Traumatic Brain Injury and High Combat Exposure Pose Additive Risk for Specific Chronic Health Conditions in Veterans Affairs Million Veteran Program Enrollees.

Traumatic brain injury (TBI) and combat exposure are common experiences among U.S. military Veterans, each linked to a wide array of chronic health conditions. Prior studies have tended to evaluate these risk factors separately; thus, their combined influence on health outcomes remains unknown. In particular, research has not systematically examined whether TBI and combat exposure interact to exacerbate risk for poor long-term outcomes, limiting our understanding of how these co-occurring experiences shape Veterans' health. This study aimed to investigate whether TBI and high combat exposure interact to exacerbate risk for chronic health conditions in Veterans. Data were drawn from 156,242 Veterans enrolled in the VA Million Veteran Program (MVP; mean age = 64.8 years [standard deviation = 12.2], 95.1% male). TBI status was determined using self-report on the MVP Baseline Survey, and combat exposure (high vs. other) was assessed using the Combat Experiences Scale of the Deployment Risk and Resilience Inventory, administered as part of the MVP Lifestyle Survey. Health outcomes included self-reported mental health/psychiatric, cardiovascular, and neurological conditions, assessed via the MVP Baseline Survey. Multivariate logistic regressions tested associations of TBI and combat exposure with each outcome, adjusting for age, sex, race, ethnicity, and service era. Additive-scale interactions were evaluated using the relative excess risk due to interaction (RERI) statistic. Veterans with a history of TBI had significantly greater odds of reporting all psychiatric, cardiovascular, and neurological conditions evaluated (odds ratios [ORs] = 1.17-6.43). High combat exposure was also significantly associated with increased odds of experiencing all psychiatric conditions and several cardiovascular and neurological conditions (ORs = 1.11-3.24). RERI analyses showed significant interactions between TBI and high combat exposure for several psychiatric and neurological conditions (RERI = 0.27-4.70), indicating that the increased probability of those conditions was even greater for Veterans with both TBI history and high combat exposure. Altogether, results showed that both TBI and high combat exposure were independently associated with increased risk for endorsing a variety of chronic health conditions. In addition, their combined exposure resulted in even greater risk for several psychiatric and neurological conditions. These findings highlight the importance of evaluating combined risk factors pertinent to Veteran health to better identify those at risk for chronic health conditions.