Coffee: Friend or Foe in Diabetes?
Ryan Bradley, ND
May 2009
Concerns about “adrenal fatigue” (a state of chronic exhaustion from accumulated stress) and negative effects on the heart have led naturopathic physicians and other complementary medicine providers to counsel abstention from coffee. This article is intended to review the somewhat contradictory recent research on the metabolic effects of coffee consumption, and will also include a brief discussion of the other well-characterized health effects of coffee. We’ll discuss the population-based research about coffee’s effects on risk of type 2 diabetes, contrast these with recent clinical trials that seem to contradict these observations, and consider coffee’s effects on cardiovascular risk factors like cholesterol and blood pressure, that are common in patients with type 2 diabetes. Throughout the discussion, we’ll consider some of the major constituents that are probably active in this drink’s effects on human health.
Coffee: A Complex Botanical Infusion
Our herbal traditions maintain that a plant medicine is more than the most active component, and this holds true for coffee. While the methylxanthine caffeine is a major constituent, coffee contains over 600 other organic compounds, notably the diterpenes kaweol and cafestol and the antioxidant phenols caffeic acid, quinic acid, and the ester of these, chlorogenic acid. In fact, coffee is a major dietary source of antioxidants (Svilaas et al.,2004; Pulido et al., 2003). Coffee is also a notable source of micronutrients, containing significant amounts of niacin, potassium, and magnesium (Dórea and da Costa, 2005). Indeed, to believe that coffee is synonymous with caffeine is a gross mischaracterization of this complex, botanical matrix of deliciously evolved constituents. And not unlike research on other botanical medicines, precision is required in the interpretation of research to separate the effects of coffee constituents, from coffee itself.
Effects of Coffee on Diabetes Risk in Large Populations
Beginning with a notable Dutch report in 2002 (van Dam and Feskens, 2002), studies conducted in very large groups of people have repeatedly showed significant, inverse associations between regular coffee consumption and the risk of type 2 diabetes, i.e. the greater the coffee consumption, the lower the risk of developing type 2 diabetes. This body of work was analyzed in a 2005 JAMA meta-analysis (van Dam and Hu, 2005), which found that coffee consumption did indeed seem to be associated with a reduced risk of type 2 diabetes. The meta-analysis examined nine studies and found that, when compared to those individuals drinking relatively small amounts of coffee (less than two cups/day), those drinking four-six cups had a 28% lower risk of type 2 diabetes, while those drinking more than seven cups per day had a 35% reduction of risk. Variations were noted between region (US vs. Europe), sex, and obesity. Regardless of these stratifications, all groups still had a significantly lower risk of type 2 diabetes with greater coffee consumption.
Further refining the observations of the JAMA meta-analysis, an interesting prospective trial of survey participants in the United States (N=7006, followed for 8.4 years) (Greenberg et al., 2005) suggested that ground (but not instant) coffee only offered a reduced risk of type 2 diabetes in people under the age of 60 who had also lost weight during the study period.
While clearly representing strong support for the position, at least observationally, that coffee seems to be protective against type 2 diabetes, there are some caveats that should be kept in mind. Most of these findings are based on populations consuming primarily paper-filtered coffee. Few studies have examined whether unfiltered coffee (pot-boiled or Greek/Turkish style) or instant coffee offers the same protection, and results are unclear (Tuomilehto et al., 2004; Reunanen et al., 2003). Similarly, these studies do not typically take into account the addition of sweeteners or lighteners (milk, cream, soy milk, etc). Finally, coffee consumption was believed to be mostly caffeinated coffee (van Dam and Feskens, 2002; Salazar-Martinez et al., 2004), so the effects of decaffeinated coffee consumption have not been fully explored.
Research on the Short-Term Effects of Coffee on Blood Sugar Regulation
The mechanisms underlying the possible protective effects of coffee are still unknown. Studies of caffeine as an isolated compound regularly show negative effects on blood glucose control (Lane et al., 2008; MacKenzie et al., 2007). Given the seeming contradiction between the impaired glucose control from caffeine, and the observation of reduced type 2 diabetes risk with coffee consumption, several recent clinical trials have sought to separate out the effects of coffee consumption vs. caffeine consumption.
A recent double-blinded randomized trial (n=11, non-diabetic males), compared the effects of caffeine and coffee on glucose after an oral glucose tolerance test (OGTT) (Battram et al., 2006). Each subject served as their own control, and on study days consumed either caffeine (in pill form), placebo (pill), caffeinated coffee, and decaffeinated coffee 60 minutes prior to a two-hour OGTT. Glucose, insulin, C-peptide, and insulin sensitivity were measured at baseline and several other intervals, in order to calculate an area under the curve (AUC) for glucose and insulin (the higher the area under the resulting curve, the higher blood sugar stayed). Results showed that caffeine induced the highest AUC for glucose, followed by placebo, caffeinated coffee, and decaffeinated coffee. AUC for insulin and C-peptide followed a slightly different pattern (again, from high to low AUC): Caffeine, caffeinated coffee, placebo, and decaffeinated coffee. The insulin-sensitivity index showed the highest value with decaffeinated coffee, followed by placebo, caffeinated coffee, and then caffeine, respectively. In other words, caffeine outside of the matrix of coffee, led to deleterious effects on glucose metabolism, and the authors surmise that some components of coffee may be responsible for attenuating these negative effects of the single alkaloid. Especially surprising is that decaffeinated coffee had a better glycemic effect than even placebo, lending further credence to this hypothesis, i.e. there appear to be protective constituents in coffee.
In an effort to more accurately describe “real life,” another group (Moisey et al., 2008) dispensed coffee with the shakes high in sugar, and then compared the effects of caffeinated coffee or decaffeinated coffee when consumed one hour before “typical” meals in non-diabetic males (n=10). The meals consisted of milk with one of two different breakfast cereals, matched by carbohydrate content but with either a high or low glycemic index (GI). The AUC for glucose, insulin, and C-peptide was higher with caffeinated coffee, and insulin sensitivity was higher with decaffeinated coffee; results held true for both meals, though the results were typically more pronounced with the low GI meal. The authors note that “although the actual GI for the low GI cereal was 50% of that of the high GI cereal, the apparent [emphasis added] GI for the low GI cereal actually exceeded that of the high GI cereal, when the former was associated with caffeinated coffee consumption.” This suggests that caffeinated coffee may impact the functional GI of meals, possible by reducing transit time in the gastrointestinal tract.
Complicating matters, yet another trial (Louie et al., 2008) compared the effects on glycemic control of caffeinated coffee, decaffeinated coffee, caffeinated coffee+sucrose, sucrose, black tea, hot water, and water in eight non-diabetic adults. This small trial found that, consistent with the above trials, caffeinated coffee increased postprandial glycemia (per AUC-glucose), caffeinated coffee with sucrose decreased post-prandial glycemia, while tea, sucrose, water, and decaffeinated coffee had no significant effect. While the findings are yet to be explained, the novelty of the study looking at the effects of sweeteners added to coffee may offer some partial explanation for the paradox observed between observational data and clinical trials.
These trials strongly suggest that other constituents of coffee may help mitigate the effects of caffeine, but also suggest that caffeinated coffee still may have deleterious affects on post-prandial glucose regulation. These findings make it difficult to reconcile the apparently protective effects of coffee found in observational studies. One possibility is that the acute effects of short-term coffee and/or caffeine consumption are muted with chronic use. Caffeine’s tendency to increase epinephrine (a stress hormone that raises blood sugar) is known to disappear within seven days of regular use (Robertson et al., 1981), and it may be that whatever dysglycemic effects that caffeine or coffee creates in a laboratory setting are similarly muted with regular consumption. Also, the observation of type 2 diabetes protection may be related to other key constituents. The polyphenol, chlorogenic acid, is thought to inhibit intestinal glucose absorption (Johnston et al., 2003; Thom, 2007) and inhibit hepatic glycogenolysis (Arion et al., 1997). When compared to decaffeinated coffee, both chlorogenic acid and trigonelline (another constituent), seemed to independently reduce glucose and insulin concentrations (van Dijk et al., 2009). Magnesium, another constituent in coffee, is considered to have potentially beneficial effects on blood glucose, including improved insulin sensitivity (de Valk, 1999). Finally, it should be noted that observational data can never determine causality, and the observational findings may be explained by coffee drinkers substituting coffee for alternative, highly caloric beverages, including sodas. Regardless, compared to other alternatives, there is little evidence, in large samples from the general population, that coffee intake has deleterious effects on diabetes risk.
Effects on Cardiovascular Risk
Apart from any effects of coffee on glycemic control, it should be remembered that many patients with diabetes frequently have related cardiometabolic conditions. Fortunately, it seems that coffee consumption does not appear to negatively impact heart disease risk, including risk of CVD-related mortality in either men or women with type 2 diabetes (Bidel et al., 2006; Zhang et al., 2009; Zhang et al., 2009). While caffeine is known to transiently increase blood pressure (Noordzij et al., 2005), habitual coffee intake probably does not raise a person’s blood pressure by more than two points (mmHg) (Gelejinse, 2008). Blood cholesterol does appear to increase from cafestol and kaweol (compounds present in boiled coffee) but paper filtering (i.e., American coffee) removes these compounds (Mensink et al., 1995), and paper-filtered does not acutely elevate lipid levels (Cheung et al., 2005). Even chronic kidney disease, a potential complication of type 2 diabetes, does not seem to be significantly impacted by moderate coffee consumption (Bolignano et al., 2007).
Additional Health Effects & Impact of Overall Mortality
It is well beyond the scope of this article to discuss other known or potential benefits of coffee, but we would be remiss in our duties if we did not highlight recent meta-analyses suggesting that coffee is largely protective against many forms of cancer. Protective associations have been noted between coffee consumption and hepatocellular carcinoma (Bravi et al., 2007), colorectal cancer (Je et al., 2009), and endometrial cancer (Bravi et al., 2009); neutral affects have been observed on gastric cancer (Botelho et al., 2006), and only a borderline significant effect on breast cancer (Tang et al., 2009). Also, the Nurses’ Health Study and the Health Professionals Follow-Up Study, which followed 86,214 women 41,736 men, respectively, for up to 24 years, demonstrated that regular coffee consumption showed a slight, dose-dependent reduction in total overall mortality (Lopez-Garcia et al., 2008).
The Real Test
As with any substance (food, medication, or nutritional supplement) the real test is how coffee impacts you and your blood sugar. If you are having difficulties reducing your blood sugar after meals, and you are a regular coffee drinker (and have removed all the major high glycemic index carbohydrate sources from your meals!) it may be prudent to reduce and/or stop your coffee for a week or so to see if there is an improvement with abstention. Stopping coffee “cold turkey” often results in a mind-numbing headache, leaving you relatively incapacitated (heavy brain fog) for up to a few days. Likely easier, is to switch to 50/50 caf/decaf blend for a week or so, and then switch to decaf coffee, for which there appears to be no concern (and it may even help lower blood sugar). If you don’t notice a change (and are following the same diet and exercise plan), then perhaps the caffeine just isn’t a big deal for you.
If you are not currently a coffee drinker, is there reason to start? Well, if you are not hooked now, I wouldn’t start drinking caffeinated coffee in order to stave off diabetes, or to try to reduce your blood sugar. However, there may be some benefits to drinking decaffeinated coffee, including increased antioxidant status, increased magnesium intake, and possibly healthful affects on digestion and detoxification.
Remember, I’m talking about pretty basic coffee in this article, maybe a little whitener or low-caloric sweetener, not your average 800-1200 calorie, 20 oz. coffee breve beverage made with half & half, sugar, and a little coffee (these beverages are clearly bad for diabetes).
Conclusions
Coffee is not the devil in disguise, but rather a complex, botanical matrix whose effects are impacted by the method of preparation, and likely the concentration of key constituents in the resulting infusion. While the short-term effects of caffeinated coffee appear potentially problematic in short trials, long-term observational data suggests that it is an overall benefit in staving off diabetes. In additional, as more precise research is conducted, we are beginning to learn that individual coffee constituents have beneficial actions on glucose regulation. Combined with the minimal effects on chronic cardiovascular pathologies, and the apparent protection coffee offers for numerous cancers, there is no strong case for getting most people to give up their morning eye-opener!
In Health-Ryan
Acknowledgement: I would like to thank Bill Walter, ND (’09) for his research and contributions to this article.
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