Coffee Consumption May Reduce Risk for Type 2 Diabetes

News Author: Laurie Barclay, MD

Diabetes Care. 2006;29:2385-2390.
Release Date: October 25, 2006

October 25, 2006 — Coffee consumption reduces the risk for type 2 diabetes by 60% compared with those who do not drink coffee, according to the results of a large prospective study reported in the November issue of Diabetes Care.

"Several recently published cohort studies suggest a significant reduced risk of type 2 diabetes in coffee drinkers," write Besa Smith, MPH, from the University of California San Diego in La Jolla, and colleagues. "The purpose of this study was to investigate the association between coffee intake and incident diabetes based on an oral glucose tolerance test (OGTT) and examine coffee habits in those with impaired glucose separately from those with normal glucose at baseline."

From 1984-1987 until 1992-1996, the investigators followed up 910 adults aged 50 years or older without diabetes at baseline. Average follow-up was 8 years after assessment of coffee intake. Logistic regression models were adjusted for sex, age, physical activity, body mass index, smoking, alcohol, hypertension, and baseline fasting plasma glucose.

Compared with those who never drank coffee, past and current coffee drinkers had a lower risk for incident diabetes (odds ratio [OR], 0.38; 95% confidence interval [CI], 0.17 - 0.87; and OR, 0.36; 95% CI, 0.19 - 0.68, respectively). The 317 participants who had impaired glucose at baseline and who were past or current coffee drinkers also had reduced risk for incident diabetes (OR, 0.31; 95% CI, 0.11 - 0.87; and OR, 0.36; 95% CI, 0.16 - 0.83, respectively).

"This study confirms a striking protective effect of caffeinated coffee against incident diabetes and extends these findings to incident diabetes based on OGTT independent of multiple plausible confounders," the authors write. "The quantity of coffee consumed daily (cup-years) did not predict diabetes risk in either those with normal or impaired glucose at baseline."

Study limitations include a predominantly middle-class, white population limiting generalizability; possible survival bias; and self-reported coffee consumption subject to recall bias.

"Given the increasing prevalence of obesity, IGT [impaired glucose tolerance], and diabetes, and the fact that the majority of adults in most of the Westernized world drink coffee daily, a coffee benefit could have widespread impact," the authors conclude. "Further investigation is warranted."

The National Institute of Diabetes and Digestive and Kidney Diseases and National Institute on Aging supported this study. The costs of publication of this article were defrayed in part by the payment of page charges, mandating that it must therefore be hereby marked "advertisement" solely to indicate this fact.

Clinical Context
C-peptide is a marker of the serum concentration of insulin, and research by Wu and colleagues, published in the June 2005 issue of Diabetes Care, demonstrates that coffee could reduce C-peptide levels by 16% when comparing subjects from the highest quintile of coffee intake vs subjects who did not drink coffee. Both caffeinated and decaffeinated coffee were associated with reduced C-peptide levels, although tea had no significant effect on C-peptide. Coffee was more powerful in lowering C-peptide levels among obese and overweight women compared with women at a normal weight.

Coffee consumption has also been linked with a decreased risk for diabetes in epidemiologic studies. However, these studies frequently used self-reported diabetes as an outcome measure. The current study relates coffee consumption to diabetes diagnosed by either self-report or OGTT.

Study Highlights
Participants were drawn from one community in Southern California. Individuals with diabetes at baseline could not participate in the study.

At baseline, subjects underwent a 75-g OGTT with measurements of fasting glucose and plasma glucose 2 hours after the glucose load. Coffee consumption habits were also assessed at baseline. A second interview to diagnose diabetes, including another OGTT, was performed after an average of 8 years.

The main study outcome was the relationship between coffee consumption and the risk of incident diabetes. Diabetes was defined by a clinical diagnosis, the use of diabetes medications, a fasting plasma glucose level of at least 7 mmol/L, or a 2-hour serum plasma glucose level at least 11.1 mmol/L on OGTT. Impaired glucose tolerance was defined by fasting glucose level between 6.1 and 7 mmol/L or 2-hour glucose level between 7.8 mmol/L and 11.1 mmol/L

593 subjects had a normal OGTT at baseline, while 317 had an impaired glucose. The mean age of subjects was 65.9 years, and 41% of the cohort was male. Only 7% of subjects were obese, but 44% of participants had hypertension. The mean fasting glucose level at baseline was 5.4 mmol/L.

72.5% of subjects reported current consumption of coffee at baseline. The average consumption of coffee among these participants was 2.8 cups/day. Participants who drank coffee were more likely to smoke and drink alcohol vs those who did not drink coffee.

Compared with subjects who did not drink coffee, current coffee consumption at baseline reduced the risk for incident diabetes by 64%. This result was similar when examining the cohort as a whole as well as the subgroup of participants with impaired glucose tolerance.

Past coffee drinking also reduced the risk for incident diabetes vs those who did not drink coffee (odds ratio, 0.38).

The average number of cups of coffee per day failed to have a significant effect on the risk for incident diabetes.

Only 12 subjects reported drinking decaffeinated coffee, making a separate analysis of this beverage impossible.

The researchers incidentally found that the risk for diabetes was lower among subjects who drank 1 to 2 alcoholic drinks per day vs higher or lower rates of alcohol consumption.

Pearls for Practice
Previous research has demonstrated that both caffeinated and decaffeinated coffee can reduce serum levels of C-peptide in women. Tea had no such effect.

In the current study, either current or past consumption of coffee reduced the risk for incident diabetes. There was no relationship between the degree of coffee intake and the risk for diabetes.

News Author
Laurie Barclay, MD is a freelance reviewer and writer for Medscape.


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