Alison Matsumoto, a 43-year-old graphic designer who lives in Los Angeles, drinks five cups of green tea every day. "I love the aroma of fresh green tea leaves. There's something magical about watching the dried leaves unfurl in my teapot and turn a vibrant green -- it's like they are coming alive." Alison also had an ulcer five years ago. "I used to live at the local Starbucks, until I got sick, that is. Now with green tea, I'm still getting some caffeine and am hopefully doing something to prevent stomach problems in the years to come."
This is not a new idea, but one that has long been a widespread belief in the Asian culture and is supported by a body of scientific research suggesting that increased consumption of green tea can lower the risk of stomach cancer.
However, a study published this month in the New England Journal of Medicine suggests otherwise. In this study, 26,311 people in Japan were given questionnaires asking about their consumption of green tea and other foods in 1984. Recently, the researchers recorded the causes of death for the participants to see if those who drank more green tea were less likely to die of stomach cancer.
The researchers found no association between green tea intake and the risk of stomach cancer. This surprising result has been reported widely by the media.
But hold on to your tea leaves -- this study has two major flaws which may explain the disappointing results.
One problem with the study is that the researchers did not obtain data on whether participants had a history of infection with Helicobacter pylori, a type of bacteria that is associated with a three- to six fold increase in the risk of stomach cancer. H. pylori infection is usually asymptomatic, although nearly all H. pylori-infected people have some sort of inflammation of the stomach. The result of this infection and inflammation is reduced acidity of stomach juices -- by the H. pylori bacteria that produces acid-neutralizing ammonia and also by the chronic inflammation that can eventually destroy the mucous lining and glands that secrete stomach acid. Decreased acidity of the stomach is associated with the formation of a greater number of cancer-causing compounds called nitrosamines in the stomach, presumably because it allows bacteria to grow in the stomach and convert dietary nitrates into these compounds.
The second problem with this study is that intake of compounds called nitrosamines were not considered as a group. Nitrosamines, which can be formed from dried, smoked, salted and preserved foods containing nitrates or introduced directly through cigarette smoke, are associated with stomach cancer. The conversion of nitrates to nitrosamines may be especially high in Japan, since inflammation of the stomach and H.pylori infection, two conditions that are very common in Japan, are known to increase this conversion. Participants who drank the most green tea also smoked more cigarettes and consumed more preserved foods, perhaps having a statistically greater intake of nitrosamines. However, nitrosamines were not considered as a group, nor was it clear whether the questionnaire included all possible sources of this group of compounds.
In addition, participants were not asked about their refrigeration and food-preservation practices. It is possible that the more green tea a person drinks, the more likely they are to have a traditional diet that includes salted, smoked and dried fish and seaweed; consumption of many preserved foods may then make them less cautious about the refrigeration of other foods. Eating partially decaying foods may be another source of nitrate-converting bacteria that can enter the stomach.
Although green tea should not be considered to be a dietary "magic bullet" for the prevention of stomach cancer, the results of a single flawed study should not be over-emphasized. Further studies that take into account nitrosamine intake, H. pylori infection, and food preservation practices are needed, since they may have countered the positive effect of green tea.
"What's worse?" asked Ms. Matsumoto, "spending a little extra money on a drink that I love? Or," she added, "finding out later on that it could have helped to prevent me from getting cancer?"
1. Fauci AS, Braunwald E, Isselbacher KJ, Kasper DL, Hauser SL, Longo DL. Harrison's Principles of Internal Medicine, Volumes I and II. McGraw Hill, 2001.
2. Reed PI, Smith PLR, Haines K, et al. Gastric juice N-nitrosamines in health and duodenal disease. Lancet 1981;8246:550-552.
3. Tsubono Y, Nishino Y, Komatsu S, Hsieh CC, Kanemura S, Tsuji I, Nakatsuka H, Fukao A, Satoh H, Hisamichi S. Green tea and the risk of gastric cancer in Japan. New England Journal of Medicine 2001;334:632-6.