Introduction
Diabetes mellitus is an autoinflammatory syndrome that is a collection of many disorders such as high blood pressure, high cholesterol, and insulin resistance. Diabetes is the fourth leading cause of death. Type 2 diabetes (T2DM) is the most common form of diabetes (about 90% of cases) and the prevalence has increased dramatically in the past decades and is estimated to double by 2030. Although genetics plays an important role in the etiology of the disease, there is increasing evidence that T2DM is associated with several modifiable risk factors, including diet and physical activity. The majority of diabetes cases could be prevented by changes in lifestyle, diet, and medication.
Coffee is one of the most widely consumed beverages around the world. Coffee exhibits its beneficial effects in both coffee drinkers and/or irregular coffee drinkers by improving mood, cognitive performance, and endurance during exercise. Coffee consumption also increases the subjective alertness, improves reaction time, and enhances the performance of manual tasks such as driving. Decaffeinated coffee drinkers reported healthier lifestyles in terms of physical activity, weight and diet. Despite some concerns about possible harmful effects of coffee, a number of roles have been identified by which coffee might affect diabetes development, and an increasing number of studies were conducted to assess the association of coffee intake with diabetes. In the past decade, multiple studies have examined the association between coffee intake and risk of type 2 diabetes: the majority of studies including several large meta-analyses have consistently found an inverse association between coffee intake and risk of diabetes.
Coffee influence on glucose, insulin, and diabetes
Impaired glucose tolerance (IGT) is a condition of hyperglycemia (high blood sugars) in which resistance to insulin is increased within body tissues in response to glucose (blood sugars). Insulin deficiency is the impaired action of insulin (hormone made by the pancreas that helps the body store and use glucose), which is responsible for high blood glucose levels. If high blood glucose levels are prolonged over time, it can result in long-term consequences like retinopathy, neuropathy, nephropathy, and atherosclerosis. To date, many studies have demonstrated that caffeinated coffee and decaffeinated coffee consumption were both associated with a lower risk of type 2 diabetes by reducing glucose plasma levels, improving insulin sensitivity, and lower 2 hour post-load glucose concentrations. Evidence suggests that caffeine induces acute increases in beta-cell insulin secretion that could be involved in the impairment of glucose tolerance and the decrease in insulin sensitivity brought on by caffeine.
Components of coffee
Since research shows both caffeinated and decaffeinated coffee consumption is associated with a lower risk of diabetes, this suggests that components of coffee other than caffeine are responsible for this putative beneficial effect. Coffee is a complex mixture of thousands of compounds, including caffeine (only 2% of coffee’s chemical profile), antioxidants, niacin, lipids, polysaccharides, minerals (magnesium, potassium, iron), carbohydrates, vitamins, and soluble dietary fiber. These may counteract the glycemic effect of caffeine, through their positive impact on glucose metabolism, insulin sensitivity, and anti-inflammatory.
Caffeine is the main component of coffee. A single cup of coffee can contain from 45-180 mg of caffeine, depending on the variety of coffee and the brewing method. Furthermore, caffeine might also protect against T2DM incidence through improving glucose tolerance and insulin sensitivity.
Coffee is a very rich source antioxidant, with the total content ranging from 200-550 mg per cup. The main phenolic compound in coffee is the antioxidant, chlorogenic acid. It has positive effects on glucose metabolism, including enhancing the antioxidant effects of coffee and decreasing glucose output in the liver.
Coffee contains relatively high concentrations of Mg (approximately 6.8 mg/240 ml cup), which may provide a protective effect through its positive effect on glucose metabolism, regulation of glucose homeostasis, and insulin sensitivity. Magnesium plays a role in regulating insulin action and is inversely associated with insulin sensitivity and type 2 diabetes. Long-term magnesium intake has been followed by a lower risk of type 2 diabetes. T2DM develops hypomagnesemia (low-levels or deficiency of magnesium). Also, it has been found that individuals without diabetes had higher serum levels of magnesium than those with diabetes.
How much coffee is recommended to make a difference?
On the basis of currently available evidence, it has been demonstrated that high consumption of coffee over the long term may reduce the risk for T2DM more significantly compared with short-term, low coffee consumption. Some studies found that glucose tolerance and insulin was impaired after short-term ingestion of coffee; but several studies demonstrated that long-term and habitual use of coffee may help increase and maintain normal glucose tolerance and improve insulin sensitivity. Compared with no coffee consumption, consumption of >3 cups/day of coffee was associated with a 33-37% lower risk of type 2 diabetes. Each additional cup of coffee was associated with a 5-10% lower risk of type 2 diabetes. Additionally, it has been reported that reduced diabetes risk for those who drank 3–4 cups of decaffeinated coffee daily.
Does Different Ethnicity, Gender, and/or Smoking Make A Difference?
Coffee consumption varies greatly in different regions of the world. The association between coffee consumption and diabetes risk was consistent for men and women as well as for European, U.S., and Asian populations. Studies indicated the inverse association with caffeinated and decaffeinated coffee and T2DM might be stronger for women than that for men.
Smoking, an established risk factor for T2DM, was also found positively associated with coffee consumption; thus, the adverse effects of smoking may cancel out the potential benefits of coffee consumption on T2DM incidence. Therefore, it is important to take into account potential confounding by smoking and other lifestyle factors when trying to establish the independent health effects of coffee.
Conclusion
In conclusion, both caffeinated coffee and decaffeinated coffee are associated with reduced diabetes risk. The protective effects of coffee are not likely due to caffeine, as there is strong evidence that it offers protective benefits as well. The studies conducted thus far provide a clear indication that healthy, habitual coffee drinkers (>3 cups/day) are more protected from the risk of contracting diabetes than individuals who do not drink coffee. On the other hand, decreasing coffee consumption by the same amount was associated with higher diabetes risk in subsequent years and could cause negative health effects. This, obviously, does not justify recommending an increase in coffee consumption as a strategy to prevent diabetes, since high consumption of coffee may have other effects on health. The study of the relationship between diet and health needs to consider dietary habits overall and should not be limited to the direct effect of a single food or beverage, in this case, item. For these reasons, the influence of coffee consumption on diabetes should always be studied within the context of healthy eating habits and lifestyle: the best way to prevent diabetes remains to combat overweight and obesity and ensure an adequate level of physical activity.
Limitations
In addition to frequency of consumption, coffee intake is determined by the size of the coffee cup and the strength of the brew. In some countries, cup size and brew strength have traditionally been standardized. Although US cup sizes are larger (240–250 ml) than those typical of European countries (~125–150 ml), Europeans tend to drink stronger brews, which may compensate for the difference. Nowadays, however, in countries such as the United States, there are a wide variety in cup sizes and preferred brew strengths. Standard questionnaires do not typically assess cup size, and brew strength is difficult to assess by self-report. However, misclassification of the amount of coffee consumed complicates the establishment of health effects for specific amounts of coffee. Because of misclassification in coffee consumption, particularly related to cup size and brew strength, it is difficult to precisely estimate reductions in risk by levels of consumption. Given the large differences in habitual coffee consumption in many populations (e.g., 6 or more cups per day vs. no consumption), it seems possible to determine the effects of coffee on disease risk. Nonetheless, it is difficult to establish the causality between coffee consumption and diabetes solely based on observational evidence. Because most of the studies coffee was assessed through self-reported dietary questionnaires, misclassification of coffee intake is inevitable.
Even within the same population, individuals have very different consumption patterns. Apart from the frequency of consumption, in fact, the type and amount of coffee consumed, the time of consumption (i.e., after a meal or between meals), and the addition of sugar, other sweeteners, or milk could influence the capacity of coffee to affect the risk of diabetes. A recent prospective study demonstrated that the pattern of coffee consumption may have an influence, suggesting that consumption after lunch further reduces the risk of developing T2D. The positive effect was also evident for decaffeinated coffee, while the addition of milk to coffee annulled the protective effect. However, other studies seem to indicate that the addition of milk or cream does not have any effect on the risk of diabetes or on insulin sensitivity. Finally, adding sugar to coffee did not seem to change the inverse association between coffee consumption and the risk of T2D, although one study suggested that the addition of sugar can decrease insulin sensitivity.
Emily F. Salas Groves RD, LD
OakBend Medical Center Clinical Dietitian
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