Teas from across the globe are becoming more and more popular in the U.S. One relative newcomer, yerba mate, is attracting fans for its allegedly jitter-free caffeine boost and high antioxidant content.
Lab research suggests some potential health benefits from drinking yerba mate, but studies of lifelong yerba mate drinkers in the tea's native South America suggest the brew increases the risk of some cancers -- a fact most marketing campaigns omit.
Yerba mate, leaves of the Ilex paraguariensis tree, is traditionally brewed and served in a dried-out gourd and sipped through a metal straw with a filter on one end to stop drinkers from ending up with a mouthful of leaves.
In the U.S., the tea is rarely served in gourds (although a few upscale tea lounges preserve the tradition). A small but growing number of companies, however, sell the loose or bagged tea leaves, and some are now offering cold bottled blends of the tea.
Ads, Web chatter and positive press promote yerba mate's clean buzz -- a caffeine high without the shakes and "crash" that sometimes follows. If that is true, it could be because yerba mate contains about 80 milligrams of caffeine per cup -- almost twice the amount in black tea but less than half that of coffee, which usually contains 100 to more than 200 milligrams per cup.
But scientists disagree about the bottom line on yerba mate, says K. Simon Yeung, clinical coordinator and research pharmacist at Memorial Sloan-Kettering Cancer Center in New York City. The tea does contain a long list of vitamins, minerals and antioxidants, including B vitamins and vitamin C; manganese, potassium and zinc; and the beneficial plant compounds quercetin, theobromine and theophylline.
Because of yerba mate's high antioxidant content, comparisons to green tea are common (one commercial yerba mate blend claims to contain 90% more antioxidants than green tea). But that is slightly misleading, says Elvira de Mejia, associate professor of food science and human nutrition at the University of Illinois at Urbana-Champaign.
The two teas have very different arrays of antioxidants: Green tea is rich in epigallocatechin gallate compounds, while yerba mate's main antioxidant is chlorogenic acid. Studies have suggested that both sets of plant compounds have the potential to reduce risk for heart disease and cancer, but the research on both is far from conclusive.
Lab tests have suggested a host of benefits from yerba mate. In test tubes, the herb and its components reduce oxidative stress on heart and liver cells, protect DNA from damage in yeast cells and kill human liver cancer cells. In lab rats, the tea improves the flow of blood through blood vessels and reduces fat accumulation.
But Yeung, who also manages Memorial Sloan-Kettering Cancer Center's "About Herbs, Botanicals & Other Products" website, cautions against drawing conclusions from such studies. For one thing, he says, "high antioxidant content doesn't always translate into a health benefit. . . . We can't rely on consumption of antioxidants as a safe way to prevent cancer" or other diseases.
He adds that effects seen in lab dishes and animals rarely hold up in human clinical trials. In fact, human studies so far have sounded a note of caution for avid mate drinkers. A 2003 review of all existing studies on yerba mate, published in the journal Head and Neck, confirmed that people who regularly drink large amounts of the tea -- as much as a liter or more each day -- had significantly increased risk of cancers of the esophagus, lungs, mouth, pharynx and larynx.
One study of roughly 1,000 adults in Uruguay, published in the journal Cancer Epidemiology Biomarkers & Prevention in 1996, found the risk of lung cancer to be 60% higher among mate drinkers. Another Uruguayan study, published in the same journal in 2003, found that in a group of about 800 adults, mate drinking tripled the risk of esophageal cancer.
A study published in the journal Epidemiology in 1994 found that drinking mate regularly increased a person's risk of respiratory or digestive cancers by 60% -- leading the authors to conclude that heavy mate consumption could be responsible for as many as 1 in 5 cases of such cancers in southern South America.
De Mejia warns against taking such studies as conclusive. She says that not all the studies were well designed and did not rule out the possibility that contaminants introduced during processing -- not yerba mate -- were behind the apparent increase in cancer risk.
Yeung says that the amount of mate consumed also appears to plays a role. "In South America, people tend to consume large amounts," he said. "The more people drink, the higher the risk of cancer. It's a dose-related response."
In the U.S., he advises, "people don't need to stop drinking it, but they do need to know something about it . . . and drink in moderation."
Given yerba mate's smoky, bitter, woody flavor -- which many have called an acquired taste -- that may not be difficult advice for most American consumers to follow.