Exploring the health-relevant poverty line: a study using the data of 663,000 individuals in Japan

BACKGROUND: Income poverty is known to be associated with poor health outcomes. However, the poverty line, which is used to calculate the poverty rate, is arbitrarily set without specific reference to health. This study explored the health-relevant poverty line to understand poverty in terms of population health. METHODS: Using repeated cross-sectional data from approximately 663,000 individuals obtained from 11 waves of nationwide population surveys conducted in Japan from 1986 to 2016, we used two methods to calculate a health-relevant poverty line: (1) We searched for a poverty line that maximized the proportion-weighted relative underperformance in health among individuals whose income was below the poverty line (Method I). (2) We searched for a poverty line that maximized the likelihood of the logistic regression model to explain poverty in terms of health using a binary variable for below-the-poverty-line income (Method II). For both methods, we considered five health outcomes: Poor/fair and poor self-rated health, subjective symptoms, problems with daily life activities, and psychological distress, along with covariates. RESULTS: Methods I and II indicated that the health-relevant poverty line should be drawn, respectively, at 72–86% and 67–69% of median income; this level is somewhat higher than the conventional 50% or 60%. CONCLUSIONS: The results suggest that there is a risk that the conventionally defined poverty line may underestimate poverty in terms of population health.