This study aimed to identify the trend of approximate participation rate in Japan's National Health and Nutrition Survey (NHNS_J). The proportion of participants among all residents of surveyed districts (estimated participation rate: EPR) was calculated by 12 regional blocks, gender, and age group, and the trend and annual percent change (APC) were clarified. Based on the 1997-2019 NHNS_J data, we created a database classified by prefecture, gender, and age group; in addition to these, the number of people per household by Population Census or population estimates were obtained from e-Stat and added to the database. All analyses were performed by regional block and gender and EPR for each year was calculated by the age group. Trends of EPR, overall and by the age group, were presented using graphs, illustrating the exponential regression curve. The graphs showed APC, standard error, and statistical significance by age group. The EPRs were declining in all the regional blocks. Additionally, the rates of decline in APC in young people under the age of 50 years were higher than those in the older age groups in 9 of 12 regional blocks. The nationwide APC in the age group <50 years was significantly larger than that in the age group ≥50 years. The declining EPR trend in NHNS_J in all regional blocks (especially among younger people) suggests the need for a strategy to improve participation rates in the future.
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