Haematocrit and risk of development of Type 2 diabetes mellitus in middle-aged Japanese men

Aims To investigate the association between haematocrit and risk of development of diabetes. Methods The study enrolled 2953 normoglycaemic [fasting plasma glucose (FPG) < 6.1 mmol/l and taking no hypoglycaemic medication] Japanese men aged 35-59 years and free of medication for hypertension and history of cardiovascular disease. FPG was measured at periodic annual health examinations from May 1994 through May 2001. Men in whom Type 2 diabetes mellitus (FPG >= 7.0 mmol/l or receiving hypoglycaemic medication) was found during repeated surveys were classified as having Type 2 diabetes. Results The estimated incidence rates for Type 2 diabetes during 7 years of follow-up were 3.1% [95% confidence interval (CI) 4.6, 4.6], 4.6% (2.8, 6.4), 5.0% (3.2, 6.9), 6.4% (4.4, 8.5), and 11.5%, (8.9, 14.2) for respective haematocrit levels of < 42.6, 42.6-44.0, 44.1-45.3, 45.4-46.8, and greater than or equal to 46.9% (the log-rank test: P < 0.001). After controlling for potential predictors of diabetes, the respective relative risks for Type 2 diabetes were 1.0 (reference), 1.52 (95% CI 0.81, 2.86), 1.24 (0.66, 2.3-1), 1.57 (0.86, 2.88), and 2.30 (1.30, 4.08) (P for trend = 0.002). From stratified analyses by presence or absence of a risk factor, a linear association of haematocrit level with risk of development of Type 2 diabetes was also observed. Conclusion These results indicate that haematocrit contributes to the risk of developing Type 2 diabetes.