Coffee Break: Studies Don't Tell Whole Story in Diabetes

Miriam E. Tucker

November 25, 2013

Drinking coffee might help prevent type 2 diabetes, according to a newly updated collection of studies compiled by the Institute for Scientific Information on Coffee (ISIC). But as ever, the devil lies in the details, with 2 experts disagreeing as to the exact conclusions that can be drawn from this work.

The ISIC report, "Good things in life: Can coffee help in type 2 diabetes reduction?" is the second to come from a roundtable discussion held at an international diabetes prevention conference in 2012. The first report summarized knowledge up to that date while the new one includes data from the past year.

The 2013 update highlights epidemiological evidence, summarizes a few clinical intervention trials, and examines data regarding a positive effect of coffee on other clinical parameters, including cardiovascular disease and the liver. The report also suggests potential mechanisms by which coffee could decrease the risk of developing type 2 diabetes.

"The main clinical message regarding coffee and type 2 diabetes is that people can drink coffee as usual," said Siamak Bidel, MD, PhD, of the diabetes unit, National Institute for Health and Welfare, and Hjelt Institute, University of Helsinki, Finland, who was cochair of the roundtable that produced the report, although he was not paid by ISIC for this role.

But James D. Lane, PhD, professor of behavioral medicine in the department of psychiatry and behavioral sciences at Duke University School of Medicine, Durham, North Carolina, disagrees.

"What they're reporting is a very optimistic view of studies that show that people who drink large amounts of coffee are less likely to develop diabetes. From this, they leap to the conclusion that it's because they drink large amounts of coffee that they have a lower risk of diabetes," said Dr. Lane, who has published a series of studies demonstrating a variety of adverse health effects of caffeine — and of positive effects from giving it up.

"The truth is, they can't show that drinking coffee reduces the risk for developing diabetes; all they can show is that there's a correlation," he told Medscape Medical News.

Both researchers acknowledge that there are important differences between studies looking at consumption of coffee and of caffeine per se, which can be found in a variety of other products such as tea, soda, and energy drinks. And results of studies involving coffee can also differ widely, depending on the type of coffee, where it's grown, the way that it's prepared, and other variables.

Dr. Bidel told Medscape Medical News, "This is very important, that we distinguish between the effects of coffee and caffeine [ingestion], which I believe are different."

According to Dr. Lane, "It's impossible to tease out. Coffee has hundreds of different chemical compounds in it other than caffeine, and it's conceivable that some of those will [also] have biological effects."

Epidemiologic Studies Cannot Establish Cause and Effect

The ISIC report summarizes recent epidemiology linking high coffee consumption to a reduced risk of developing type 2 diabetes. A 2009 meta-analysis of 18 studies demonstrated that drinking 3 to 4 cups of coffee (with or without caffeine and including tea) per day was associated with an approximately 25% lower risk compared with consuming zero to 1 cup per day.

And a 2012 systematic review found that individuals who habitually drank 4 or more cups of coffee a day had a lower risk for type 2 diabetes compared with those who drank less than 2 cups per day (Current Diabetes Reviews. 2012;8:162-168).

Other epidemiological evidence includes the finding of a dose-response relationship between coffee consumption and reduced type 2 diabetes risk (Eur J Nutr. 2013;DOI:101007.s00394-013-0603-x) as well as reductions in diabetes risk factors such as C-peptide (Diabetes Care 2005;28:1390-1396) and adiponectin (Diabetes Care 2008;31:504-507) with increased coffee consumption.

But Dr. Lane countered, "They can't establish that the reason heavy coffee drinkers have less disease is because of the coffee they drink. To me, people who drink 4 to 6 cups of coffee every day are different from people who don't drink coffee in a host of ways. First, people who are feeling chronically ill are less likely to drink 6 cups of coffee."

He added that there may be other disease precursors, including those for type 2 diabetes, that might lead people to avoid caffeine and coffee. Pregnancy — during which many women develop an aversion to coffee — is one example, he noted. "We know there are biological factors that affect a person's motivation to drink coffee."

Acknowledging this limitation, Dr. Bidel said epidemiologic studies "do not provide the answer but create questions that might find an answer." However, when studies with large databases from diverse countries produce the same result, it is more likely the finding is a true one, he noted.

Clinical Intervention Trials Contradictory

In terms of intervention studies, the new ISIC publication cites just a handful that demonstrate a benefit for coffee, including 2 from Asia showing glycemic improvements with long-term coffee consumption (Eur J Clin Invest. 2011;41: 659-666).

In fact, the report also cites a recent review of randomized controlled trials that found that caffeine has a negative effect on blood glucose control among people diagnosed with type 2 diabetes (J Hum Nutr Diet. 2013;26:111-125). But, it notes that those authors "concluded that further work was required."

Dr. Lane's own work has shown a consistent acute worsening of glycemia with caffeine consumption. Most recently, in a small pilot study, he showed that 3 months of total caffeine abstinence improved glycemic parameters among 12 established coffee drinkers with type 2 diabetes (J Caffeine Res. 2012;2: 45-47). "Here, all they did was quit taking in caffeine, and their A1c improved as much as if they were taking in another medication," he told Medscape Medical News.

And in a prior review article, Dr. Lane cites 17 studies finding that caffeine exaggerates the rise in glucose after carbohydrate ingestion even in healthy, nondiabetic adults (J Caffeine Res. 2011;1:23-28 ).

But, Dr. Bidel points out, "There is a difference between acute and chronic effects of coffee on glycemia and insulin sensitivity."

Many Mechanisms Could Be at Play

Dr. Lane said his findings apply to all sources of caffeine, not just coffee. Caffeine raises levels of epinephrine (adrenaline), which also are known to stimulate the liver to produce glucose in a "fight-or-flight" response.

"We know caffeine has certain effects. It raises epinephrine and blood pressure. It can make you jittery and sweaty. For some people that's bad, for others it's just an annoyance… I would say people who have type 2 diabetes would be better off if they didn't drink coffee."

On the other hand, the ISIC report notes that those same effects of coffee could reduce the risk for developing type 2 diabetes.

One theory is simple calorie replacement: Choosing coffee over sugary drinks leads to reduced calorie consumption, and coffee also speeds up the metabolism.

And there may be ingredients in coffee — apart from caffeine — with beneficial effects. According to the ISIC report, coffee contains components with potential anti-inflammatory, antioxidant, and hormonal properties that could improve insulin sensitivity. Other coffee ingredients could chelate iron, which also may improve insulin sensitivity or cardiovascular health.

Interestingly, Dr. Bidel told Medscape Medical News that coffee might have a mechanism similar to that of current glucose-lowering medications.

"Coffee consumption increases production of the incretin hormone glucagonlike peptide-1 [GLP-1], possibly owing to an inhibitory effect of [chlorogenic acid] on glucose absorption."

In all, he said that coffee's effects could be due to several of its bioactive compounds working in concert and that the type of coffee and the way it's prepared can influence these effects as well.

"One may work in some people better than the others, or all work together to obtain the observed results. It depends on the pathophysiologic state of the body and organs involved in glucose metabolism," he noted.

He recommends that people drink coffee to their own tastes, yet in moderation: "People who drink loose coffee can drink more and stronger coffee consumers a lesser amount."

Dr. Lane told Medscape Medical News that he has had difficulty obtaining grants to further his research. "Critics say it's not worth funding because you can't get people to stop drinking coffee, which we actually did in the pilot study... Even scientists reviewing research grants seem to love their coffee so much they don't want to see research against it."

Neither Dr. Bidel nor Dr. Lane has reported any relevant financial disclosures.

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