Association Between Hemoglobin Concentration and Metabolic Syndrome in a Japanese Population

Hemoglobin (Hb) concentration has been associated with metabolic abnormalities, but its relationship with metabolic syndrome (MetS) remains incompletely characterized. We examined these associations in a Japanese health checkup population. This cross-sectional study included 17,567 adults (10,175 men and 7392 women) examined between 2007 and 2024. MetS was defined according to the Japanese criteria. MetS analyses included 10,169 men and 7389 women after excluding nine participants with missing medication data. Sex-stratified regression, sensitivity, restricted cubic spline, and exploratory receiver operating characteristic analyses were performed. MetS was present in 1973 men (19.4%) and 310 women (4.2%). Higher Hb quartiles were generally accompanied by less favorable metabolic profiles. In adjusted linear models, Hb was positively associated with homeostasis model assessment of insulin resistance (HOMA-IR), triglycerides, fasting plasma glucose, waist circumference, and body mass index in both sexes; the largest standardized association was with HOMA-IR (men, β = 0.264; women, β = 0.150). In the principal confounder-adjusted logistic model, the odds ratios for MetS per 1 g/dL higher Hb were 1.441 in men and 1.668 in women. Associations remained significant in all sensitivity analyses. Restricted cubic splines indicated significant nonlinearity in both sexes. The areas under the curve for Hb alone were 0.579 in men and 0.676 in women. Higher Hb levels were associated with prevalent MetS and adverse metabolic parameters in both sexes. Hb may provide ancillary metabolic information but is insufficient for standalone MetS screening. The cross-sectional design precludes causal inference.