Body mass index (BMI) distribution and its impact on cardiovascular disease (CVD) vary between Asian and western populations. The study aimed to reveal time-related trends in the prevalence of obesity and underweight and safe ranges of BMI in Japanese patients with CVD. We analyzed 5,020,464 records from the national Japanese Registry of All Cardiac and Vascular Diseases-Diagnosis Procedure Combination dataset over time (2012-2019) and evaluated BMI trends and the impact on in-hospital mortality for six acute CVDs: acute heart failure (AHF), acute myocardial infarction (AMI), acute aortic dissection (AAD), ischemic stroke (IS), intracerebral hemorrhage (ICH), and subarachnoid hemorrhage (SAH). Patients were categorized into five groups using the WHO Asian-BMI criteria: underweight (< 18.5 kg/m2), normal (18.5-22.9 kg/m2), overweight at risk (23.0-24.9 kg/m2), obese I (25.0-29.9 kg/m2), and obese II (≥ 30.0 kg/m2). Age was significantly and inversely related to high BMI for all diseases (P < 0.001). The proportion of BMI categories significantly altered over time; annual BMI trends showed a significant and gradual increase, except AAD. In adjusted mixed models, underweight was significantly associated with a high risk of in-hospital mortality in all CVD patients (AHF, OR 1.41, 95% CI 1.35-1.48, P < 0.001; AMI, OR 1.27, 95% CI 1.20-1.35, P < 0.001; AAD, OR 1.23, 95% CI 1.16-1.32, P < 0.001; IS, OR 1.45, 95% CI 1.41-1.50, P < 0.001; ICH, OR 1.18, 95% CI 1.13-1.22, P < 0.001; SAH, OR 1.17, 95% CI 1.10-1.26, P < 0.001). Moreover, obese I and II groups were significantly associated with a higher incidence of in-hospital mortality, except AHF and IS. Age was associated with in-hospital mortality for all BMI categories in six CVD patients. BMI increased annually in patients with six types of CVDs. Although underweight BMI was associated with high mortality rates, the impact of obesity on in-hospital mortality differs among CVD types.
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