Coffee consumption, serum γ-glutamyltransferase, and glucose tolerance status in middle-aged Japanese men

Background: Recently, coffee consumption has been related to decreased risk of type 2 diabetes mellitus (DM) among those with high levels of serum γ-glutamyltransferase (GGT). We examined the association between coffee and glucose tolerance, determined by a 75 g oral glucose tolerance test, and the effect modification of serum GGT on the association. Methods: The study subjects were 5320 men aged 46-60 years who received a health examination at two Self-Defense Forces hospitals from January 1997 to March 2004. Those medicated for DM were excluded. Coffee consumption was classified into < 1, 1-2, 3-4, and ≥5 cups/day Statistical adjustment was made for age, body mass index, smoking, alcohol use, leisure-time physical activity, green tea consumption, parental diabetes, hospital, and rank in the Self-Defense Forces. Results: Men with normal glucose tolerance, isolated impaired fasting glucose (IFG), isolated impaired glucose tolerance (IGT), combined IFG/IGT, and type 2 DM numbered 3384, 398, 790, 348, and 400, respectively. The prevalence odds of isolated IGT, combined IFG/IGT, and type 2 DM, but not of isolated IFG, decreased with increasing consumption of coffee. An inverse association with coffee was observed for isolated IGT in both low (≤40 IU/L) and high (> 40 IU/L) GGT groups, and for combined IFG/IGT and type 2 DM in the latter group. Conclusions: Coffee drinking is protective against glucose intolerance. A possible effect modification of GGT on the coffee-DM association warrants further studies.