Credit Fred R. Conrad for The New York Times
An influential panel of experts convened by the World Health Organization concluded on Wednesday that regularly drinking coffee could protect against at least two types of cancer, a decision that followed decades of research pointing to the beverage’s many health benefits. The panel also said there was a lack of evidence that it might cause other types of cancer.
The announcement marked a rare reversal for the panel, which had previously described coffee as “possibly carcinogenic” in 1991 and linked it to bladder cancer. But since then a large body of research has portrayed coffee as a surprising elixir, finding lower rates of heart disease, Type 2 diabetes, neurological disorders and several cancers in those who drink it regularly.
In their report, the scientists did identify one surprising risk for coffee and tea drinkers. They said that drinking “very hot” beverages was “probably carcinogenic,” because the practice was linked to esophageal cancer in some studies.
Much of the evidence for coffee’s health benefits stems from large epidemiological studies, which cannot prove cause and effect. But the favorable findings on coffee consumption have been so consistent across numerous studies in recent years that many health authorities have endorsed it as part of a healthy diet.
Last year, a panel of scientists that shaped the federal government’s 2015 dietary guidelines said there was “strong evidence” that three to five cups of coffee daily was not harmful, and that “moderate” consumption might reduce chronic disease. Another group, the World Cancer Research Fund International, reported that coffee protects against multiple types of cancer. And several systematic reviews of studies involving millions of people have found that regular coffee drinkers live longer than others.
In its report, published Wednesday in Lancet Oncology, the World Health Organization’s International Agency for Research on Cancer said it had assembled a team of 23 international scientists who reviewed more than 1,000 studies. The agency said the evidence showed that drinking coffee was unlikely to cause several types of cancer, including breast, prostate and pancreatic cancers, and that it was associated with a lower risk of uterine and liver cancers. For 20 other types of cancer there was “inadequate” evidence of a link to cancer, said Dana Loomis, the deputy head of the agency’s program that classifies carcinogens and the first author of the report.
Decades ago, the group listed coffee as a “possible carcinogen” – along with lead and diesel fuel – because of studies that suggested a weak link to bladder and pancreatic cancer. But those early studies did not adequately account for higher smoking rates among coffee drinkers and, since then, more rigorous and better-quality studies have become available, Dr. Loomis said. “There is less of a concern today than there was before,” he added.
In its report, the group cited evidence, for example, that coffee drinkers’ risk of liver cancer decreases 15 percent “for each one cup per day increment.” Still, the group did not give coffee a ringing endorsement. It placed coffee in its Group 3 category for things with “inadequate” evidence of carcinogenic potential, such as fluoride, low frequency electric fields, and toluene, a solvent used to make nail polish.
Geoffrey Kabat, a cancer epidemiologist at the Albert Einstein College of Medicine in New York, said he felt that the agency did not go far enough in its report. He said that coffee had been studied in populations across the globe and that studies now show a clear lack of evidence of harm. He said the agency tends to give greater weight to studies showing harm, even when they are outnumbered by many more showing benefit.
“What the evidence shows over all is that coffee drinking is associated with either reduced risk of several cancers or certainly no clear increase in other cancers,” he said. “There’s a strong signal that this is probably not something that we need to be worrying about.”
Since 1971, the cancer research agency has evaluated nearly 1,000 environmental and lifestyle factors to determine the extent to which they can promote cancer. About 120 have been categorized as carcinogenic to humans, including asbestos, cigarettes and plutonium. Other things it has classified as carcinogenic include wood dust, salted fish and processed meats such as bacon, ham and hot dogs.
But the agency’s reports have at times been controversial. Critics have called the agency’s ranking system arcane and confusing because it classifies things according to the strength of the overall research, not their actual level of danger. Dr. Kabat said the assessment system was useful for cancer researchers but that it served no utility for the public.
“I really feel that it’s not contributing to the public good because it’s stoking these concerns,” he said.
Still, the news on coffee is likely to be welcomed by many Americans – about 130 million of whom drink coffee every day. Around the world, more than 1.6 billion cups of coffee are consumed daily, making it one of the world’s most popular drinks behind tea.
The agency said it was not clear why coffee seems to protect against at least two types of cancers. But it noted that drinking coffee produced “strong antioxidant effects” in clinical trials, and that it promoted the death of cancer cells in laboratory studies.
The report’s concerns about “very hot” beverages included mention of mate, a type of tea traditionally consumed in South America, the Middle East and some parts of Europe, often at high temperatures. The agency said that regular consumption of beverages hotter than 149 degrees Fahrenheit was “probably carcinogenic” based on a small number of studies showing a link between the practice and esophageal cancer.
One reason is that, over time, scalding hot beverages may injure cells that line the throat, setting the stage for rare cancers. But Dr. Loomis cautioned that the increased risk was seen in people who regularly drink their tea or mate at very high temperatures, typically at 158 degrees Fahrenheit or greater.
That is very different from the average tea or coffee drinker in the United States and Europe, where coffee is generally consumed at temperatures below 140 degrees Fahrenheit. It is also an issue that can be easily avoided.
“Those who are concerned should just wait a few minutes more before drinking their drink,” Dr. Loomis said.