Depending on whom you ask, coffee is either the next super food or the next public health hazard. Consult the latest news headlines or scientific studies, and you’ll find that coffee “fights heart disease,” reduces the risk of breast cancer, colon cancer, and multiple sclerosis, and “could keep Alzheimer’s at bay.” One recent study by researchers at the National Institute of Health and Yale University found that people who drank four or more cups of coffee a day were a whopping 20 percent less likely to develop melanoma, a type of skin cancer, than people who didn’t drink coffee.
But dig around a little, and you will learn that coffee could also raise your risk of contracting various illnesses. A 2010 meta-analysis of 13 studies, for example, found that drinking two cups of coffee a day increased a person’s risk of lung cancer by 14 percent. More recently, a 2014 meta-analysis suggested a link between coffee consumption and cancer of the larynx.
Why the conflicting results? A partial answer may be that some components of coffee are beneficial while others are damaging. For example, coffee contains antioxidants, which help protect cells against damage by unstable molecules, called free radicals, that have been linked to cancer. At the same time, some researchers speculate that drinking hot coffee may increase the risk of esophageal cancer because it can burn the esophagus.
Most of the confusion, though, is likely due to so-called confounding variables—extraneous factors, such as diet or genes, that can influence a study’s outcome. The more variables there are, the more difficult it is to deduce cause-and-effect relationships with much confidence.
Take coffee. First, people brew it in all sorts of ways—dripped, pressed, percolated, cold-brewed, pressurized in a vacuum pot, or, simply stirred, “cowboy style.” Your brew method of choice may change the way the beverage acts on your body. Preparing coffee in a drip filter, for example, strips away some of the lipids (fats) in the beans, including cafestol and kahweol. These compounds can increase your levels of low-density lipoprotein (LDL), also known as “bad cholesterol.” For this reason, drip coffee may have a more positive impact on heart health than, say, coffee made in a French press.
Lifestyle factors can also confound study results. Marilyn Cornelis, an assistant professor of preventative medicine at Northwestern University, points out that heavy coffee drinkers are often also heavy smokers and alcohol imbibers, and that they tend to get stressed out—all habits that can negatively impact health independent of coffee consumption. Even seemingly innocuous routines, such as adding cream or sugar to your coffee, can muddy an investigation. Although most recent studies use statistical tools to control for these variables, many older analyses did not.
And let’s not forget about genes, which regulate how people metabolize caffeine and other molecules in a cup of joe. Unfortunately, there aren’t many studies investigating the role of DNA in the health effects of coffee, primarily because scientists discovered some of the genes associated with caffeine metabolism only within the past decade. Combining this information with environmental factors could go a long way toward reconciling some of the discrepancies between study outcomes, Cornelis wrote in a 2015 review.
In the meantime, if you’re conflicted about refilling your mug, the current consensus is this: don’t fret. Most public health experts agree that the benefits of coffee outweigh the risks. One of the strongest pieces of evidence for this comes from a 2014 meta-analysis of cancer, heart disease, and mortality studies from 1996 to 2013, which concluded that long-term coffee consumption is, overall, either benign or beneficial. So go ahead, fellow coffee lover, and take another sip, guilt-free.
Jess Scanlon is a freelance science writer based in central New Jersey.