"Eating a Mediterranean diet 'cuts deadly breast cancer risk by 40%' in postmenopausal women," says the Mail Online of a widely reported study carried out by researchers in the Netherlands.
The researchers looked at data from a study involving more than 60,000 women aged 55-69 over a 20-year period.
At the start of the study, details of the women's diet, physical activity and other cancer-related risk factors were collected.
The researchers then compared the diets of more than 2,000 women who went on to develop breast cancer with a selected group of similar women who didn't develop the cancer.
Overall, there was no link between a Mediterranean diet and breast cancer risk.
However, the researchers found women whose diet was most like a Mediterranean diet were 40% less likely to develop one particular type of breast cancer: oestrogen receptor-positive breast cancer.
As with all studies of this type, it's difficult to separate out the effects of diet and other lifestyle factors, such as exercise and smoking. This makes it difficult to be certain that the differences in risk are the result of the Mediterranean diet alone.
The researchers tried to take into account other factors that could affect the risk of breast cancer, but it's difficult to identify all possible contributing factors.
The Mediterranean diet has been linked to many health benefits. Sticking to a healthy, balanced diet with plenty of fruit and vegetables, wholegrains and some fish, along with a low intake of red meat and sugary foods, is in line with current government recommendations for healthy eating, as set out in the Eatwell Guide.
Where did the story come from?
The study was carried out by researchers from Maastricht University Medical Centre in the Netherlands.
Funding was provided by the Wereld Kanker Onderzoek Fonds Nederland, as part of the World Cancer Research Fund International grant programme.
This study has been reported in a number of media sources, who presented the main findings of this story reasonably. But most failed to make it clear in their headline that the link was only found with one type of breast cancer.
What kind of research was this?
This study analysed data from women who had participated in the Netherlands Cohort Study to investigate the link between a Mediterranean diet and a reduced risk of postmenopausal breast cancer (ER-negative breast cancer).
The analysis was effectively a nested case control study, where women in the cohort who developed breast cancer were compared with a selected group of controls from the cohort who didn't develop breast cancer.
As well as looking at all types of breast cancer together, the researchers looked at different types of breast cancer separately.
Around 70% of breast cancers are oestrogen receptor positive (ER positive). This means there are a significant number of oestrogen receptors in the breast cancer tissue. This type of breast cancer can respond well to hormonal treatments.
If oestrogen receptors aren't present in large numbers, it's known as oestrogen receptor-negative (ER negative) breast cancer.
The Mediterranean diet has been linked to better health for a number of years. It's thought the diet may reduce the risk of cancer because of its high fibre and antioxidant content, and because it helps maintain a healthy body weight.
This type of study is good for identifying possible links between lifestyle factors and disease. But the main limitation is that groups of people who have different diets may also differ in other ways that affect their risk of disease, and it's difficult to separate out the effects of all of the contributing factors.
Researchers will often use various statistical techniques to try to take potential confounding factors into account, but it's difficult to be sure this has been completely successful.
What did the research involve?
The researchers used data from women participating in the Netherlands Cohort Study (NLCS).
At the start of the study, the women completed a questionnaire about their cancer risk factors.
The questionnaire collected data on the following:
- dietary intake
- smoking habits
- physical activity
- body measurements
Dietary information was collected using a 150-item semi-quantitative food frequency questionnaire for the year prior to joining the study. This was checked using a nine-day diet record.
This dietary data was used to calculate how close the women's diets were to the typical Mediterranean diet pattern.
Although drinking a moderate amount of alcohol is a normal part of a Mediterranean diet, the researchers didn't consider alcohol to be part of the Mediterranean diet in their analysis because alcohol consumption is a risk factor for breast cancer.
The researchers identified the women who developed cancer by looking at the Netherlands Cancer registry records and the nationwide Dutch Pathology Registry (PALGA).
Once the researchers identified the women who developed cancer during the study, they compared their diet with that of a randomly selected group of women from the cohort who had no history of cancer (except some cases of skin cancer) at the start of the study, and whose diet information was complete.
What were the basic results?
A total of 62,573 women aged 55-69 were followed for an average of 20.3 years (1986-2007). During the follow-up period, 3,354 women developed breast cancer.
Women whose diets were more like the Mediterranean diet were generally more physically active, educated to a higher level, and more likely to have taken oral contraceptives at some stage.
Those whose diets were less like the Mediterranean diet tended to be older, were less likely to have had any children, and more likely to be current smokers and have a family history of breast cancer (in some analyses).
No significant association was seen between sticking to a Mediterranean diet and overall risk of breast cancer or ER-positive breast cancer.
Women whose diets most closely resembled a Mediterranean diet were 40% less likely to develop ER-negative breast cancer than women whose diet was least like a Mediterranean diet (hazard ratio [HR] 0.60,95% confidence interval [CI] 0.39 to 0.93).
How did the researchers interpret the results?
The researchers concluded that, "Our findings support an inverse association between [Mediterranean diet] adherence and, particularly, [oestrogen] receptor-negative breast cancer.
"This may have important implications for prevention because of the poorer prognosis of these breast cancer subtypes."
This study aimed to assess whether sticking to a Mediterranean diet was associated with a reduction in breast cancer risk for postmenopausal women.
The researchers found following a Mediterranean diet was indeed associated with a reduction in breast cancer risk – but only for ER-negative breast cancer.
This study has both strengths and weaknesses. Its large, prospective design and long period of follow-up are strengths.
The typical weakness of this type of study is that many factors are likely to contribute to risk, and it's very difficult to be sure the factor in question – in this case, eating a Mediterranean diet – is wholly responsible for the differences seen.
The researchers did take other factors into account in their analysis, but it's possible that the effects of unknown or unmeasured factors remain.
The researchers also note other possible limitations, including:
- They didn't know the ER status of all of the breast cancer cases, so they had to exclude some from their analysis.
- It's possible there's an element of inaccuracy in the responses from the food frequency questionnaires, as is often the case when participants are asked to recall information.
- The women may have altered their diet or physical habits during the study period, meaning information collected at the start of the study no longer accurately reflected their lifestyles, and this was not accounted for.
Although this study has limitations, the Mediterranean diet has been linked to many health benefits, including a healthier heart.
A Mediterranean diet is similar to the government's healthy eating advice set out in the Eatwell Guide, which also involves eating plenty of fruit and vegetables, and not too much red meat or sugary foods.