A new round of dietary do’s and don’ts accompanied last month’s scientific report on the latest food research, summarizing everything from aspartame to saturated fats. The report puts eggs back on the menu. High dietary cholesterol is no longer linked to blood cholesterol in most healthy people. But what grabbed the headlines? Coffee, of course.
Many of us are happy to raise a mug to our legal stimulant of choice, especially with the report’s suggestion that three to five cups of joe get a pass. But where do these numbers come from? What science do nutrition experts take into account to determine whether coffee is harmful or safe? And — perhaps the most important question — what does “three to five cups” really mean?
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Science, not policy
The good news for coffee comes from the 2015 Dietary Guidelines Advisory Committee, a group of experts in nutrition and health appointed by the Department of Health and Human Services and the U.S. Department of Agriculture to review the science behind what Americans should eat. The report, released February 19, is not the be-all-end-all of what should be on our plates and in our cups. Instead, it’s a scientific report intended to help the HHS and USDA make policy decisions for the next edition of the Dietary Guidelines for Americans, due out later this year.
This is the first time the U.S. Dietary Guidelines have addressed coffee at all. But now, there is enough science on coffee to make a closer look worthwhile, says Tom Brenna, a food scientist at Cornell University and a member of the Committee. “There was so much evidence out there,” he says. “Instead of just five or six papers on the subject, there’s a huge number.”
And coffee is one of the most widely consumed beverages in the United States and it’s the primary contributor to caffeine consumption, notes Frank Hu, a member of the Committee who studies nutrition and epidemiology at Harvard University. He’s also concerned about whether we might be drinking a little too much caffeine. “There’s been a lot of media attention to overconsumption of caffeine, especially among young adults,” he notes.
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After weighing the recent studies, the committee offered the following guidance on coffee: “Strong evidence shows that consumption of coffee within the moderate range (3 to 5 cups per day or up to 400 milligrams per day caffeine) is not associated with increased long-term health risks among healthy individuals.” They also noted that really, most of us aren’t pushing that 400-milligram limit. Instead, healthy adult men and women average around 200 milligrams of caffeine per day. Pregnant women, the report notes, should probably stick below 200 milligrams per day.
The scientific brew
The committee pooled a large number of scientific reports, from studies of how much caffeine Americans consume by age group to studies evaluating coffee and its association with cancer, type 2 diabetes, cardiovascular disease, Parkinson’s disease and even Alzheimer’s disease.
The gold standard for medical studies is the randomized controlled trial, in which participants are randomly assigned to receive one of two interventions without knowing what group they are in. This works well for short-term studies of caffeine, but is problematic in studies that run many months or years. You can’t really randomly assign people to a caffeinated or caffeine-free lifestyle, and forget trying to do so without the participants catching on, Brenna notes.
So the committee considered short-term studies, where people come into a laboratory, receive a single or a few doses of caffeine and are then examined for everything from blood pressure to brain function. The group also looked at cohort studies, in which researchers follow groups of people who do or do not drink coffee over many years and track any differences in certain health outcomes. “The advantage is that you can look directly in humans and over long periods of time,” Brenna explains. “You can get associations.” But, he notes, you can’t really prove with a cohort study that coffee or caffeine causes a specific effect.
Many of the studies the committee ended up focusing on were meta-analyses. These are not original research. Instead, they combine the results of different studies — in this case, several clinical trials and cohort studies on coffee and caffeine. For a meta-analysis, researchers comb the literature for all the studies assessing, say, the link between coffee drinking and heart attacks. They then select the studies that have similar populations of people who consume similar doses of coffee, pool the results and try to determine if, overall, coffee increases or decreases the risk of heart attack.
By combining many studies together, a series of small studies with 50, 100 and 150 participants becomes a study with 300. “It’s a way of doing a larger study with many participants that might not otherwise be feasible,” says Shahla Wunderlich, who studies food production at Montclair State University in Montclair, N.J.
But Wunderlich notes that meta-analyses should be interpreted very cautiously. “Each of these studies can be very different, there may be a different population size, and so on,” she explains. “Combining all these studies and coming up with a conclusion may not be meaningful. It’s not the same as a single experimental study.” While the people in the studies might be of similar age, weight, education and health, they are probably not in the same location or even in the same country. It’s very difficult to conduct truly rigorous meta-analyses with proper statistics. And although the committee’s methods for selection were extremely careful, it’s still important to make conclusions carefully.
And of course, different participants, even within the same study, are probably not consuming the same coffee. “The caffeine content in coffee can range dramatically,” says Elizabeth Bertone-Johnson, an epidemiologist at the University of Massachusetts at Amherst. “It means we’re not as precise as we’d like to be.”
It’s also important to note that most of the science, and most of the meta-analyses, are on coffee, not caffeine. “Among adults, coffee is the lion’s share of caffeine intake,” says Bertone-Johnson. “Coffee is also complicated. It contains more than 1,000 different chemicals.” She notes that some studies try to tease out the effects of caffeine alone by comparing regular coffee to decaf. But there are very few long-term studies looking at caffeine alone, without the other components of soda, coffee or tea. The high levels of caffeine in energy drinks are another matter entirely, and there just wasn’t enough data for the committee to weigh in on caffeine itself.
The dose makes the poison
In the end, the committee’s assessment of the coffee literature was that there is “strong” evidence that three to five cups of coffee per day, or less than 400 milligrams of caffeine, does not increase the risk of cardiovascular disease or cancer. They also note that the same dose of coffee or caffeine might slightly decrease the risk of type 2 diabetes, cardiovascular disease and Parkinson’s disease, but this got a “moderate,” not a “strong” ranking. Evidence for Alzheimer’s disease risk and cognitive decline was too limited to draw conclusions.
As for the high caffeine intake in young people from sodas and energy drinks? The drinks are too new, and more research is needed before experts can give evidenced-based guidance. But in the meantime, the committee settled on limited to no consumption of energy drinks for youth until their health effects are better understood. Put down the energy drinks, kids.
“They are fairly straightforward recommendations in terms of the quality of the evidence,” says Bertone-Johnson. “It’s reassuring; the science is really showing there’s no adverse health consequences.”
But the dose makes the poison. And what is the dose? Three to five “cups” of coffee could mean different things to different people. The committee defined a cup as eight ounces containing roughly 100 milligrams of caffeine. But most small coffees at coffee shops start at 12 ounces. They go up from there. A 20-ounce Pike Place from Starbucks clocks in at 415 milligrams.
Of course, most people probably don’t look up the caffeine content online before they order their coffee, let alone know the caffeine content of their home-brewed cup of joe. “I’m concerned the public is going to interpret [the report] badly,” Wunderlich says. “Giving people a blank check about coffee is concerning because it’s a stimulant that may affect people differently.”
The committee wants to be sure that coffee doesn’t get that blank check. “The results don’t suggest that if you don’t drink coffee you should,” notes Alice Lichtenstein, a nutritional biochemist at Tufts University in Medford, Mass., and a member of the committee. “It’s really indicating that you drink moderate amounts of coffee there are not adverse outcomes. It’s an important distinction.”
What sweetens up that moderate-sized cup is also an important point. The group had a lot to say about added sugar and fat, and extended those cautions all the way to the three pumps of vanilla syrup and the whole milk in your latte. “I don’t think it hurts to add a little cream and sugar,” Hu says. “But I think people should pay attention to large amounts of added calories when they go to a coffee shop.”
It’s nice to get that extra boost of confidence that at least our coffee habits are (probably, most likely, generally speaking, based on a lot of meta-analyses) not going to kill us. And it’s far more optimistic news than the gloomy fingers pointing at our sugary treats. “It was a pleasant surprise,” Brenna notes. “It’s nice not to be delivering bad news all the time.” So coffee drinkers, rejoice. At least, as long as you’re drinking less than 40 ounces of coffee, or 400 milligrams per day. And if you don’t already, you might want to try learning to like it black.