Can a plant-based diet help prevent breast cancer?

New evidence highlights how diet quality, not just plant-based eating, can influence breast cancer risk, particularly in postmenopausal women and those with lower BMI.

Study: The association between plant-based diet indices and the risk of breast cancer: a case-control study. Image Credit: Alkema Natalia / Shutterstock

A case-control study conducted on Iranian women highlights the benefits of consuming a healthy plant-based diet in terms of preventing breast cancer risk, especially in postmenopausal women. The findings are published in the Journal of Health, Population, and Nutrition.

Background

Breast cancer is the most common cancer in women worldwide, with over two million cases diagnosed annually. Certain factors, including genetics, age, and reproductive history, increase the risk of developing breast cancer.

Besides these non-modifiable risk factors, there is a range of modifiable factors that can potentially increase breast cancer risk, including alcohol consumption, smoking, body mass index (BMI), and dietary habits.

Dietary modification is one of the major lifestyle interventions that has been found to reduce breast cancer-related morbidity and mortality by one-third. In this context, studies have highlighted the preventive effects of healthy dietary patterns, such as the Mediterranean and Dietary Approaches to Stop Hypertension (DASH) diets, on breast cancer. Similarly, the adverse impact of Western dietary patterns on breast cancer risk has been pointed out in several studies.

The health benefits of the Mediterranean and DASH diets are primarily associated with a higher intake of plant-based foods and a limited or no intake of animal-based foods. Asian women are known to have lower breast cancer risk than Western women, mainly because of a higher intake of vegetarian diets.

Most previous studies investigating the impact of a plant-based diet on breast cancer risk have not considered the varying nutritional values and quality of plant foods. To address this gap in research, the current study aimed to evaluate the impact of different qualities of plant-based diets on breast cancer risk in Iranian women.

The prevalence of breast cancer in Iranian women is estimated to be 23.6%. Despite relying primarily on a plant-based diet, a significant portion of their daily energy comes from refined grains such as bread and white rice. The Iranian population, therefore, presents a valuable opportunity to differentiate the effects of unhealthy and healthy plant-based diets on breast cancer risk.

The study

The study was conducted on 133 Iranian women with breast cancer and 265 healthy women without breast cancer. The participant’s dietary intake information was obtained through a food frequency questionnaire.

The quality of plant-based foods was assessed using a general plant-based diet index (PDI), healthful PDI (hPDI), and unhealthy PDI (uPDI) scoring approach. A higher score in each index indicated greater adherence to the dietary pattern.

All foods were divided into three main classes, including animal foods, unhealthy plant foods (sugar-sweetened beverages, sweets or desserts, fruit juices, refined grains, and potatoes), and healthy plant foods (fruits, nuts, tea or coffee, vegetables, legumes, vegetable oils, and whole grains).

Study findings

The study found that a higher intake of a healthy plant-based diet significantly reduces the risk of breast cancer in Iranian women, especially in postmenopausal women. Specifically, after adjusting for confounding factors, women in the highest tertile of the healthy plant-based diet index had about half the odds of developing breast cancer compared to those in the lowest tertile. In premenopausal women, the study found that a higher intake of an unhealthy plant-based diet increases the risk of breast cancer.

The healthful plant-based diet index refers to the consumption of nutritious plant foods, such as vegetables, fruits, and whole grains. These foods contain higher amounts of dietary fiber, phytochemicals, antioxidants, vitamins, and minerals, and lower amounts of total and saturated fats, unhealthy carbohydrates, and cholesterol.

The nutritional profiles of these foods may be responsible for their antioxidant, anti-inflammatory, DNA-protective, cell-cycle-regulatory, and angiogenesis-modulatory properties, which in turn are associated with a reduced breast cancer risk.

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.

Notably, the study finds an inverse relationship between the intake of a healthy plant-based diet and the risk of breast cancer in women with a BMI of less than 25. Participants’ lower BMIs may partly be attributed to their higher adherence to a healthy, plant-based diet, which is known to reduce body fat accumulation, improve weight management, and lower the risk of obesity.

Plant-based diets are known to improve blood glucose regulation and insulin resistance, which are associated with estrogen receptor (ER)-negative breast cancer. However, it is important to note that the current study did not assess the tumors’ hormone receptor status. Instead, the study references prior research (such as Shah et al., 2023), indicating that a higher intake of a healthy plant-based diet has also been found to reduce the risk of ER-positive breast cancer by 15%.

These observations highlight the need for considering tumor type (ER-positive or negative status), hormonal therapy, and consumption of estrogen-containing contraceptives while evaluating the effect of plant-based diet on breast cancer risk.

However, this study did not consider the tumors’ hormonal status, which might be a potential limitation. Another limitation is self-reporting of dietary intake, which might introduce recall bias. Additionally, the hospital-based design and relatively small sample size are noted as further limitations in the original study. The authors emphasize that future studies should directly analyze tumor subtype, including hormone receptor status, to clarify these associations.

Considering the study findings, researchers recommended that future prospective cohort studies should consider the type of breast cancer and the menopausal status of patients for a more conclusive interpretation.