Is coffee bad for your heart? The answer is complicated. We examine media stories on the issue, comparing the information to evidence from the actual medical trials to determine whether we can make a case for coffee’s health risks.
Heart Disease: The Leading Cause of Death
If there’s a health problem the media ought to be covering accurately, it’s cardiovascular disease. This is the leading cause of mortality in the United States and worldwide, accounting for about one in four deaths.
That’s ahead of all forms of cancer, combined. About 610,000 Americans die of heart disease each year, or about one every 50 seconds.
Accurate and timely health reporting on the issues surrounding cardiovascular health—what we can do to identify heart and blood vessel diseases early, or to prevent or reverse them—should be a crucial part of keeping all of us healthy.
How Heart Disease Affects You
When we talk about cardiovascular health, we’re referring to diseases that affect the heart and the blood vessels.
Many of these conditions have at their core a process called atherosclerosis. That’s when plaque forms on the inside of blood vessels, blocking flow.
When atherosclerosis affects the coronary vessels—that means the blood vessels that supply blood and oxygen to the heart muscle itself—this is called coronary artery disease. The end result of blocked flow can be a myocardial infarction, or a heart attack.
When atherosclerosis affects the blood vessels that lead to the brain, a stroke can occur—that’s more formally called a “cerebrovascular accident,” or a CVA. There are also milder degrees of illness related to atherosclerosis, like the chest pain with exertion called angina pectoris, or a kind of limb pain called claudication.
This is a transcript from the video series The Skeptic’s Guide to Health, Medicine, and the Media. Watch it now, on The Great Courses Plus.
Chronic congestive heart failure is often related to atherosclerotic disease, as are some kinds of dementia or memory loss. When you put all of these problems together, you can see that cardiovascular disease is at the root of some of our biggest health problems.
We lump these together because the risk factors, the things that increase a person’s risk for heart attacks and stroke and all of the other kinds of cardiovascular disease, overlap. They’re basically the same things, which makes sense, because all of these conditions are usually caused by atherosclerosis.
Prevent atherosclerosis, and you’ve prevented the most common killers in our population. It would be nice to have agreement on the best ways to prevent or reverse cardiovascular disease.
Learn more about cardiovascular health
Coffee and Heart Health
A seemingly straightforward question to start with: Is coffee good or bad for your heart?
The answer is complicated; the best answer has changed over the last 20 years of study. Even what is know now isn’t really based on rock-solid evidence.
From 2002, from WebMD.com, a headline read “Is Caffeine Bad for Your Heart?” with a subtitle “New research suggests caffeine elevates blood pressure, stress.”
First, special kudos goes to WebMD for putting an explicit message, right under that headline, that reads “from the WebMD archives,” and right under that, the date of publication of the article, August 1, 2002. In old-school newspaper lingo, that date printed at the top of a story is called the dateline, and it’s a traditional part of any newspaper article.
That publication date is crucial. Unfortunately, some sites leave it off, or include just a month and day with no year. With no dateline, old news often recirculates, and may mislead people, especially when we’re talking about health news, or news about a “new” research study, as suggested by the headline of this article.
The story itself is about a study that looked at blood pressure and other measurements in a small number of coffee drinkers—47 adults. Over three days, the study participants were given either a capsule of caffeine equivalent to about 4 cups of coffee or a capsule of placebo; on the other day, they got the other capsule.
When participants serve as their own controls, it is often called a crossover study. The researchers found that on the caffeine day, average blood pressures were higher by about three or four points, going from (just for example) 120/80 to 124/83.
At the same time, the average heart rate actually decreased by two beats per minute—a finding that was largely ignored in the press, but that reflects decreased stress on the heart. The article says, “The researchers concluded that the equivalent of four cups of coffee raises blood pressure for many hours. Although the increases appear modest, they are large enough to affect heart attack and stroke risk.”
What do you think? Are you convinced that coffee is bad for your heart?
Well, first, the study was quite small, involving all of 47 people. Small studies are not strong studies.
And it only documented one day of very modestly increased blood pressure. When we talk about the increased risk of heart disease related to high blood pressure, we’re talking about a risk that develops over many, many years.
One day of higher blood pressure honestly doesn’t matter. What could matter is years and years of high blood pressure, but that’s not what was measured in this study.
We don’t know that caffeine raises blood pressure on day two or seven or 100 of coffee drinking, or much less after 10 years of coffee drinking.
Heart Health or Blood Pressure: What’s Actually Being Measured?
But most important, this study looked at what’s called a surrogate marker rather than a real clinical endpoint. The headline of the story was “Is Caffeine Good for Your Heart?” but to be more honest, what the study was all about was a measurement called blood pressure.
They weren’t looking at heart damage or heart disease. Now, we do know that over time, a long period of hypertension, or an elevated blood pressure, is a risk factor for heart disease.
But they’re not one and the same. Not everyone with high blood pressure gets heart disease, and not everyone with heart disease has high blood pressure.
It may be an important observation that caffeine elevates blood pressure. But showing the rise in blood pressure is not the same as showing that caffeine raises the risk of heart disease or death, or anything that’s significantly important.
Always be wary of studies that look at a surrogate marker, like a lab measurement or a finding like blood pressure. Those measures might be important, but what you really want to know isn’t what a lab or vital sign shows, but what the effect of the intervention is on health itself.
This consideration is crucial, and headlines do not typically make this clear. If you’re asking, “Does coffee make your blood pressure go up?” that’s an interesting question, looking at a surrogate marker and risk factor for cardiovascular health.
But the question “Does coffee make you more likely to have a heart attack or die?” That’s much more important. And the effect of coffee on heart health is much more involved than just changes in blood pressure.
Learn more about understanding real clinical endpoints
Another Coffee Study Misses the Big Picture
Here’s another headline about coffee and heart health, again from WebMD, this one from 2004: “Coffee May Raise Heart Disease Risk.” The article is about a study done in Greece involving over 3,000 adults—that’s much bigger than the 2002 study of 47 adults, so that’s a good start.
The participants, none of whom had any history of heart disease, were asked about how many cups of what kinds of coffee they drank, and blood was drawn to test for what are called “markers of inflammation.” These are blood tests that show that some kind of inflammatory process is underway, and previous research had shown that these markers correlate with heart disease risk.
And they did find that the more caffeine consumed from coffee, the higher these markers turned out to be. The conclusion from the article was, “Drinking even moderate amounts of coffee may raise your risk of heart disease.”
The article and the headline didn’t make it clear, but this was another surrogate marker study. The study didn’t examine if the participants developed heart disease; it only measured these inflammatory markers.
Though the inflammatory markers indicate a risk factor, they’re only one of many risk factors. Maybe the coffee also helped the participants lose weight, which would be a marker for improved heart health.
Or maybe coffee prevented diabetes, also helping the heart. Or maybe coffee drinkers were less likely to overindulge in alcohol—and that would protect the heart, too.
Conversely, maybe coffee increases alcohol consumption. We just don’t know from this study.
The bottom line is that looking at one surrogate marker never really tells you the big picture and it doesn’t necessarily correlate with what you really want to know.
When you’re judging the strength of a study, keep an eye on the endpoint, or what, exactly, was measured. Stronger studies measure exactly what’s most important, not only surrogate markers.
There’s another problem with this study that wasn’t mentioned in the news article. It’s subtle, but it’s important: The participants were asked to remember what kind of coffee they consumed, and how much, and how often.
Did they remember correctly? Could it be that some over- or underestimated their consumption? That is pretty likely.
Studies that rely on subjects remembering what they’ve done are far less strong than studies that collect data as it happens, by having people fill out a daily diary, or even better, by using an electronic device to count how often they use their coffee machines.
None of these criticisms are meant to be aimed at the study designers and researchers. Doing a perfect study on many subjects over a long period of time to look at a clinically relevant endpoint is difficult and expensive.
But the news articles should highlight a study’s strengths and weaknesses. In this case, the headline and text of this article oversimplified what the study found and what that finding meant.
Though imperfect, the evidence so far was stacking up on the anti-coffee side. These studies of surrogate markers drove headlines and created a media impression that coffee of any kind was bad for your heart, at least, in 2004.