• Taku Saito, Jun Shigemura, Masato Kitano, Fumiko Waki, and Masanori Nagamine.
• Division of Behavioral Science, National Defense Medical College Research Institute, National Defense Medical College, Tokorozawa City, Saitama 359-8513, Japan.
• Mil Med. 2024 Sep 23.

IntroductionThe "healthy warrior effect" (HWE), in which deployed military personnel are healthier than those not deployed, is known to be associated with deployment. However, the HWE associated with deployment to United Nations (UN) peacekeeping operations (PKO) has not been examined. This study examined the HWE associated with deployment to UN PKO by examining the impact of pre-deployment mental health on the likelihood of deployment and whether this difference persisted after deployment.Materials And MethodsThis retrospective cohort study included 2,712 Japan Ground Self-Defense Forces personnel from four engineering units who consented to participate in annual mental health checks conducted between August and October 2015. The Kessler Psychological Distress Scale (K6) was used to assess general psychological distress before and after deployment. Logistic regression analysis adjusted for age, sex, and rank was conducted with deployment as the dependent variable and the pre-deployment K6 total score as the independent variable. Additionally, a mixed-effects model was performed with K6 total scores as the dependent variable, time and deployment as fixed effect factors, and the individual as the random effect factor.ResultsOf the participants, 254 were in the deployed group, and 2,458 were in the non-deployed group. The pre-deployment K6 total score was significantly lower (t = 5.2, P < .001) for the deployed group (0.67 ± 1.60) than for the non-deployed group (1.26 ± 2.41). Logistic regression analysis showed an odds ratio for the K6 total score before deployment as 0.87 (95% CI = 0.79-0.94). The mixed-effects model exhibited a significant negative main effect of being deployed (t = -5.1, P < .001) and a significant interaction effect between time and being deployed (t = -2.5, P = .012).ConclusionsPsychologically healthier personnel were more likely to be deployed for UN PKO and their better mental health status persisted after deployment, reflecting the HWE. However, personnel in good health may develop new mental health issues after their deployment, and more attention to health management will be essential before and after deployment.© The Association of Military Surgeons of the United States 2024. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site–for further information please contact journals.permissions@oup.com.

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