Befitting its status as one of the most commonly consumed beverages in the world, coffee has long been a topic of study among medical researchers.
Yet, according to Walter C. Willett, MD, DrPH, professor of epidemiology and nutrition at Harvard T.H. Chan School of Public Health and professor of medicine at Harvard Medical School, many early efforts at determining just what all those cups of java were doing to our health were held back by design flaws that made establishing correlations difficult.
"Publishing a paper on coffee is almost a rite of passage for epidemiologists," says Willett. "Importantly, the quality of studies has greatly improved with time, in part because they are now almost all prospective, are much larger, and follow-up is longer."
This, in turn, has led to a significant change in how we think about coffee's effects on human health.
"The consensus around coffee has evolved over time from being concerned about increased risks of cancer and cardiovascular disease in the 1970s and '80s, to being quite confident of no overall increase in risk of noncommunicable diseases, and a good likelihood of benefit for cardiometabolic risk," he said.
As Willet and colleagues noted in a recent review paper in the New England Journal of Medicine, habitual coffee consumption (around 3 to 5 cups a day) has now been consistently linked to the prevention of several chronic conditions, acting on various organ systems as both a potent delivery system for caffeine and a dietary cornerstone with plenty of uniquely advantageous properties of its own.
Here's a look at some of the more convincing data of coffee's proposed benefits.
The last decade has seen convincing data from several large-scale population analyses that drinking coffee reduces all-cause mortality. This association has been reported by investigators in Japan, the United Kingdom, and the United States, and is apparent even after stratifying for ethnicity.
A 2019 meta-analysis found that coffee consumption's mortality reducing effects occurred even after accounting for risk factors such as smoking and alcohol use, age, and weight status. Benefits are observed in those consuming up to 8 cups of coffee a day, regardless of whether it was caffeinated or decaffeinated, instant or ground, or consumed by people with genetic polymorphisms coding for slower or faster caffeine metabolism.
Every additional cup of coffee consumed daily may contribute to as much as a 7% reduction in excess risk of diabetes. Conversely, decreasing consumption over 4 years was linked to a higher risk of type 2 diabetes. Coffee's protective effects may be strongest when consumed at lunchtime.
A variety of biological pathways have been proposed to account for coffee's purported antidiabetic effects. Both caffeinated and decaffeinated coffee appear to attenuate hepatic insulin resistance and offer protection against deterioration of glucose tolerance. Additional analyses have also linked coffee consumption with improvements in adipocyte and liver function, reductions in fat mass, and advantageous modulations of lipid metabolism.
Benefits may also be derived from coffee's unique compositional properties, said Marilyn Cornelis, PhD, associate professor of preventive medicine in the division of nutrition at Northwestern University Feinberg School of Medicine, whose research has focused on the role genetics plays in coffee consumption.
"What's unique to coffee are the polyphenols that it contains, more specifically chlorogenic acid," Cornelis told Medscape. "Coffee is actually the major contributor of these particular polyphenols to the diet. That polyphenol has been linked to some benefits on glucose control that might possibly underlie the positive associations we see with coffee and type 2 diabetes."
Cardiovascular and Cerebrovascular Considerations
Coffee drinkers are reported to be significantly less likely to develop cardiovascular disease, with the most benefit shown in those who regularly consume 3 to 5 cups a day. Meta-analyses of prospective studies have linked its consumption to decreased risk of coronary disease, heart failure, and atrial fibrillation.
Regularly drinking coffee doesn't appear to impact blood pressure, and may actually lead to a modest reduction in hypertension risk. Chlorogenic acid may be a factor in this as well, as it leads to significant decreases in systolic and diastolic blood pressure.
Coffee consumption may reduce oxidation of low-density lipoprotein (LDL) cholesterol, inflammatory markers, and levels of protein markers linked to cardiovascular disease, such as leptin and chitinase-3 like-protein-1.
Whether coffee consumption meaningfully reduces the risk of stroke is somewhat less definitive. A 2014 meta-analysis reported that the association was weak, whereas a recent meta-analysis drawing from 2.4 million participants found a 21% reduction in stroke risk among those consuming 3 to 4 cups of coffee a day.
Coffee's impact on cholesterol appears highly dependent on what form you take it in. Unfiltered coffees (eg, French press) are particularly high in cafestol, which has been called "the most potent cholesterol-elevating compound known in the human diet." In a 2020 study, Cornelis and colleagues showed that while unfiltered espresso formulations resulted in higher LDL levels, this effect wasn't observed among those drinking standard ground or instant coffee.
Coffee's potential to limit rates of several cancers has gained considerable attention in the literature. A 2020 umbrella review of 28 meta-analyses on this topic indicated that the strongest associations were with endometrial cancer and liver cancer. Willett cited a 2017 study from Bravi and colleagues, which found the risk of hepatocellular carcinoma significantly decreased when coffee consumption went up, as what he considered "the most important relatively new evidence" on the health benefits of coffee. Although endometrial cancer rates appear to decrease with both caffeinated and decaffeinated coffee, only the former appears to have this effect in hepatocellular carcinoma.
More moderate inverse associations have also been reported between coffee consumption and the risk of developing estrogen-receptor negative and postmenopausal breast cancer, melanoma, basal cell carcinoma, cancer of the oral cavity and pharynx, colorectal cancer, and both lethal and localized prostate cancer.
Neurological and Psychiatric Effects
In the first wide-ranging study of chronic coffee consumption's effects on human brain networks, investigators reported that habitual coffee drinkers exhibited functional changes indicative of enhanced concentration and improved motor control and alertness, as compared with non-coffee drinkers. Other results have not found a link between coffee and improved cognition.
Coffee's mood-boosting properties may have a role in alleviating mental health disorders. Both low and high levels of coffee consumption yield comparably substantial reductions in depression rates of approximately 30%. In one notable study, risk of suicide decreased by around 45% in those drinking 2 to 3 and 4 or more cups of coffee a day, compared with drinking 1 or fewer cups per week.
Coffee consumption has been proposed to offer protective benefits against a variety of neurodegenerative conditions. Coffee drinkers with mild cognitive impairment were shown to have an associated decreased risk or delayed onset of dementia. Coffee consumption has also consistently been shown to reduce the risk of Parkinson's disease, and may have a role in preventing Lewy body formation.
Yet such observed effects may be primarily due to coffee's role as a delivery system of caffeine, which has known neuroprotective properties and impacts short-term mood by altering serotonin and dopamine activity. Notably, decaffeinated coffee hasn't shown protective value in Parkinson's disease and suicide risk, or as an enhancer of cognitive performance.
A 2017 umbrella review of meta-analyses investigating coffee's impact on multiple health outcomes concluded that the highest-magnitude of positive associations related to liver outcomes.
Increasing consumption by 2 cups daily greatly reduces rates of alcoholic cirrhosis and related mortality. Coffee drinkers not only significantly decrease their risk of developing nonalcoholic fatty liver disease (NAFLD), but they also are better protected against liver fibrosis if they already have NAFLD. It also appears to slow the progression of liver disease in patients with advanced hepatitis C.
There are a variety of potential mechanisms underlying these effects. Coffee is a conduit for caffeine, which stimulates tissue remodeling by acting as an adenosine receptor antagonist, thereby potentially preventing hepatic fibrosis, and also limits the ability of collagen to deposit itself in the liver.
The consensus that coffee can improve multiple areas of human health has mounted alongside the arrival of higher-quality data. However, it will take considerably more work to determine which of the more than 1000 known compounds contained in coffee are to be credited with these effects.
"We need to keep in mind that coffee is a seed, and a key function of a seed is to protect the living plant embryo in the germ," said Willet. "Thus, seeds in general have many components that have antioxidant and anti-inflammatory effects, and coffee seeds are unusually high in polyphenols and other antioxidants. These may be responsible for the apparent benefits, but we are not yet sure."
And coffee consumption isn't without risks. Caffeinated coffee can potentially increase glaucoma risk, elevate blood pressure and heart rate in those unaccustomed to drinking it, impair sleep quality, and induce feelings of anxiety, dysphoria, and — in rare cases — psychotic or manic symptoms. Caffeine intake should be decreased or eliminated during pregnancy, given its link to lower birth weight and pregnancy loss. And it's important to remember that all coffee is not created equal. A heavily sugared confection from your local chain store is not the same as a hot cup of filtered black coffee.
Coffee's health effects are also unclear in certain populations, said Cornelis.
"We really don't know the impact that caffeine has on people that already have adverse health, or on the developing brains of children."
Most of us, however, can continue to enjoy our coffee breaks without concern.
"For patients who raise concern about their coffee consumption, I think physicians can convey reassurance to those who enjoy this beverage," said Willett.
Those cups of joe may be doing a whole lot more than simply getting you through the daily grind.
John Watson is a freelance writer in Philadelphia, Pennsylvania.
Medscape Medical News © 2021
Cite this: John Watson. More Than Just a Hill of Beans: The Health Effects of Coffee - Medscape - Jul 12, 2021.