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.

The Japanese government approved the use of oral contraception (OC) in 1999, but OC users remain a small minority in Japan. Using the results of an online survey conducted in 2010, I examine the factors determining Japanese women's choice of contraceptive method by estimating multinomial choice models. The estimation results indicate that OC use is positively associated with age, willingness to pay for contraceptive effectiveness, frequency of intercourse and experience with abortion or emergency contraception. These findings suggest that OC use increases as women learn from experience and that the low and declining frequency of intercourse in Japan offers one explanation for the slow diffusion of OC. Additionally, the findings indicate that OC use is more prevalent among women with a higher risk of unintended pregnancies. Subjective probabilities regarding each contraceptive method's contraceptive effectiveness, disruption of romantic moods, partner disapproval, side benefits and minor, non-life-threatening side effects are important determinants of contraceptive choice. The perceived risk levels of OC side effects are significantly higher than the population-based probabilities, implying that increased medical knowledge might increase OC use.

The body mass index (BMI) of the Japanese is significantly lower than is found in other high-income countries. Moreover, the average BMI of Japanese women is lower than that of Japanese men, and the age-specific BMI of Japanese women has decreased over time. The average BMI of Japanese women at age 25 decreased from 21.8 in 1948 to 20.4 in 2010 whereas that of men increased from 21.4 to 22.3 over the same period. We examine the long-term BMI trend in Japan by combining several historical data sources spanning eleven decades, from 1901 to 2012, to determine not only when but also how the BMI decline among women began: whether its inception was period-specific or cohortspecific. Our nonparametric regression analysis generated five findings. First, the BMI of Japanese women peaked with the 1930s birth cohort. This means that the trend is cohort-specific. Second, the BMI of men outpaced that of women in the next cohort. Third, the BMI of Japanese children, boys and girls alike, increased steadily throughout the 20th century. Fourth, the gender difference in the BMI trend is due to a gender difference in the weight trend, not the height trend. Fifth, these BMI trends are observed in urban and rural populations alike. We conclude that the BMI decline among Japanese women began with those who were in their late teens shortly after World War II. (C) 2015 Elsevier B.V. All rights reserved.

Children play a key role in supporting elderly parents, and the literature has consistently found reciprocity whereby parents compensate their children for providing care and attention. To understand how the mode of compensation is related to the characteristics of parents and children, we studied the determinants of transitions to parent-child coresidence in Japan. The results conformed to the hypothesis that the mode of reciprocity depends on the costs and benefits of coresidence for each family member. Parental assets and care needs were associated with coresidence. Additionally, transitions to coresidence with married parents were characterized by young, unmarried children and the presence of parental housing assets, whereas transitions to coresidence with widowed mothers were characterized by mothers' non-housing assets.

Previous studies have consistently found evidence of an income gradient in health among children in various countries, and studies in Anglo-Saxon countries have found that this gradient increases with child age. Using nationally representative individual-level data, I examine the association between child health and parental income in Japan. Japan has a child poverty rate that is similar to the rate of many countries that have been studied previously, but Japan outperforms those countries on most health indicators. I find that an income gradient exists in child health in Japan, but that it is limited to specific health measures and symptoms, and that it is weaker overall in that respect than the gradient found in other countries or among Japanese adults. Moreover, I find no evidence that the gradient increases with child age. The fact that children in low-income families have relatively modest and non-accumulating health disadvantages may contribute to the overall health of the Japanese population. Nevertheless, there is a statistically significant negative association between parental income and the incidences of asthma, hearing problems, and dental symptoms in children, implying that future efforts to improve the health of underprivileged children should focus on the prevention and control of these diseases. (C) 2014 Elsevier Inc. All rights reserved.