The Health Effects of Coffee – A Comprehensive Summary

Fitting its status as the world's most consumed beverage after water, coffee has long attracted medical researchers' attention.

According to Dr. Walter C. Willett, professor of epidemiology and nutrition at Harvard Medical School, there have been many early attempts to determine what all this coffee does to health, but study design flaws have prevented meaningful, convincing conclusions.

Willett says, "Publishing a paper about coffee is almost a rite of passage for epidemiologists," adding, "Importantly, study quality has improved greatly over time, partly because almost all are now prospective, much larger and have longer follow-up."

This has led to a significant shift in how we think about coffee's effects on human health.

Debate around coffee has continued to evolve since the 1970-80s, when cancer and cardiovascular disease became more common. Cancer and cardiovascular diseases are noncommunicable diseases occupying the top two places among all-cause deaths and affecting tens of millions worldwide each year. Coffee appears to have a complex relationship with both disease groups, although the view that it is beneficial predominates.

Scientific Research on Coffee's Health Effects

In the world's leading scientific medical journal, New England Journal of Medicine(NEJM), Willet and colleagues published in July 2020 the paper “Coffee, Caffeine, and Health”, which states that coffee consumption reaching 3-5 cups daily is now a cornerstone of our diet. As a result, our organ systems are virtually bathed in caffeine, which has many health benefits and risks. Ultimately, this level of coffee consumption is thought to be linked to preventing many chronic diseases.

Let us examine the more convincing data on coffee's proposed benefits:

Coffee and the Risk of All-Cause Death

Over the last decade, several large population analyses have provided convincing data that coffee drinking reduces all-cause mortality. Researchers in Japan, the United Kingdom and the United States have reported this association, and the benefit remains apparent even after stratification by ethnicity.

A meta-analysis published in 2019 found that coffee's mortality-reducing effects occurred even after accounting for risk factors such as smoking, alcohol use, age and weight status. Benefits are observed among people consuming up to 8 cups a day, whether caffeinated or decaffeinated, instant or filtered/ground, and among people with genetic polymorphisms encoding slower or faster caffeine metabolism.

Diabetes and Coffee

Each additional 1 cup of coffee consumed daily may contribute to diabetes risk reduction of up to 7%. Conversely, reducing consumption over 4 years was linked to a higher risk of type 2 diabetes. Coffee's protective effects may be strongest when consumed at lunch.

Various biological pathways have been proposed to explain coffee's possible antidiabetic effects. Both caffeinated and decaffeinated coffee appear to reduce hepatic (liver) insulin resistance and protect against impaired glucose tolerance. Further analyses have linked coffee consumption with improved fat-cell and liver function and reduced fat mass.

What is distinctive about coffee is its polyphenols, particularly chlorogenic acid. Coffee is actually the largest dietary source of these particular polyphenols. This polyphenol is linked to benefits in glucose control that may underlie the positive associations observed between coffee and type 2 diabetes.

Cardiovascular and Cerebrovascular Diseases and Coffee

Coffee drinkers are reported to have a significantly lower likelihood of developing cardiovascular disease, with the greatest benefit observed among those regularly consuming 3 to 5 cups of coffee daily. Meta-analyses of prospective studies have linked coffee consumption to lower risks of coronary disease, heart failure and atrial fibrillation.

Regular coffee drinking does not appear to affect blood pressure and may actually lead to a modest reduction in hypertension risk. Chlorogenic acid may also be involved, as it leads to significant reductions in systolic and diastolic blood pressure.

Coffee consumption may reduce oxidation of low-density lipoprotein (LDL, bad cholesterol), inflammatory markers and protein markers linked to cardiovascular disease, such as leptin and chitinase-3-like protein-1.

Whether coffee consumption significantly reduces stroke risk is less certain. A meta-analysis conducted in 2014 reported a weak association, whereas a more recent meta-analysis of 2.4 million participants found a 21% reduction in stroke risk among those consuming 3 to 4 cups of coffee daily.

Coffee's effect on cholesterol appears to depend largely on the form consumed. The unfiltered coffees, such as espresso and Turkish coffee, are particularly high in what has been called "the most powerful cholesterol-raising compound known in the human diet": kafetol. In a 2020 study, Cornelis and colleagues showed that although unfiltered espresso preparations resulted in higher LDL levels, this effect was not seen among people drinking standard ground or instant coffee.

Cancer and Coffee

Coffee's potential to reduce the risk of various cancers has attracted considerable attention in the literature. On this topic, a 2020 umbrella review of 28 meta-analyses showed that the strongest associations were with uterine and liver cancer. Willett called a 2017 study by Bravi and colleagues, which found substantially lower hepatocellular carcinoma (liver cancer) risk as coffee consumption increased, "the most important relatively new evidence" of coffee's health benefits. Although endometrial (uterine) cancer rates appear to decline with both caffeinated and decaffeinated coffee, only the former appears to have this effect in hepatocellular carcinoma.

Moderate links between coffee consumption and reduced risk of various cancers have been reported for: estrogen receptor-negative and postmenopausal breast cancer, melanoma, basal cell skin cancer, oral cavity and pharyngeal cancer, colorectal cancer and prostate cancer.

Neurological and Psychiatric Effects of Coffee

The first large-scale study on this topic reported that habitual coffee consumers showed better concentration, motor control and alertness than nonhabitual drinkers. Earlier studies found no link between coffee and improved cognitive function.

Coffee's mood-enhancing properties may play a role in alleviating mental health disorders. Both low and high consumption levels provide relatively substantial reductions of approximately 30% in depression rates. In one notable study, compared with people drinking 1 or fewer cups per week, those consuming 2 to 3 and 4 or more cups daily had an approximately 45% lower suicide risk.

Coffee consumption has been proposed to protect against various neurodegenerative conditions. In people with mild cognitive impairment, coffee drinking has been associated with lower dementia risk or delayed onset. Sustained consumption has been shown to reduce Parkinson's disease risk and may help prevent Lewy bodies.

These observed effects may nevertheless arise mainly from caffeine, which has known neuroprotective properties and affects short-term mood by altering serotonin and dopamine activity. Notably, decaffeinated coffee showed no protective value for Parkinson's disease or suicide risk, nor a cognitive performance benefit.

Liver Disease and Coffee

A 2017 umbrella review of meta-analyses investigating coffee's effects across health outcomes concluded that the strongest positive associations involved liver outcomes.

Increasing consumption by 2 cups daily greatly reduces alcoholic cirrhosis and related mortality. Coffee drinkers not only reduce their risk of nonalcoholic fatty liver disease substantially, but people with that disease are also better protected against any liver fibrosis. Coffee also appears to slow liver disease progression in patients with advanced hepatitis C.

Potential mechanisms behind these effects appear to include the following: Coffee is a major source of caffeine, which acts as an adenosine receptor antagonist stimulating tissue remodeling, potentially preventing hepatic fibrosis and limiting collagen's ability to accumulate in the liver.

COVID-19 and Coffee

In the United Kingdom, in May 2021, Nutrients published a study finding 10% lower COVID-19 risk in people drinking 1 or more cups daily compared with nondrinkers. Coffee also reduces pneumonia risk in older people. Although not a prospective study, this is a positive result worth investigating.

Why Coffee?

Consensus that coffee may improve many aspects of human health has grown as higher-quality data have emerged. However, identifying which of coffee's more than 1000 known compounds are associated with these effects will require much more work.

Willet said, "We should remember that coffee is a seed, and a seed's basic function is to protect the living plant embryo from microbes", adding, "Seeds therefore generally have many components with antioxidant and anti-inflammatory effects, and coffee seeds are unusually rich in polyphenols and other antioxidants. These may explain the apparent benefits, but we are not yet sure."

Coffee Consumption Is Not Risk-Free

Coffee consumption is not risk-free. Caffeinated coffee may increase glaucoma risk, raise blood pressure and heart rate in people unaccustomed to it, impair sleep quality, and cause anxiety, dysphoria and, rarely, psychotic or manic symptoms. Given its association with low birth weight and pregnancy loss, caffeine intake should be reduced or eliminated during pregnancy. It is also important to remember that coffees are not all created equal. A very sugary confection from a local chain store is not the same as a hot cup of filtered black coffee.

We also do not really know caffeine's effects on people already in poor health or on children's developing brains.

Most of us, however, can continue enjoying our coffee breaks without worry.

Willett said, "For patients voicing concerns about coffee consumption, I think doctors can reassure those who enjoy this beverage".

A cup of coffee may do much more than relieve the day's pressures and wake you up.