Sayaka Nakamura

Importance Chronic kidney disease (CKD) is a major public health issue, affecting 850 million people worldwide. Although previous studies have shown the association between socioeconomic status and CKD, little is known about whether this association exists in countries such as Japan where universal health coverage has been mostly achieved. Objective To identify any association of income-based disparity with development of impaired kidney function among the working population of Japan. Design, Setting, and Participants This was a nationwide retrospective cohort study of adults aged 34 to 74 years who were enrolled in the Japan Health Insurance Association insurance program, which covers approximately 40% of the working-age population (30 million enrollees) in Japan. Participants whose estimated glomerular filtration rate (eGFR) had been measured at least twice from 2015 to 2022 were included in the analysis, which was conducted from September 1, 2021, to March 31, 2023. Exposure Individual income levels (deciles) in the fiscal year 2015. Main Outcomes and Measures Odds ratios were calculated for rapid CKD progression (defined as an annual eGFR decline of more than 5 mL/min/1.73 m²), and hazard ratios, for the initiation of kidney replacement therapy (dialysis or kidney transplant) by income level deciles in the fiscal year 2015. Results The study population totaled 5 591 060 individuals (mean [SD] age, 49.2 [9.3] years) of whom 33.4% were female. After adjusting for potential confounders, the lowest income decile (lowest 10th percentile) demonstrated a greater risk of rapid CKD progression (adjusted odds ratio, 1.70; 95% CI, 1.67-1.73) and a greater risk of kidney replacement therapy initiation (adjusted hazard ratio, 1.65; 95% CI, 1.47-1.86) compared with the highest income decile (top 10th percentile). A negative monotonic association was more pronounced among males and individuals without diabetes and was observed in individuals with early (CKD stage 1-2) and advanced (CKD stage 3-5) disease. Conclusions and Relevance The findings of this retrospective cohort study suggest that, even in countries with universal health coverage, there may be a large income-based disparity in the risk of rapid CKD progression and initiation of kidney replacement therapy. These findings highlight the importance of adapting CKD prevention and management strategies according to an individual’s socioeconomic status, even when basic health care services are financially guaranteed.

Women have a lower BMI than men in developed countries, yet the opposite is true in developing countries. We call this the gender BMI puzzle and investigate its underlying cause. We begin by studying time trends in Japan, where, consistent with the cross-country puzzle, the BMI of adult women has steadily decreased since the 1950s, whereas the BMI of adult men has steadily increased. We study how changes in energy intake and energy expenditure account for the over-time gender BMI puzzle using the Japanese National Nutrition Survey from 1975 to 2010, which provides nurse-measured height and weight and nutritionist-assisted food records. Because long-term data on energy expenditure do not exist, we calculate energy expenditure using a steady-state body weight model. We then conduct cross-country regression analysis to corroborate what we learn from the Japanese data. We find that both energy intake and energy expenditure have significantly decreased for Japanese adult men and women and that a larger reduction in energy expenditure among men than women accounts for the increasing male-to-female BMI gap. Trends in BMI and energy expenditure vary greatly by occupation, suggesting that a relatively large decrease in physical activity in the workplace among men underlies the gender BMI puzzle. The cross-country analysis supports the generalizability of the findings beyond the Japanese data. Furthermore, the analysis suggests the increasing male-to-female BMI gap is driven not only by a reduction in the energy requirements of physically demanding work but also by weakening occupational gender segregation. No support is found for other explanations, such as increasing female labor force participation, greater female susceptibility to malnutrition in utero, and gender inequality in nutrition in early life.