**2. Materials and Methods**

### *2.1. Subject and Setting*

Originally, the aim of the survey was to investigate the relationship between oral health and systemic health in 80-year-old adults. This survey is known as the 8020 Data Bank Survey. A 20-year follow-up study was conducted from 1996 to 2017 with subjects 80 years of age (born in 1917) residing in the 8 districts served by one health center in Iwate Prefecture. The sampling method was cluster sampling, and the sampling frame was a complete count survey for all subjects. For the baseline survey, based on residential registration, public health nurses visited 944 homes where 80-year-olds lived, and 666 subjects participated in these checkups.

The follow-up survey was conducted by the resident register with surviving subjects who participated in the baseline survey. After 20 years, 12 subjects survived. Among them, 5 lived in their own home with their families; the others lived in nursing homes. Four subjects who lived in their own home agreed to participate in the survey. We visited their homes at 10:00 in the morning to minimize the effect of food intake. Details of the survey were described in our previous report [27,28,37,38].
