Why It's So Hard to Determine If Food and Drinks Cause Cancer

Good news, coffee addicts, your daily cup of joe doesn't cause cancer, according to the World Health Organization. Or, well, sometimes it might, but overall it doesn't. In fact it can prevent some negative health effects. We think. Probably.

On Wednesday, the WHO's cancer research arm—the International Agency for Research on Cancer—announced it had taken coffee off its list of items considered possibly carcinogenic to humans, where it was classified since 1991. However, the group said it was adding "hot beverages," defined as 65 degrees Celsius (149 Fahrenheit) or hotter, to the list as probably carcinogenic due to a link to esophageal cancer. One former Starbucks barista claimed the company's acceptable range for beverages was 145-165 degrees F, and even the coffee machine at my office clocked in at 160 F when I tested it Wednesday. So should we all switch to iced coffees?

"There are these kinds of back and forths on nutrition information, because nutrition research is really hard to do and it's hard to do well," said Timothy Caulfield, a Canada Research Chair in Health Law and Policy and a Professor in the Faculty of Law and the School of Public Health at the University of Alberta, and an author of several books on nutrition.

It seems that studies on coffee—like wine, chocolate, and so many of our favorite vices—are published every other day conflicting one another. Why is it so hard to get conclusive research when it comes to nutrition?

"Humans are incredibly complicated organisms that live in incredibly complicated environments and we make lots of different decisions throughout the day related to our health and some of them we're not even aware of," Caulfield told me. "So in order to isolate the effect or impact of one compound, or one food, it can be really difficult. There are so many variables at play."

Caulfield said part of the problem is that the bulk of nutritional studies are observational, rather than randomized controlled trials—considered the gold standard in clinical study. The results of observational studies are correlative, but can't necessarily show causation. He told me this doesn't mean observational studies are useless, and said comparing a large body of studies that all point to a similar result is a good measure of a substance's effect, but it does make it tricky to draw conclusions from any individual study.

A nutritional randomized controlled trial would be very difficult, and very expensive, because you'd need to control exactly what people are eating and ideally without them realizing, for a long period of time. This is much harder than testing, say, a new medication, because people eat anyway, but they usually don't take new pills. There's also a risk of bias in nutritional studies due to funding from food companies, and these studies tend to get overrepresented in the media because, unlike a new treatment for a specific disease, nutrition news affects all of us—everybody eats.

The good news is Caulfield said we ought not to be too worried about the hot beverage listing.

"A lot of these risk increases are only detectable when looking at large populations. Individual risk is usually not huge," Caulfield said. "What really matters is living a healthy lifestyle. For most people the details don't matter, it's the big picture: lots of fruits and vegetables, avoiding junk food, getting exercise, not smoking. That's what people should focus on."