Background This study investigated the cumulative impact of persistent poverty on dental caries among elementary schoolchildren in Japan.

Methods Data were derived from four-wave longitudinal data of children in all public elementary schools in Adachi City, Tokyo, Japan, from 2015 to 2020 (n=4291, response rate: 80.1%–83.8%). Poverty status, defined as annual household income <JPY3 million, material deprivation or payment difficulties for lifeline utilities, was assessed by caregiver questionnaires when the children were in the first, second, fourth and sixth grades. School dentists assessed dental caries. We estimated the difference in the number of primary and permanent teeth with incidences of dental caries from second to sixth grade by persistent poverty and never having experienced poverty. Targeted maximum likelihood estimation was used to consider baseline and time-varying confounders.

Results Children with persistent poverty experienced more dental caries (mean: 3.81, SD: 3.73) than children who had never experienced poverty (mean: 2.39, SD: 3.27). After controlling for confounders, being in persistent poverty was significantly associated with having more dental caries than never being in poverty (mean difference: 1.54, 95% CI 0.60, 2.48). The magnitude of the association was greater than that of poverty assessed at first grade only (mean difference: 0.75, 95% CI 0.35, 1.16) or experience of poverty at any of the four waves (mean difference: 0.69, 95% CI 0.39, 0.99).

Conclusion The cumulative impact of persistent poverty could be larger than the poverty assessed at a single time point.