Does Green Tea Aid in Cancer Prevention?
Page 1 of 3
A review of the published scientific literature indicates that green tea has a strong protective effect against cancer and other diseases. One study, however, failed to demonstrate that green tea drinkers have a lower risk of stomach cancer. The media used this study to attack the value of green tea. In this article, we discuss the one negative report and point to numerous positive studies that document green tea’s anti-cancer properties. We also reveal findings showing that stomach cancer is a preventable disease.
By Ivy Greenwell
The anti-cancer benefits of green tea against a variety of cancers appeared to be well-established. However, a Japanese study published in the March 1, 2001 issue of the New England Journal of Medicine casts doubt upon the efficacy of green tea in lowering the risk of stomach cancer. Because this was a prospective epidemiological study using a fairly large sample over an eight-year period, its findings and conclusions deserve to be seriously examined. After an adjustment for sex and age (male sex and older age are risk factors), history of peptic ulcer (a rough indication of Helicobacter pylori infection), smoking status, alcohol consumption and certain dietary components, persons who drank less than one cup of green tea a day were found to have approximately the same risk of gastric cancer as those who drank more than five cups a day (Tsubono 2001). The media publicized these negative findings, making some consumers wonder whether green tea offered any protection against any kind of cancer.
However, it is unwise to draw firm conclusions on the basis of only one study, especially if several other studies disagree. Before we review studies that found a protective effect of green tea against gastric cancer, let’s take a look at the various risk factors for this type of cancer.
Gastric cancer risk factors
Although the rates of gastritis and stomach cancer have declined in this century, stomach cancer is the second most common cancer in the world, after lung cancer. It remains the most common cause of cancer death in Japan and Korea. Stomach cancer is also the most common cancer in China, among both men and women.
Interestingly, the geographic distribution of stomach cancer shows that it is very rare in the Western world, with rates of less than 1 per 100,000, as opposed to Eastern Europe, China, Korea and Japan, with rates of over 40 per 100,000 in some areas. What is even more revealing is the fact that stomach cancer used to be a lot more common in both the Western world and Japan, but there has been a dramatic decline in its incidence that appears to parallel the introduction of refrigeration. Even though gastric cancer is still the leading cause of cancer-related mortality in Japan, between 1970 and 1995 its mortality rate declined from 88.9 to 45.4 per 100,000 among Japanese men, and from 46.5 to 18.5 per 100,000 among Japanese women (note the wide gender difference).
A great deal of epidemiological evidence points to certain components of a traditional diet that appear to be causally involved in carcinogenesis: very salty foods, smoked foods and pickled foods. Many Japanese enjoy salty treats such as salt-cured dikon with every meal. Contrary to the sentimental view, not all traditional ethnic foods are good for health. The pre-refrigeration practice of preserving foods by smoking and/or heavily salting them has a great potential for harm. A diet high in sodium and smoked and pickled foods, especially if combined with heavy smoking and drinking, has been repeatedly associated with increased risk of stomach cancer. High sodium intake alone is estimated to increase the risk of stomach cancer up to six times (Lam 1999). Thus, reducing salt intake is the cornerstone of gastric cancer prevention.
There is a parallel in the incidence of stroke and gastric cancer: both decline as the intake of sodium goes down. As one reviewer put it, “High salt intake is associated with high blood pressure and with stomach cancer, especially with inadequate intake of potassium from fruits and vegetables, and of calcium from certain vegetables and low-fat dairy products” (Weisburger 2000). Inadequate intake of potassium generally goes hand in hand with inadequate intake of antioxidants.
Food preserved with nitrites is also under suspicion as a carcinogen that could lead to stomach cancer. Nitrites can be converted to nitrosamines, known to be carcinogenic. In some rural regions, drinking water is contaminated with nitrogen compounds due to infiltration of nitrogen fertilizers. A Spanish study showed that gastric cancer mortality correlated with increasing exposure to nitrates (under some conditions, nitrates may be converted to nitrites).
Stomach cancer also shows a strong male prevalence, which may be connected to estrogens being protective in women. Phytoestrogens, including the catechins found in green tea, very likely play a similar protective role. Soy also contains powerful phytoestrogens; the confounding factor here is that many Asian soy products are heavily salted. A couple of studies hint, however, that nonsalted soy products (e.g. plain tofu) help protect against gastric cancer. In addition, genistein and daidzein have been found to inhibit the growth of stomach cancer cells in vitro. Another phytoestrogen, kaempherol, has also been found to provide protection. Various other phenolic compounds, including anthocyanins (found in berries and dark grapes), have also been shown to be protective.
Yet another culprit is infection with Helicobacter pylori, often associated with gastric and duodenal ulcers (the infection is found in 80% of ulcer cases). H. pylori infection leads to an inflammation of the stomach lining, and can ultimately result in chronic gastritis (a precancerous stomach lesion), which may then progress to gastric cancer. H. pylori infection is seen significantly more frequently among gastric cancer patients than in healthy controls (Konturek 2000). According to most studies, H. pylori doubles or triples the risk of gastric cancer; one author estimates that H. pylori increases the risk of gastric cancer five times if the infection continues for 10 years (Lam 1999). Likewise, the presence of H. pylori infection was found to be associated with ten-times the risk of chronic atrophic gastritis. It is the moderately atrophied gastric mucosa that are most likely to become sites of malignancy. It is important to note that green tea has been found to be protective against chronic atrophic gastritis (Kuwahara 2000; Setiawan 2001).
In addition, H. pylori causes reduced bioavailability of vitamin C, and produces a cytotoxic protein known as CagA, which recruits immune cells that further destroy the stomach lining. While H. pylori infection is common also in the Western world, it appears to be kept at lower severity due to better nutrition, especially higher antioxidant status. Antioxidant deficiencies are suspected to favor the acquisition of H. pylori infection and increase its severity. Treatment with antioxidants, on the other hand, appears quite effective in reducing the bacterial load and the degree of gastric inflammation. Green tea polyphenols, especially epigallocatechin gallate, are known to be potent antioxidants. While Western populations may not be drinking much green tea, they obtain antioxidants from year-round fruit and vegetables, wine, coffee, black tea (also an excellent source of polyphenols, mainly the more complex type known as theaflavins), chocolate and nutritional supplements.
It should be pointed out that an infection with H. pylori, quite common around the world, including the Western world, does not mean that a person will go on to develop gastric cancer. It is estimated that half of the world population is infected to some degree with H. pylori. In Western countries, H. pylori infection is more likely to produce chronic early-stage gastritis or ulcers rather than gastric cancer. A diet low in sodium and rich in antioxidants appears to play a major role in arresting the infection at an early stage and limiting the damage it can do. Thus, even without eradicating H. pylori with antibiotics, it should be relatively easy to keep the infection from leading to precancerous chronic gastritis and cancer. A recent Chinese study put a special emphasis on smoking and vitamin C deficiency, in addition to H. pylori, as factors leading to progression from gastritis to gastric cancer. The risk of progression to cancer was only 20% in individuals in the highest tertile of baseline vitamin C level compared with the lowest tertile (You 2000).