Life would be so much easier if coffee were all good or all bad (bummer, but at least we’d know). The truth is much more nuanced, though mostly good (yay!). Here’s the skinny: Moderate coffee drinking (of 2-3 cups per day), consumed by healthy individuals, has been shown to significantly lower the likelihood of developing type 2 diabetes. On the other hand, coffee can also raise blood sugar levels (at least acutely), making it somewhat more challenging for people who already have diabetes to know whether they’re OK drinking it. Preliminary research findings suggest that while coffee clearly disturbs glucose metabolism and insulin sensitivity over the short-term (first few hours), longer-term, daily coffee consumption does not appear to adversely affect either of those important metabolic processes — suggesting that those with diabetes needn’t be overly concerned with moderate coffee consumption. (However, as with all dietary choices, individual responses vary greatly and you are urged to consult with your own medical team before making decisions.)

What is coffee?

Coffee is an agricultural product that contains myriad bioactive compounds that can differ based on species, roasting method, and preparation. Besides being a natural source of caffeine, brewed coffee contains chlorogenic acids (CGAs), trigonelline, melanoidins, tannins, magnesium, and hundreds of other compounds that might act independently or synergistically on pathways linked to disease prevention (or progression).

Caffeine

An alkaloid belonging to a class of compounds called methylxanthines, caffeine occurs naturally in coffee beans and green tea. Acting as a stimulant, caffeine speeds up the central nervous system, which typically improves concentration and mental alertness, and increases blood flow and heart rate. 

Caffeine is one of the most comprehensively studied ingredients in the food supply. Yet, despite our considerable knowledge of caffeine and centuries of safe consumption in foods and beverages, questions and misperceptions about the potential health effects associated with caffeine persist. Overall, moderate consumption of caffeine is considered safe for the average healthy individual. The Food and Drug Administration (FDA) has deemed it safe to consume even up to 4-5 cups (400 milligrams) of coffee per day. 

Research even points increasingly to certain beneficial effects of drinks containing caffeine, particularly coffee, for lowering the risks of developing a range of disorders that includes cancers (particularly endometrial and prostate), cardiovascular disease, strokes and even Alzheimer’s and Parkinson’s diseases. However, consumption of caffeine in high quantities can lead to some diseases in sensitive populations such as lactating and pregnant women, children, and aged people. Excessive caffeine consumption has been associated with certain diseases and conditions, including hypertension, cardiovascular diseases, bone density, various cancers, mental and behavioral disorders, and reproduction and developmental abnormalities.

Caffeine’s effect on individuals within specific disease populations, such as diabetes, has also been studied extensively, yet questions remain there, too… and scores of additional clinical research studies are currently underway across the world.

Other bioactives

Coffee contains not only caffeine, but a long list of bioactives — and analyzing the effects of each, separately, has proven challenging in the scientific community. 

While much research remains to be done on the non-caffeine compounds in coffee, it is thought that polyphenols may be the most important to study, as they exhibit potent antioxidant properties. As we detailed in our recent report, What Are Polyphenols?, studies link polyphenols to lower blood pressure and reduced bad cholesterol levels as well as a reduced risk of death from heart disease. These cardiovascular benefits of antioxidants are what laymen have heard the most about. And while those properties are found in greater quantities in foods such as whole grains and flax seeds, they are nevertheless contained (in smaller quantities) in coffee, as well. 

But in addition to the cardiovascular benefits, polyphenols also appear to improve glucose metabolism by increasing glucose disposal or insulin sensitivity. Of the polyphenols found in coffee, chlorogenic acids (CGAs) appear to be the most potent, and they have been shown to be protective against metabolic processes that could lead to inflammation and ensuing disorders such as diabetes. Interestingly, coffee represents the richest dietary source of CGAs.

This 2019 systematic review discusses CGAs and the research done on the compounds in coffee to date, including initial observations on how they work and what remains to be studied.

Coffee also contains the minerals magnesium and chromium. While increasing magnesium intake has been linked to lower rates of type 2 diabetes, the quantity of magnesium in coffee is small. Furthermore, the caffeine in coffee may wipe out any benefit from the magnesium, as caffeine can cause nutrient depletion of important nutrients, like vitamin B6, and interfere with nutrient absorption of essential minerals, including calcium, iron, magnesium and B vitamins.

Coffee and diabetes: What we know so far

For those individuals concerned with diabetes, the two most important clinical findings to date concerning coffee consumption are these: 

1. In healthy individuals, coffee drinking reduces the risk of developing type 2 diabetes.

Back in 2013, a large (over 100,000 subjects) prospective study concluded that when healthy individuals drink black coffee — both regular and decaf — they enjoy a significant reduction in the risk of developing type 2 diabetes (T2D). (Unsurprisingly, the researchers found the direct opposite if people added sugar to their coffee.)  People who increased the amount of coffee that they drank by more than 1 cup per day over a 4-year period had an 11% lower risk of developing type 2 diabetes than those who made no changes to their coffee intake. In addition, people who decreased their coffee consumption by more than 1 cup per day had a 17% higher chance of developing type 2 diabetes — the latter finding leading authors to claim new evidence that “changes in coffee consumption habit can affect type 2 diabetes risk in a relatively short period of time.” 

That next year (2014), a meta-analysis of >25 prospective cohort studies across the United States, Europe, and Asia supported this same dose–response reduction in type 2 diabetes (T2D) risk with coffee consumption. Each additional cup of regular or decaffeinated coffee consumed per day was associated with a 9% and 6% reduction in T2D risk, respectively. As a 2020 editorial in the American Journal of Clinical Nutrition remarks, “that consistency is remarkable considering a ‘cup of coffee’ can differ in so many ways not captured in detail by observational studies.” While this same research group cautions that “of course association does not imply causality and confounding can never be completely discounted,” they go on to state that “it is difficult to imagine an overlooked factor strong enough to explain the dose–response protective association observed across different populations.” 

Given that the benefits were present with both regular and decaffeinated coffee, the researchers hypothesize that it could be the other bioactives (many of which are antioxidants) in coffee, rather than the caffeine, that protects against type 2 diabetes.

2. In obese individuals, those with prediabetes, or those with type 2 diabetes, coffee can cause blood glucose levels to rise, but mostly over the short term. However, longer-term consumption of both caffeinated and decaffeinated coffee appears to have no significant deleterious impact on either insulin sensitivity or glucose metabolism.

Much of the early lab and clinical research on the effects of drinking coffee looked at coffee’s primary chemical substance: caffeine. In the early 2000s, certain research studies (including one widely cited study out of Duke University) demonstrated that “the acute administration of caffeine impaired postprandial glucose metabolism in diabetic patients,” and the authors concluded that “such effects could have implications for the clinical management of type 2 diabetes.” However, the study’s authors cautioned that the study was small in size and tested the effects of caffeine only, and not the effects of actually drinking coffee or tea. “Both beverages contain numerous organic compounds, some of which might augment or offset the effects of caffeine.”

A 2019 meta-analysis, which reviewed all of the more worthy clinical studies published up through 2017, helped reverse most earlier fears that coffee-drinking is dangerous for those with diabetes. This review concluded that while coffee impairs your glucose response acutely, i.e., over the short term (hours), coffee actually improves glucose metabolism when consistently consumed over long periods (2 to 16 weeks): the glucose curve is reduced, glucose transport and disposal improve, and the insulin response increases. Neither did long-term coffee consumption result in any significant changes in insulin sensitivity. 

Similarly, A 2020 study presenting the results of the Coffee for Metabolic Health (COMETH) Study, a 24-week parallel randomized clinical trial involving 126 healthy but overweight Asian coffee drinkers, concluded that coffee intake had no effect on insulin sensitivity nor on any of the secondary outcomes, exceptions being a ∼4% loss of fat mass and 21% reduction in urinary creatinine compared with the placebo. (This latter benefit of coffee and caffeine on body weight is considered plausible by many clinicians and has been reported previously.)

What about decaffeinated coffee and diabetes?

The studies summarized above found similar beneficial effects of habitual coffee consumption, relative to both the prevention of type 2 diabetes and the management of blood glucose and insulin levels in those individuals already dealing with diabetes, whether drinkers were consuming caffeinated or decaffeinated coffee. This again is suggestive of the idea that it’s the other bioactives in coffee, such as the antioxidant polyphenols, that provide the “protective” effect. Accordingly, some clinicians suggest that decaf is the safest option because it provides the same benefits without the potential risks of caffeine ― which include certain cardiovascular issues, like rapid or irregular heartbeats, especially when caffeine is consumed in large quantities.

Interestingly, however, a study comparing 16-week consumption of caffeinated vs. decaf coffee found that while, as other studies have shown, both drinks (consumed habitually, but in moderation) were associated with lowered levels of the postload glucose and thus protective against deterioration of glucose tolerance, waist circumference (an important metabolic marker) in the decaf group increased, whereas that same measurement decreased in the caffeinated coffee group. The authors speculate that this unwanted effect of decaf may have related to some subjects in that group exercising less. In the caffeine group, one could speculate that the caffeine may have increased energy levels, leading to higher exercise frequency and intensity in that group, which in turn benefited waist circumference.

What about tea and diabetes?

While this report is focused on coffee consumption, here’s a brief word about tea. Some clinical studies have not shown that tea and other caffeine-containing beverages are as consistently associated with lower risk of T2D as is coffee. However, the large 2013 prospective trial, which had longer-term follow-up, did show that 4 or more cups of caffeinated tea was associated with a 20% lower risk of T2D. The authors theorize that the flavonoids in tea act as antioxidants, which may beneficially interrupt the pathogenesis of diabetes.

Watch out what you put in both drinks

Most studies that looked at the impact of added sugar or creamer to coffee have documented increases in blood sugar levels, so those with diabetes (and all of us seeking to prevent its onset) should pay close attention to those added ingredients. It’s probably not the black coffee that should worry us as much.

A note on exercise and coffee

A small pilot study showed that drinking caffeine before exercising may reduce blood sugar levels in people with type 2 diabetes, and another study showed that people with type 1 diabetes might benefit from taking a caffeine supplement to lower blood sugar during exercise. However, these research studies were small and preliminary, and researchers rightly caution that those with diabetes also need to be concerned about blood sugar levels falling too precipitously. See our article: What is hypoglycemia?

Key takeaways

Much about coffee remains to be studied. Future research will hopefully shed new light on such issues as: how much individual response variability is there that can affect how each of us reacts to coffee? What impact does the microbiome have on coffee metabolism? Are there genetic variants associated with coffee- or caffeine-drinking behavior and concentrations of caffeine in the blood that affect its metabolism from person to person? What we know now are these two main points: moderate coffee consumption actually decreases your risk of getting type 2 diabetes; and if you already have diabetes, moderate coffee consumption appears to do no major harm, and may even have modest benefits to your overall glucose and weight management regimens.

If you're looking for more information of coffee's impact on glucose, check out our guide, "Does coffee raise blood sugar?"