Adherence to Mediterranean dietary quality index and risk of breast cancer in adult women: a case-control study

In this hospital-based case-control study, we found a significant inverse association between MedDQI scores and odds of BC among Iranian women. This inverse association was also seen in postmenopausal women after controlling for energy intake and age. No significant relation was found between MedDQI scores and BC in premenopausal women. This is the first study looking at the link between the MedDQI scores and the risk of BC in Iranian women. BC is the most common malignancy in women [1] Statistics demonstrate a significant increase in the incidence of BC over the past 25 years worldwide [24]. Diet plays a considerable role in the primary prevention of BC. In the present study, adherence to a diet with low MedDQI scores was inversely associated with odds of BC. The effects of a non-Mediterranean diet in the incidence of breast cancer is also well established [25]. Consumption of fresh fruits and vegetables increase the consumer polyphenols that help in fighting against tumorigenesis. In the other hand increased intake of fiber and carbohydrate may be associated with the breast cancer prognosis. Soybean proteins consumption are involved in breast cancer risk reduction and lowering the chances of breast cancer reoccurrence. Ethanol consumption exert their carcinogenic effect on breast tissues and mediate breast cancer development. Processed meat releases the carcinogens compound likes heterocyclic amines, which mediate the onset of breast cancer. High saturated fat diet increases receptor-positive cancer, particularly ER + risk of breast cancer [25]. This relationship was found in postmenopausal women after taking potential cofounders into account. Studies looking at the link between healthy eating habits and the BC have produced conflicting results [26‐29]. Similar to our research, several studies have found an inverse relationship between the risk of BC and good eating habits, or diets with low MedDQI scores [26]. Similar to our study, a recent meta-analysis revealed that following a Mediterranean-style diet may help lower the risk of breast cancer, albeit this link was not significant in premenopausal women [30]. Moreover, in an updated meta-analysis conducted by Morze et al. the highest adherence to MedDiet was inversely associated with cancer mortality and BC risk [31]. An increased adherence to the prudent or similar dietary patterns, those rich in fruit, vegetables, fish, whole grains, and low-fat dairy products, were strongly associated with a reduced risk of BC, according to a recent meta-analysis of 32 observational studies looking at the associations between various dietary patterns and odds of BC. These connections were similarly significant in premenopausal women, despite our study, though. Additionally, a Turati study revealed that following a Mediterranean diet was linked to a lower incidence of BC in both pre- and post-menopausal women [12]. The small sample size (n = 300) in this subgroup could be the reason why no correlation between MedDQI scores and risk of BC in premenopausal women was found. We did not look at the associations between BC subtypes. The relationship between following a diet with low Med-DQI scores and lowered risk of a specific subtype of BC has been discussed in several papers [12, 32, 33] In a prospective cohort study, a Mediterranean diet was associated with decreased risk of estrogen and progesterone receptor-negative (PR- ER-) BC [12]. According to another cohort study, following a Mediterranean diet reduced women’s likelihood of developing (ER-)-type BC [32]. A cohort study that involved 49,258 generally healthy women showed that adopting the Mediterranean diet did not significantly lower the risk of BC, in contrast to the current study [28]. Some research in this area has not discovered any conclusive links between healthy eating habits or other similar dietary patterns and the risk of BC [34‐36]. The lack of several confounders being controlled for in some research may account for this disparity. Additionally, various dietary evaluation methods and components used in different research may contribute to the contradictory results. Based on the case-control nature of this study, the patients might have reported their current dietary intakes rather than their usual diet. This would result in a biased association between the MedDQI scores and risk of BC compared to those reported in prospective cohort studies, in which the exposure has been measured prior to disease incidence. Second, we ascertained the MedDQI scores based on dietary intakes of both cases and controls, while cases might had changed their dietary intakes after disease manifestation. This would lead to a higher adherence to MedDQI scores in cases than controls and would eventually result in a biased relationship. Finally, no information about subtypes of BC is collected in this study. It has been indicated that diet with low MedDQI scores may have an impact on some subtypes of BC and no effect on others. The underlying mechanisms for the possible favorable of the diet with high MedDQI scores and odds of BC are not completely known. The MedDQI scores is a valuable tool to predict dietary quality and has been already verified using nutritional biomarkers [17]. This index was founded on the recommendations by the National Research Council (NRC) and American Heart Association (AHA) about the diet and health [37]. The intake of 30% or less of the daily total energy from fat, 10% or less of the total energy derived from saturated fat, 30 mg/d or less from cholesterol, 55% of energy from complex carbohydrates and 5 servings or more from fruits and vegetables have been taken into account by NRC and AHA. Fruit, vegetables and whole grains are good sources of antioxidants, dietary fiber and polyphenols, which can mediate the inverse association between the Dietary Approaches to Stop Hypertension (DASH) diet in relation to BC [38]. The consumption of olive oil, due to its monounsaturated fatty acids, have shown to be beneficial associated with some types of BC prevention and survival [39]. In spite of the fact that this study has demonstrated that dietary habits and lifestyle have an impact on the incidence of breast cancer, there is no doubt that hormones are the main cause of the disease. Efforts must also be made to encourage a healthy lifestyle, with special emphasis on the importance of eating a diet consisting primarily of fruits, vegetables, and whole grains with a low intake of red meat and saturated fats. To build the foundation for future advances in evidence-based public health efforts in this region, continued and expanded research on diet, lifestyle and breast cancer risk is urgently necessary.

Suitable sample size, careful assessment of confounding elements and their controlling in the analyses and being the first investigation in Middle East women could be considered as the strengths of the present study. However, some limitations should be considered. First, the case control design of the study does not permit us inferring causality. In addition, these kinds of studies are subject to selection and recall bias. In case-control studies, cases may report their past diet correctly due to their cancer diagnosis. This can attenuate this association. Another concern for case-control studies is that cases may have changed their diet before diagnosis due to early symptoms of the disease. To reduce this error, we recruited newly-diagnosed cases in the study. Furthermore, we used FFQ for assessing dietary intakes which can result in misclassification in our study participants. In addition, an energy adjusted MedDQI scores was applied which could reduce the possibility of subject misclassification. However, we used a qualified questionnaire to evaluate dietary intakes. Finally, we did not collect information about estrogen or progesterone receptor status of study patients.