veterans by Goss Gilroy Inc. (1998) and several studies of UK Gulf War veterans, but, although helpful, they used self-reports of symptoms and medical conditions to reach their conclusions. The committee did not consider symptom reporting alone to be sufficient evidence of a health effect in veterans; it required objective measures of a medical condition or a physician’s diagnosis or assessment.
The experimental literature on stress in both animals and humans suggests that exposure to a stressor initiates a cascade of biologic mechanisms that result in short-term and long-term consequences. In most people, once the stressor is removed, the stress response ceases and the body returns to normal; we consider such a stress response “adaptive.” In some people, however, the stress response does not turn off when the stressor is no longer present, and it becomes “maladaptive” because the body continues to produce stress hormones and other chemical mediators that eventually result in deterioration of normal physiologic processes. The prolonged or chronic stress response can affect virtually all organ systems. Most research has focused on effects in the brain, the cardiovascular system, the gastrointestinal tract, the endocrine system, and the immune system, and the literature on the effects of stress in the general population suggests that chronic stress results in adverse health effects. The epidemiologic literature