Wednesday March 18 2009
The results do not "prove that eating mushrooms will stop cancer"
“Eating mushrooms daily 'may cut breast cancer risk by two-thirds'” reported The Daily Telegraph. It said a study in over 2,000 Chinese women found that women who ate a third of an ounce or more of fresh mushrooms every day reduced their risk of breast cancer by 64%. It also found that women who ate fresh mushrooms and drank green tea reduced their risk by 90%. According to the newspaper, the researchers say that their results “do not prove eating mushrooms will stop cancer, and more studies are needed to confirm the results”.
The cautious conclusions of the researchers are sensible, as this study has a number of limitations and cannot prove that mushrooms and green tea alone were responsible for the effects on breast cancer rates. In addition, these findings may not apply to women with different ethnic backgrounds and nationalities, and it is worth pointing out that Chinese women are reported to have a lower risk of cancer than women in some western countries. It is unlikely that a single ‘super-food’ which prevents cancer will be discovered. Women should aim to have a healthy balanced diet, of which mushrooms can be a part.
Where did the story come from?
Dr Min Zhang and colleagues from the University of Western Australia and Zhejiang University in China carried out this research. The work was funded by the National Health and Medical Research Council in Australia. The study was published in the peer-reviewed International Journal of Cancer.
What kind of scientific study was this?
This case-control study looked for differences in diet between women who had breast cancer and those who did not. The researchers were particularly interested in the effects of mushrooms and green tea consumption, as they have both been reported to have anticancer effects. Previous research found that mushroom extract inhibited the activity of the aromatase protein in breast cancer cells grown in the laboratory, which works similarly to the breast cancer drugs known as aromatase inhibitors.
The researchers studied women in the city of Hangzhou in southeastern China between 2004 and 2005. They included 1,009 women aged between 20 and 87 years with breast cancer or in situ carcinoma, from four teaching hospitals (cases). These women had been newly diagnosed with invasive ductal carcinoma (a cancer in the cells lining the milk ducts that has escaped into the breast tissue) or in situ carcinoma of the breast (abnormal cancer cells that stay within the milk ducts). Women whose cancer started elsewhere in the body and then spread to the breast were not included in the study. The researchers managed to identify and include 98.8% of eligible women with breast cancer in their study.
To act as controls, the researchers also enrolled 1,009 women from the same hospitals who had attended the breast clinic for routine preventive care. These women did not have breast cancer, benign breast disease, or any other cancer. Each control was matched by age to one of the cases, with a maximum five year age difference.
The participants were told the general aims of the study, and given a face-to-face interview. The interview collected information about the women’s demographic characteristics, lifestyle, use of vitamin and mineral supplements, and hormonal status (including menstrual history, menopausal status, reproductive history, oral contraceptive use and family history of breast cancer).
The women’s dietary intake was assessed using a standard food frequency questionnaire (FFQ) which asked questions about 100 food items, including fresh and dry mushrooms, and green tea. The type of mushrooms the women ate were mainly fresh white button mushroom (Agaricus bisporus) and dry fragrant mushrooms (Lentinula edodes). The participants rated how often they ate a particular food item based on nine categories, from never or hardly ever, to three or more times a day. They were also asked how much of the item they ate per meal, and how it was cooked. The participants were also asked about their habitual diet, with particular reference to the previous year. If participants recently changed their diet, then only information for the period before this change was used in the study. Total energy intake for each participant was estimated based on the FFQ and Chinese food composition tables.
The researchers then compared the consumption of mushrooms between cases and controls, while taking into account factors that might affect the results (confounders), such as age, area of residence, body mass index (BMI) five years before diagnosis (for the cases) or interview (for the controls), education, age at first period, use of oral contraceptives or HRT, family history of breast cancer, menopausal status, total energy intake, and lifestyle factors (smoking, passive smoking, alcohol use, physical activity, tea consumption). The researchers also looked at the effect of combined mushroom and green tea intake.
What were the results of the study?
The majority of cases in the study had invasive ductal carcinoma (97%). On average, the cases (women with breast cancer) had fewer years of education, and were less likely to have used oral contraceptives or HRT than the controls (women without breast cancer). Cases also had higher BMIs in the past (five years ago) and higher total energy intake. They were also more likely than controls to have had their first period before the age of 13, to be exposed to passive smoking, and to have a mother or sister with breast cancer.
Compared to controls, the cases had a lower average intake of fresh mushrooms, but had the same average dried mushroom intake. When the researchers compared women with different levels of mushroom intake, and adjusted for possible confounders, they found that the more mushrooms a woman ate, the less likely she was to have breast cancer.
Women who reported eating 10g or more of fresh mushrooms a day were 64% less likely to have breast cancer than women who reported eating no fresh mushrooms. Women who reported eating 4g or more of dried mushrooms a day were 47% less likely to have breast cancer than women who reported eating no dried mushrooms. Similar results were found if pre- and post-menopausal women were analysed separately.
Drinking green tea was also associated with a reduced risk of breast cancer, with the effect increasing with the amount of green tea drunk. Women who drank high levels of green tea (1.05g or more of green tea leaves a day) and ate high levels of fresh mushrooms (7g a day or more) were 89% less likely to have breast cancer than women who did not drink green tea or eat fresh mushrooms. Similar results were found for dried mushrooms.
What interpretations did the researchers draw from these results?
The researchers concluded that “higher dietary intake of mushrooms decreased breast cancer risk in pre- and postmenopausal Chinese women, and an additional decreased risk of breast cancer from joint effect of mushrooms and green tea was observed”. They say that further research to investigate this link is needed.
What does the NHS Knowledge Service make of this study?
There are a number of factors to consider when interpreting this study:
- In this type of study, it is possible that the differences between cases and controls in the factors of interest (mushroom and green tea consumption) are not the factors contributing to the outcome of interest (breast cancer). The researchers took into account (adjusted for) other factors that could have affected the results (confounders), but there may still be residual confounding factors. In particular, the analyses did not adjust for other types of food intake. People who eat a lot of mushrooms may also eat a lot of other vegetables and less meat, and these other differences in diet could be contributing to the different rates of breast cancer.
- In this type of study, the way in which the controls are selected is very important. The controls should be as similar as possible to the general population from which the cases are drawn. The selection of controls from women attending hospital for routine preventive care may mean that these women were more careful about their health than the general population, which may include women who did not attend routine screening. This could mean that differences between the case and control women arose because of the way in which the controls were selected.
- The study assessed food intake retrospectively, and this may result in inaccurate reporting, which could affect results. This would particularly be a problem if women with breast cancer recall their food intake differently to control women. For example, women who had breast cancer might feel that a poor diet may have contributed to their cancer, and might therefore be likely to report their diet as less healthy than it was. The authors considered this possibility, but thought it unlikely that the women would perceive a link between their mushroom consumption and breast cancer.
- The results from this Chinese study are reportedly supported by case-control studies looking at the effect of mushroom consumption in Korean women. However, these results may not apply to women from different countries and with non-Asian ethnic backgrounds.
The results of this study will need confirmation from other studies, preferably where food intake is assessed in a prospective fashion. Mushrooms can form a part of a healthy balanced diet, and such a diet can improve many aspects of health.