by Craig Weatherby
Last week, three significant new tea-and-health studies were tossed over our digital transom, so we’ve summarized their findings here.
Brief #1: Green Tea May Boost Elders’ Brains
Widely hailed as a helpful health habit, a daily cup or two of tea is thought to offer added protection against cardiovascular disease, obesity, and certain cancers.
But while animal studies have shown potential for tea to protect the brain, it hasn’t been studied for its brain-preserving potential in people.
In the first study of its kind, Japanese researchers reported that green tea may reduce the risk of mental decline among the aged (Kuriyama S, 2006), with the lowest risk of mental decline being associated with drinking at least two cups of green tea per day.
The study involved 1,000 participants aged 70 or older, and its results showed a dose-response effect, which bolsters the idea that something in tea protects brain functions as we age:
- Compared with those who drank green tea three times a week or less, those who reported drinking two cups a day were 54 percent less likely to show age-related declines in memory, orientation, ability to follow commands, and attention.
- Declines in brain function were 38 percent less likely among those who drank a cup of green tea four to six times a week, compared with participants who reported drinking green tea fewer than three times a week.
The likely brain-protective agents in green tea are its catechin-class polyphenol antioxidants, which demonstrate beneficial brain-function effects in animal tests.
To account for distortions caused by likely confounding factors, the researchers adjusted for the deleterious brain effects of diabetes, smoking, and advanced age, as well as for brain-boosting health habits such as greater physical activity, social ties, fish and vegetable consumption, and overall health.
As with all such population studies, it is possible that something else about higher green tea consumption might have helped the brain.
However, given the positive results of animal studies—which reveal how tea exerts positive effects on brain function—it seems likely that tea is responsible for the brain-preserving effects recorded among Japanese elders.
Brief #2: Black Tea May Cut Risk of Ovarian Cancer
Green tea gets most of the cancer-prevention attention, and for good reason, since it is higher than black tea in the catechin-class antioxidants—particularly one called EGCG—shown to exert the strongest anti-cancer effects in lab tests.
While black tea contains much lower levels of catechins, it is abundant in other beneficial antioxidants created when the fermentation process that turns green tea black converts catechins to related compounds. This probably explains why black tea appears to offer substantial protection against cardiovascular disease.
Now, the results of a long-term study, published late last year, suggest that black tea may reduce women’s risk of ovarian cancer (Larsson SC, 2005).
The researchers followed a cohort of 61,057 Swedish women aged 40 to 76 years, whose dietary patterns were captured in food-consumption questionnaires.
Most of the tea drinkers in the study consumed black tea—the kind favored in Europe, Oceania, Africa, and South Asia—rather than green tea, which is more popular in East Asian countries. In fact, black tea accounts for about 78 percent of tea consumption, worldwide. (This week's recipe section includes one for Indian Railway Tea, with Darjeeling black tea and spices.)
At the end of the 15-year study period, those who drank at least two cups of tea per day had the lowest rates of ovarian cancer, but even one cup a day decreased the risk by 24 percent.
The tea-drinkers tended to have healthier lifestyles, so adjustments were made for factors influencing risk of ovarian cancer, such as weight, age, past pregnancies and consumption of calories, fruits, vegetables and alcohol.
It is interesting to note that the researchers found no link between coffee consumption and the risk of ovarian cancer, despite coffee’s very considerable antioxidant content.
The new findings are supported by those of an earlier Chinese case-control study (Zhang M, 2002), which compared 254 women with ovarian cancer to a group of 652 healthy women. In that investigation, daily tea drinkers were 61 percent less likely to develop ovarian cancer, and the cancer risk was 77 percent lower among those who had sipped tea for more than thirty years.
The Chinese researchers adjusted for possible confounding demographic, lifestyle, and familial factors, including hormonal status and family ovarian cancer, age, weight status, past pregnancies, exercise and alcohol consumption. (They did not adjust for dietary differences between tea drinkers and non-tea drinkers, because most of the participants ate similar, relatively anti-cancer diets.)
And, women in the study with ovarian cancer who drank at least one cup of green tea a day were more than twice as likely to survive, compared with non-tea drinkers.
Laboratory studies suggest that tea could protect against ovarian cancer, as well as some other cancers, by inhibiting both the initiation of cancer and the growth and spread of cancer cells, and by increasing the rates at which cancer cells commit pre-programmed “suicide” (apoptosis).
The Chinese team could come to no firm conclusion concerning the potential protective effects of black or oolong tea, because so few of the 254 women with ovarian cancer in the study drank those types, compared with green tea.
Untangling Study Terms: Clinical, Cohort, Case-Control, or Population?
Here’s a pocket primer on the types of studies scientists can conduct, listed in descending order of reliability:
Controlled Clinical Trial: Subjects are divided into an “active” group, who receive the drug or nutrient being tested for its therapeutic/preventive effects, and a “control” group, who receive an identical-looking/tasting placebo (inactive) drug or nutrient. The most reliable controlled trials are “double-blind”, which means that neither the researchers nor the subjects know which group is getting the treatment being tested.
Prospective Cohort Study: Researchers follow a group of participants over time, recording data on rates of the disease being studied and possible disease factors (diet, lifestyle, etc.)
Case-Control Study: Researchers collect data on a group of patients and look for unusual diet or other lifestyle patterns, in comparison with a group of healthy but otherwise similar people.
Population (Epidemiological) Study: Researchers collect data on a large population group, looking for correlations between rates of a given disease and diet or other lifestyle factors.
Brief #3: New Analysis Counters FDA on Green Tea’s Breast-Cancer Benefits
Late last year, the FDA approved a breast-cancer-prevention claim for green tea so negative that many wondered why the agency issued label language few tea makers would want to put on their products.
One problem facing the FDA was that the existing evidence comes mostly from population and experimental studies, rather than controlled clinical trials, which provide more reliable results.
But the authors of a new meta-analysis—that is, a study analyzing existing studies—looked at the same evidence the FDA considered, and came to a different, more positive conclusion.
The new meta-analysis, conducted by researchers at the University of Minnesota Cancer Center (Sun CL, 2005), examined 13 studies from eight countries, each of which provided data on consumption of green tea, black tea, or both in relation to breast cancer risk.
The investigations examined in the new meta-analysis included three cohort studies and one population-based study for green tea, plus five cohort and eight case-control studies regarding black tea and breast cancer (see sidebar titled “Untangling Study Types”).
The Minnesota team found that women who drank the most green tea enjoyed a 20 per cent reduction in risk of breast cancer.
However, no significant protective effect was found among black tea drinkers. All of the case-control studies showed a small decreased risk, but one cohort study indicated a small increased risk.
The prevention gap seen between green and black tea in this study probably stems from the fact that, compared with black tea, green tea is about ten times higher in the catechin-class antioxidants known to promote programmed cancer-cell death (apoptosis).
Unfortunately, it is unlikely that this new meta-analysis will change the FDA’s weak-to-negative cancer-prevention claim for green tea, because, as the University of Minnesota team noted, “…the number of published studies on this topic is too small for the results to be conclusive.”
The University of Minnesota analysts called for longer term studies, to clarify the differing degrees of protection offered by green and black tea.
Tea and breast cancer prevention: a possible genetic distinction
The potential of green tea to protect against breast cancer may depend in part on a woman’s personal genetic profile. Variations in the gene controlling a chemical called angiotensin-converting enzyme (ACE) appear to affect the development of breast cancer, and survival rates.
Women possessing the “low activity” form of the ACE gene appear to have only half the risk of breast cancer, compared with women who have the “high activity” ACE genotype, which results in increased production of cell-damaging free radicals.
ACE-induced free radicals promote cancer development and growth, so some of green tea’s apparent protective effect may stem from the ability of its catechin-class polyphenols to neutralize ACE-induced free radicals.
Accordingly, studies in China show that women with the more dangerous “high activity” ACE genotype get more protection from green tea, compared with women with the safer, “low activity” ACE genotype.
- Kuriyama S, Hozawa A, Ohmori K, Shimazu T, Matsui T, Ebihara S, Awata S, Nagatomi R, Arai H, Tsuji I. Green tea consumption and cognitive function: a cross-sectional study from the Tsurugaya Project 1. Am J Clin Nutr. 2006 Feb;83(2):355-61.
- Larsson SC, Wolk A. Tea consumption and ovarian cancer risk in a population-based cohort. Arch Intern Med. 2005 Dec 12-26;165(22):2683-6.
- Goldbohm RA, Hertog MG, Brants HA, van Poppel G, van den Brandt PA. Consumption of black tea and cancer risk: a prospective cohort study. J Natl Cancer Inst. 1996 Jan 17;88(2):93-100.
- Zheng W, Doyle TJ, Kushi LH, Sellers TA, Hong CP, Folsom AR. Tea consumption and cancer incidence in a prospective cohort study of postmenopausal women. Am J Epidemiol. 1996 Jul 15;144(2):175-82.
- Michels KB, Holmberg L, Bergkvist L, Wolk A. Coffee, tea, and caffeine consumption and breast cancer incidence in a cohort of Swedish women. Ann Epidemiol. 2002 Jan;12(1):21-6.
- Larsson SC, Holmberg L, Wolk A. Fruit and vegetable consumption in relation to ovarian cancer incidence: the Swedish Mammography Cohort. Br J Cancer. 2004 Jun 1;90(11):2167-70.
- Zhang M, Binns CW, Lee AH. Tea consumption and ovarian cancer risk: a case-control study in China. Cancer Epidemiol Biomarkers Prev. 2002 Aug;11(8):713-8.
- Sun CL, Yuan JM, Koh WP, Yu MC. Green tea, black tea and breast cancer risk: a meta-analysis of epidemiological studies. Carcinogenesis. 2005 Nov 25; [Epub ahead of print]
- Yuan JM, Koh WP, Sun CL, Lee HP, Yu MC. Green tea intake, ACE gene polymorphism and breast cancer risk among Chinese women in Singapore. Carcinogenesis. 2005 Aug;26(8):1389-94. Epub 2005 Mar 31.
- Koh WP, Yuan JM, Van Den Berg D, Lee HP, Yu MC. Polymorphisms in angiotensin II type 1 receptor and angiotensin I-converting enzyme genes and breast cancer risk among Chinese women in Singapore. Carcinogenesis. 2005 Feb;26(2):459-64. Epub 2004 Oct 21.
- De Bacquer D, Clays E, Delanghe J, De Backer G. Epidemiological evidence for an association between habitual tea consumption and markers of chronic inflammation. Atherosclerosis. 2006 Jan 25; [Epub ahead of print]