Coffee Drinking Reduces Diabetes Risk

The ability of coffee to lower the risk of women developing type II diabetes has been linked to an increase in plasma levels of the protein sex hormone–binding globulin (SHBG).

Testosterone and estradiol circulate in the bloodstream, bound mostly to SHBG and to some degree bound to serum albumin. Only a small fraction is unbound, or "free," and thus biologically active and able to enter a cell and activate its receptor. Thus, bioavailability of sex hormones is influenced by the level of SHBG. SHBG is produced mostly by the liver and is released into the bloodstream. Other sites that produce SHBG include the brain, uterus, testes, and placenta.

Previous studies have firmly established an inverse relationship between coffee drinking and diabetes risk. However, a paper published in the January 2011 edition of the journal Diabetes may be the first to identify the molecular basis for this effect.

Investigators at the University of California, Los Angeles (USA) compared SBGH and sex hormone levels in a group of 359 postmenopausal women with newly diagnosed type 2 diabetes to those in a matched control group. The subjects were normalized according to age, race, duration of follow-up, and time of blood draw.

Results indicated that caffeinated coffee was positively associated with SHBG but not with sex hormones among women consuming at least four cups per day. In contrast, neither decaffeinated coffee nor tea was associated with SHBG or sex hormones. The four cups per day group had significantly higher levels of SHBG than nondrinkers did and were 56% less likely to develop diabetes than were nondrinkers.

"It was thought that coffee may improve the body's tolerance to glucose by increasing metabolism or improving its tolerance to insulin,” said senior author Dr. Simin Liu, professor of epidemiology and medicine at the University of California, Los Angeles, "but exactly how is elusive. Although we now know that this protein, SHBG, is critical as an early target for assessing the risk and prevention of the onset of diabetes."

"It seems that SHBG in the blood does reflect a genetic susceptibility to developing type II diabetes," said Dr. Liu. "But we now further show that this protein can be influenced by dietary factors such as coffee intake in affecting diabetes risk — the lower the levels of SHBG, the greater the risk beyond any known diabetes risk factors."

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University of California, Los Angeles