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The association between plant-based diet indices and the risk of breast cancer: a case-control study

Journal of Health, Population and Nutrition volume 44, Article number: 127 (2025) Cite this article

Abstract

Background

Previous studies examining the relationship between plant-based diets and breast cancer (BrC) have provided conflicting evidence. To address these inconsistencies, we aimed to evaluate the association between the plant-based diet index (PDI), healthful PDI (hPDI), and unhealthy PDI (uPDI) with the odds of BrC in Iranian women.

Methods

The current case-control research was performed on 133 Iranian women with BrC and 265 controls. The study subjects were selected from hospitals in Tehran. PDI, hPDI, and uPDI were categorized into eighteen food groups based on nutrient composition similarity. The relationship between PDIs and BrC was assessed using logistic regression.

Results

After adjusting for confounding factors, the chance of developing BrC was lower in the highest tertile of hPDI compared to the lowest tertile (odds ratio (OR) = 0.495; 95% confidence interval (CI): 0.274–0.891; P = 0.019). In addition, postmenopausal women in the second and last tertiles of hPDI had lower odds of BrC than those in the first tertile (T) (T2: OR = 0.342; 95% CI: 0.141–0.828; P = 0.017– T3: OR = 0.262; 95% CI: 0.107–0.639; P = 0.003) in the adjusted model. Furthermore, in premenopausal women in the highest tertile of uPDI, the odds of BrC were increased compared to the lowest tertile (OR = 2.546; 95% CI: 1.051–6.167; P = 0.038) in the adjusted model.

Conclusions

Adherence to a healthy plant-based dietary pattern, including vegetables, fruits, whole grains, nuts, and legumes, seems to be beneficial for BrC prevention, particularly in postmenopausal women. Future prospective cohort studies that consider menopausal status and the type of BrC are needed to support these findings.

Introduction

Breast cancer (BrC) is the most common cancer in women all over the world [1]. While certain risk factors for BrC, including genetics, age, and reproductive history, are considered non-modifiable, several factors, such as alcohol consumption, smoking, body mass index (BMI), and dietary habits, are known to be potentially modifiable targets for BrC prevention [2]. The prevalence of this cancer in Iranian women is estimated to be 23.6% [3].

Lifestyle modifications, particularly dietary modifications, are estimated to decrease morbidity and mortality related to BrC by one-third [4]. However, results from epidemiological studies investigating the association between individual nutrients, foods, and the risk of BrC are inconsistent [5, 6]. These inconsistencies may be explained by the complex interactions between various food components [7, 8]. Therefore, to address this issue, rather than focusing on the effects of individual nutrients or food items, dietary patterns are defined and applied to capture the interactions and combined effects of dietary components [9].

Research has shown that several dietary patterns, such as the Mediterranean [10] and Dietary Approaches to Stop Hypertension (DASH) [11] diets, are protective against BrC, while the Western dietary pattern [12] increases its risk. Additionally, the lower prevalence of BrC among the Asian population, who have a higher intake of plant-based foods compared to women from Western countries, suggests the potential role of a vegetarian diet in BrC prevention [13].

Previous methods used to investigate the impact of plant-based diets on BrC did not consider the different nutritional values and quality of plant foods [14]. To assess the quality of plant-based diets, Satija et al. suggested using a general plant-based diet index (PDI), healthful PDI (hPDI), and unhealthy PDI (uPDI) scoring approach [15]. Plant-based diets are characterized by the occasional consumption of animal-based foods and the consumption of mostly plant-based foods [16]. The Middle Eastern diet, specifically in Iran, is mostly plant-based. However, a significant portion of an individual’s daily energy comes from refined grains such as bread and white rice [17]. Therefore, this population presents a valuable opportunity to differentiate the effects of unhealthy and healthy plant-based diets on the risk of BrC. Previous studies examining the relationship between the effects of plant-based diets and BrC have provided conflicting evidence in Iranian women [18,19,20]. To address these inconsistencies, the present study aimed to investigate the relationship between PDI, hPDI, and uPDI with the odds of BrC in Iranian women.

Methods

Study design

The current case-control study was conducted on Iranian women with BrC. The participants, aged between 30 and 65 years, were selected from two general hospitals in Tehran (Imam Hossein and Shohadaye Tajrish hospitals). The case group consisted of women who had been diagnosed with BrC within the past six months histologically. The control group comprised women with non-neoplastic diseases who were also from the same hospitals. Two controls were randomly selected for each case from a single medical center. The sample size of the study was determined based on previous research (odds ratio (OR) = 0.47, confidence interval (CI) = 0.24–0.94, power β = 0.20, and α = 0.05) [21].

The current study’s participation rate was 92%. Participants who did not complete more than

50 items of the food frequency questionnaire (FFQ) and those with an energy intake lower or higher than the mean ± three standard deviations (SD) were excluded [22]. As a result, 265 controls and 133 cases were included in the final analysis. All individuals provided written informed consent. The present study was approved by Shahid Beheshti University of Medical Sciences (Research Institute of Nutrition and Food Sciences).

Dietary assessment and measurements

A reliable and valid 168-item FFQ was applied to evaluate participants’ dietary intake [23]. Their consumption was converted to daily frequency, and using the household measures handbook, these intake frequencies were then translated into grams per day [24]. The United States Department of Agriculture (USDA) food composition table was utilized to calculate the foods’ nutrients and energy.

Three models of plant-based diets, including uPDI, hPDI, and PDI, were categorized into eighteen food groups based on similarities in nutrient composition [25,26,27]. All foods were divided into three main classes: animal foods (various animal-based foods, meat, fish/seafood, dairy, eggs, and animal fat), unhealthy plant foods (sugar-sweetened beverages, sweets/desserts, fruit juices, refined grains, and potatoes), and healthy plant foods (fruits, nuts, tea/coffee, vegetables, legumes, vegetable oils, and whole grains). In hPDI and PDI, the maximum intake of healthy plant foods and total plant foods received a score of 10, while the minimum intake received a score of 1. In uPDI, the lowest and highest intakes of unhealthy plant foods scored 1 and 10, respectively. The total score of each index was between 18 and 180 [26,27,28,29]. A higher score in each index indicated greater adherence to the dietary pattern. The details of food items and index scores are shown in Supplementary Table 1.

Socio-demographic and anthropometric assessments

Trained dietitians conducted all measurements and questionnaires during a single interview. A validated questionnaire was utilized to assess physical activity, with results reported in terms of metabolic equivalent of task (MET)-hours per day (MET-h/d) [30]. A non-stretchable tape measure fixed to a wall was used to measure participants’ height with an accuracy of 0.5 cm. Participants’ weight was measured using a digital scale with an accuracy of 0.1 kg while they were dressed in minimal clothing and without shoes. BMI was calculated by dividing weight (kg) by the square of height (m²). We also used a checklist to collect participants’ clinical information, lifestyle details, and socio-demographic-economic factors. This included family history of cancer (yes/no), nighttime bra wearing (yes/no), duration of breastfeeding (in months), age at marriage (in years), age (in years), age at first pregnancy (in years), smoking history (yes/no), menopausal status (pre-menopause or post-menopause), and whether they take supplements or medications (yes/no).

Statistical analysis

For statistical analysis, SPSS (version 26.0) was utilized. To determine the normal distribution of variables, the Kolmogorov-Smirnov test was applied. The level of significance was a p-value of less than 0.05. In the present study, the independent samples T-test and chi-square test were used for parametric continuous and categorical variables, respectively. The Mann-Whitney U test was used for non-parametrically distributed continuous variables. The relationship between PDIs and BrC, as well as the relationship between PDIs and BrC by menopausal status and BMI, was evaluated using logistic regression in two crude and adjusted models (adjusted for age (years), BMI (kg/m2), family history of cancer (yes/no), energy intake (kcal/day), fiber intake (g/day), menopausal status (pre-/postmenopausal), vitamin D supplement (yes/no), physical activity (MET-h/day), and smoking (yes/no)). Covariates were selected based on previous studies on BrC that identified contributing factors to the condition [31,32,33].

Results

The baseline characteristics of the current study illustrated that the mean age of cases was higher than that of the controls (P = 0.030). However, the hPDI score of the control group was higher than that of the cases (P = 0.012). There was a significant difference between the control and case groups in terms of family history of cancer (P = 0.047) and taking vitamin D supplements (P = 0.037) (Table 1).

The relationship between PDIs and BrC is presented in Table 2. In model 1 of the hPDI, the odds of developing BrC in the second and last tertiles were lower than those in the first tertile (T) (T2: OR = 0.591; 95% CI: 0.360–0.970; P = 0.037– T3: OR = 0.523; 95% CI: 0.310–0.884; P = 0.015). After adjusting for age (years), BMI (kg/m2), family history of cancer (yes/no), energy intake (kcal/day), fiber intake (g/day), menopausal status (pre-/postmenopausal), vitamin D supplement (yes/no), physical activity (MET-h/day), and smoking (yes/no), this relationship remained significant in the last tertile (OR = 0.495; 95% CI: 0.274–0.891; P = 0.019).

Table 3 shows the relationship between PDIs and BrC based on menopausal status. In postmenopausal women, lower odds of BrC were observed in the second and last tertiles of hPDI in comparison to the first tertile in the crude model (T2: OR = 0.419; 95% CI: 0.197–0.889; P = 0.023– T3: OR = 0.425; 95% CI: 0.209–0.866; P = 0.018). After adjusting for the role of some potential confounders, the association remained significant between hPDI and BrC risk in postmenopausal women (T2: OR = 0.342; 95% CI: 0.141–0.828; P = 0.017– T3: OR = 0.262; 95% CI: 0.107–0.639; P = 0.003). Furthermore, in premenopausal women in the highest tertile of uPDI, the odds of BrC were increased compared to the lowest tertile (OR = 2.244; 95% CI: 1.061–4.744; P = 0.034) in the crude model. This association remained significant after adjusting for potential confounders (OR = 2.546; 95% CI: 1.051–6.167; P = 0.038). No significant relationship was observed in other indices in post- and premenopausal women in the adjusted models.

Table 4 shows the relationship between PDIs and BrC based on BMI category. In individuals with a BMI ofless than 25 kg/m2, lower odds of BrC were found in the highest tertile of hPDI in comparison to the lowest tertile in both crude and adjusted models (crude model: OR = 0.146; 95% CI: 0.029–0.736; P = 0.020– adjusted model: OR = 0.113; 95% CI: 0.017–0.760; P = 0.025). No significant relationship was observed in other indices in both groups.

Table 1 The baseline characteristics of the study participants
Full size table
Table 2 Association between plant-based diet indices and breast cancer
Full size table
Table 3 Association between plant-based diet indices and breast cancer by menopausal status
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Table 4 Association between plant-based diet indices and breast cancer by BMI category
Full size table

Discussion

In the present case-control research, we found a significant inverse relationship between hPDI and the risk of BrC. However, no significant association was observed between PDI and uPDI with the risk of BrC. Furthermore, the analysis based on menopause status revealed an inverse relationship between hPDI and BrC risk in postmenopausal women, while a direct relationship was observed between uPDI and BrC risk in premenopausal women.

The hypotheses concerning the main environmental factors that promote carcinogenesis emphasize lifestyle choices, especially diet. Individuals in developed countries usually consume diets that are energy-dense and high in fat and animal products, while their intake of vegetables, fruits, and fiber is lower. In contrast, people living in countries with low BrC incidence and mortality, such as those in Asia and Africa, tend to have different dietary patterns [34].

In the current study, a higher hPDI, which is a measure of following a high-quality plant-based diet, was related to lower odds of BrC. Similar results were also reported in Sasanfar et al.’s study [20]. In addition, Rigi et al.’s study showed a negative relationship between hPDI and the risk of BrC in a case-control design [19]. Moreover, the results of other studies showed an inverse relationship between hPDI and BrC risk [35, 36]. In contrast to our findings, a hospital-based case-control study conducted by Payandeh et al. did not show a significant relationship between hPDI and BrC [18]. The hPDI emphasizes the consumption of nutritious plant foods, such as vegetables, fruits, and whole grains, which are linked to better health outcomes [15]. The findings of a systematic review on dietary patterns and the risk of BrC revealed that dietary patterns emphasizing higher intake of legumes, whole grains, vegetables, and fruits were inversely related to the risk of BrC [37]. Observational studies have shown that a prudent diet, which includes high poultry, low-fat dairy, fruits, fish, whole grains, and vegetables, is related to an 18% reduction in BrC risk [12]. These diets are rich in fiber, phytochemicals, antioxidants, and vitamins E and C while being low in total and saturated fat and cholesterol [38]. These nutrients may have antioxidant, anti-inflammatory, and deoxyribonucleic acid (DNA) protective effects, improving cell signaling, cycle regulation, and angiogenesis related to cancer initiation [39]. Additionally, the high fiber content of these diets may reduce gastrointestinal transit time and the contact of carcinogens with the colonic mucosa while increasing short-chain fatty acid production [40].

The findings of the current study did not reveal a significant association between PDI and uPDI with BrC in Iranian women. Previous studies examining this relationship have reported inconsistent findings [18,19,20]. Consistent with our results, a case-control study by Payandeh et al. involving Iranian women found no significant association between PDI and uPDI with the risk of BrC [18]. Similarly, another case-control study among Iranian women did not show a significant relationship between PDI and uPDI with BrC odds [20]. The results obtained from the prospective NutriNet-santé cohort showed no significant link between a pro-plant-based dietary score and the risk of BrC [40]. Moreover, results obtained from a large prospective study (more than 90% of the study population were premenopausal women) did not show a significant linear association between greater adherence to pro-vegetarian diets (PVG), including diets emphasizing plants and some animal products, and BrC incidence [41]. Conversely, a population-based case-control study by Rigi et al. identified a significant inverse relationship between PDI and uPDI with the risk of BrC [19]. The lack of a relationship between uPDI and the risk of BrC can be attributed to the fact that the consumption of refined grains such as bread, white rice, potatoes, and sweets, which are considered as unhealthy plant-based foods, is the primary source of energy in the Iranian population. This might reduce the chance of detecting an existing inverse relationship between uPDI and BrC [17, 42]. Plant-based diets can enhance glycemic control and insulin resistance, which are linked to estrogen receptor (ER)-negative BrC [15]. However, in recent years, despite the increase in ER-positive BrC, the incidence of ER-negative BrC has decreased [37]. This might explain why studies often fail to detect the relationship between plant-based diets and BrC without considering tumor type. Therefore, due to the important role of estrogen in BrC, the type of tumors (ER-negative or -positive), consumption of estrogen-containing contraceptives, and hormone therapy need to be considered in future studies. As hormonal factors greatly affect BrC, the detection of the effects of dietary factors might be challenging.

In the present study, we observed a protective effect of hPDI on BrC in postmenopausal women. Additionally, a direct relationship between uPDI and BC risk was found in premenopausal women. The results of a prospective cohort study showed that there is an inverse relationship between hPDI and the risk of BrC in postmenopausal women [35]. Also, a case-control study conducted by Rigi et al. showed an inverse relationship between hPDI and BrC risk only in Iranian postmenopausal women. However, their study did not show a significant relationship between uPDI and BrC risk in premenopausal women [19]. Moreover, the findings of the study by Sasanfar et al. indicated the inverse relationship between hPDI and the odds of BrC in postmenopausal women and the lack of relationship between uPDI and the risk of BrC in premenopausal women [20]. Shah et al. discovered that participants in the highest quintile of the hPDI had a 15% lower risk of developing ER-positive BrC compared to those in the lowest quintile. Similarly, greater adherence to the uPDI was associated with a 20% increased odds of ER-positive BrC [36]. Therefore, hormone receptor status is a crucial prognostic and diagnostic characteristic of breast tumors and should be carefully considered.

The findings of this study revealed an inverse relationship between hPDI and the risk of BrC in individuals with a BMI of less than 25. However, based on BMI classification, no relationship between other indicators and BrC risk was observed for individuals with a BMI of less than 25 or greater than 25. Limited studies have examined the relationship between these indices, based on BMI, and the risk of BrC. In the study conducted by Payandeh et al., no relationship was observed between any of the PDIs and the risk of BrC in individuals with a BMI of less than or greater than 25 [18]. Adhering to a plant-based diet, particularly one that is abundant in nutritious plant foods, is associated with reduced body fat, improved weight management, and a lower risk of obesity [43]. BMI is one of the modifiable risk factors associated with BrC risk [44]. The potential inverse relationship between hPDI and BMI in women with BrC may be partly due to a higher intake of beneficial components found in plant-based foods. This suggests that a healthful plant-based diet offers improved diet quality compared to an unhealthy plant-based diet [45]. The primary sources of vegetable proteins include nuts, legumes, soybeans, vegetables, and fruits, all of which provide antioxidants and fiber. Additionally, nuts are a significant source of polyunsaturated fatty acids. Furthermore, tea and coffee are excellent sources of antioxidants [46, 47]. As a result, healthy plant-based diets can contribute to reducing the risk of BrC by lowering BMI and providing beneficial substances with cancer-preventive properties.

This study has several strengths and limitations. Subgroup analysis based on the menopausal status of women and BMI is one of the strengths of this research. Considering the quality of plant-based diets and including diets with limited consumption of animal foods rather than vegetarian diets in which animal foods are completely eliminated is more practical to differentiate between plant-based foods and those who preferentially consume more plant-based foods and fewer animal-based foods but are not vegetarian. Due to self-reporting in completing the FFQ, the study is prone to recall bias. However, to minimize this limitation, a valid questionnaire with high validity and repeatability was used in the Iranian population. Also, the hospital-based design was one of the main limitations of the current study. Moreover, this study did not consider the hormone receptor status of patients’ tumors. In addition, the small sample size can be mentioned as another limitation of the current research. Finally, even though we considered various confounding factors, due to the observational nature of the study, the effects of residual confounding factors could not be entirely excluded.

Conclusions

Overall, we did not find any significant relationship between PDIs and the risk of BrC, except for hPDI. However, adhering to a healthy plant-based dietary pattern, which includes vegetables, fruits, whole grains, nuts, and legumes, seems to be beneficial for preventing BrC in postmenopausal women. Additionally, we found a significant association between uPDI and BrC odds in premenopausal women. It is recommended that future prospective cohort studies take into account the type of BrC and menopausal status to further support these findings.

Data availability

No datasets were generated or analysed during the current study.

Abbreviations

BrC:

Breast cancer

PDI:

Plant-based diet index

hPDI:

Healthful plant-based diet index

uPDI:

Unhealthy plant-based diet index

OR:

Odds ratio

CI:

Confidence interval

T:

Tertile

BMI:

Body mass index

DASH:

Dietary approaches to stop hypertension

FFQ:

Food frequency questionnaire

SD:

Standard deviation

USDA:

United States department of agriculture

MET:

Metabolic equivalent of task

PVG:

Pro-vegetarian diets

ER:

Estrogen receptor

DNA:

Deoxyribonucleic acid

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Acknowledgements

We sincerely thank all field investigators, staff, and participants of the present study.

Funding

Not applicable.

Author information

Authors and Affiliations

  1. Department of Clinical Nutrition, School of Nutrition and Food Sciences, Shiraz University of Medical Sciences, Shiraz, Iran

    Fateme Souni & Fatemeh Mansouri

  2. Department of Nutritional Sciences, The Pennsylvania State University, University Park, PA, USA

    Fatemeh Jafari

  3. Department of Prosthodontics, Faculty of Dentistry, Tabriz University of Medical Sciences, Tabriz, Iran

    Reza Sharifi

  4. Department of Food Science and Technology, Science and Research Branch, Islamic Azad University, Tehran, Iran

    Sepideh PourvatanDoust

  5. Department of Nutrition and Biochemistry, School of Medicine, Ilam University of Medical Sciences, Ilam, Iran

    Zainab Shateri

  6. Infertility and Reproductive Health Research Center, Health Research Institute, Babol University of Medical sciences, Babol, Iran

    Mehran Nouri

  7. Department of Community Nutrition, Faculty of Nutrition and Food Technology, National Nutrition and Food Technology Research Institute, Shahid Beheshti University of Medical Sciences, Tehran, Iran

    Bahram Rashidkhani

Authors
  1. Fateme Souni
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  2. Fatemeh Mansouri
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  5. Sepideh PourvatanDoust
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  6. Zainab Shateri
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  7. Mehran Nouri
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Contributions

F.S, F.M, F.J, R.S and S.P; Contributed to writing the first draft. M.N, Z.S and B.R; Contributed to all data and statistical analysis and interpretation of data. M.N and B.R.; Contributed to the research concept, supervised the work, and revised the manuscript. All authors read and approved the final manuscript.

Corresponding authors

Correspondence to Mehran Nouri or Bahram Rashidkhani.

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Ethics approval and consent to participate

This study was conducted in accordance with the ethical standards of the Declaration of Helsinki and was approved by the Research Institute of Nutrition and Food Sciences of Shahid Beheshti University of Medical Sciences. All participants read and signed the informed consent form.

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Not applicable.

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The authors declare no competing interests.

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Souni, F., Mansouri, F., Jafari, F. et al. The association between plant-based diet indices and the risk of breast cancer: a case-control study. J Health Popul Nutr 44, 127 (2025). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s41043-025-00879-2

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  • DOI: https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s41043-025-00879-2

Keywords

Journal of Health, Population and Nutrition

ISSN: 2072-1315

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