Background: Socioeconomic status (SES) imbalances in developed and developing countries may result in individuals being overweight and obese. However, few studies have investigated this issue in Japan. We herein examined the relationship between SES and being underweight, overweight or obese according to sex and age groups (20–64 or ≥65 years) in Japan. Methods: In 2010, we established a cohort of participants in the National Health and Nutrition Survey of Japan. We divided 2,491 participants (1,081 men and 1,410 women) according to the WHO definitions of underweight, overweight or obesity and performed multinomial logistic analyses using BMI <18.5 kg=m2 (underweight), BMI 25.0–29.9 kg=m2 (overweight), and BMI ≥30.0 kg=m2 (obese) versus BMI 18.5–24.9 kg=m2 (normal) as the outcome, with SES groups as the main explanatory variables. Results: In adult men, a lower education level relative to a higher education level was inversely associated with obesity after adjustments for other SESs (odds ratio [OR] 0.41; 95% confidence interval [CI], 0.18–0.96). However, in adult women, lower education level was positively associated with being overweight and obese (OR 1.67; 95% CI, 1.07–2.49 for overweight and OR 2.66; 95% CI, 1.01–7.01 for obese). In adult women, a lower household income was positively associated with being overweight and obese (obese: OR 4.84; 95% CI, 1.36–17.18 for those with a household income <2 million JPY relative to those with ≥6 million JPY). Conclusions: In adult women, a lower education level and lower household income were positively associated with being overweight or obese. In contrast, in adult men, a lower education level was inversely associated with obesity. Gender and age differences in SESs affect the prevalence of being overweight or obese.
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