Plant-based diets and cancer

Introduction

The prevalence of cancer has been increasing steadily over the last decades, representing one in six deaths globally in 2019 compared to one in eight in 1990 (1). Due to improved detection and treatment for certain types of cancer, more people are surviving and living with cancer but the condition still represents 10% of Disability Adjusted Life Years (DALYs) globally and 20% in the European Union.

According to the Global Burden of Disease study, there were over 85 million cases of cancer diagnosed in 2019. In the European Union, cancer prevalence was 3.6% of the population, representing 28.7% of deaths. Therefore, identifying risk factors for developing cancer is one of the key public health challenges of today.

Lifestyle factors and cancer risk

The relationship between lifestyle factors such as physical activity, sedentary behaviour, and diet and cancer risk are being increasingly studied. There is general agreement that lifestyle, including diet, impacts cancer risk, although the extent to which this is true varies greatly among specific types of cancer.

The World Cancer Research Fund (WCRF) recommends the following behaviours for cancer prevention:(2)

• Maintain a healthy weight

• Be physically active

• Eat a diet rich in wholegrains, vegetables, fruit and beans

• Limit consumption of processed foods high in sugar, starches and fat

• Limit consumption of red and processed meat

• Limit consumption of sugar sweetened beverages

• Limit alcohol consumption

These behaviours work synergistically together and are intended to be adopted as an overall healthy lifestyle which promotes reduction in cancer risk as well as cardiovascular disease and other chronic conditions.

Diet and cancer

The WCRF dietary recommendations reflect a nutrient-dense diet, high in fibre and low in saturated fat, salt and sugar (2). Plant-based dietary patterns conform more closely to these dietary recommendations aimed at reducing cancer risk in comparison to typical Western diets.

Eating this way may reduce cancer risk directly or indirectly by reducing incidence of obesity which is a key risk factor for certain cancer types. People following plant-based eating tend to have a lower BMI and reduced incidence of obesity, which is positively associated with a number of cancers.

Read more about plant-based eating and weight maintenance.

Plant-based diets and cancer

Epidemiological studies show that diets including plant-based foods such as wholegrains, fruits, and vegetables are associated with reduced cancer risk.(3) However the precise understanding of the relationship between diet and risk of specific cancers remains elusive.

Despite this, observations from migration studies, changes in cancer rates over time within a population, and differences in cancer rates within the same genetic population living in different geographical regions, all lend support to the link between lifestyle factors and cancer risk.(4)

Plant-based diets are associated with overall health benefits. Therefore, the lack of robust data in support of cancer-protective effects should not dissuade health-conscious individuals from following a plant-based dietary pattern.

Why is there limited evidence relating to diet and cancer risk?

The limited evidence may be due in part to the fact that cancer is not a single disease but encompasses a multitude of different types of cancers, each with different etiologies. Many different factors play into the risk of developing cancer, including lifestyle, genetic and environmental factors.(2)

Moreover, it is possible that the impact of diet on cancer risk occurs early in life. For example, there is evidence that soya consumption during childhood and/or teenage years may be the determining factor for the reduced breast cancer risk observed in soya consumers.(2) Additionally, a diet high in fruits, vegetables, fibre and carotenoids during adolescence has been associated with reduced incidence of breast and colorectal cancer later in life.(3)

Early dietary habits may be especially relevant since the transformation of a normal cell into a cancer cell and eventually a tumor is believed to develop over decades. While diet may impact this process at many different stages, this also means that only long-term vegetarians are likely to benefit from the protective effects of a plant-based diet. Epidemiologic studies often do not take early dietary habits into account.

Still, there is evidence of a relationship between dietary factors and incidence of several types of cancer.

Plant-based diets and colorectal cancer

The WCRF report concluded that there is convincing evidence that physical activity and dietary fibre protect against colorectal cancer.(2) Furthermore, consumption of red and processed meat and alcohol as well as excess body and abdominal fat is associated with increased risk of colorectal cancer.

In 2015, two publications studied the relationship between plant-based dietary patterns and incidence of colorectal cancer. The data from a large prospective cohort study (US Adventist Health Study) demonstrated a 22% lower incidence of colorectal cancers amongst vegetarian participants compared to non-vegetarians.(5)

A Canadian case-control study demonstrated dietary patterns high in meat and sugary foods to be associated with an increased risk of colorectal cancer, whereas a plant-based dietary pattern of fruits, vegetables and whole grains was associated with 45% lower risk of colorectal cancer.(6)

In 2018, Schwingshackl et al carried out a systematic review and meta-analysis of dietary factors and their association with colorectal cancer risk.(7) The meta-analysis demonstrated an inverse association between cancer risk and whole grains (-5% per 30g daily intake), fruit and vegetables (-3% per 100g daily intake), and dairy (-7% per 200g daily intake). Conversely, a linear relationship was identified with red meat consumption (12% increased risk for every 100g daily intake) and processed meat (17% increase per 50g daily intake).

A year later, the same team published their analysis of European food consumption data across 16 countries and association between different disease outcomes and consequent contribution to disability adjusted life years (DALYs). Once again, higher intakes of healthy plant foods (whole grains, nuts, fruit and vegetables) were associated with lower DALYs due to colorectal cancer whilst higher intakes of processed and red meat were linearly associated with colorectal cancer related DALYs.(8)

Based on the existing evidence, a plant-based diet which limits red and processed meat is optimal for reducing the risk of colorectal cancer.

Plant-based diets and breast cancer

Breast cancer is the most commonly diagnosed cancer in women across the global accounting for 1 in 4 cancers in women.(9) In the Nurses’ Health Study I and II (with over 170,000 participants), women who scored highly on a healthy plant-based dietary index were at a modestly lower risk of developing breast cancer with an 11% lower incidence rate during the follow up period.(10)

The WCRF report highlights that there is some evidence that consuming non-starchy vegetables and foods high in carotenoids decrease risk whilst high calcium foods (including dairy), may also decrease the risk of premenopausal breast cancer. Consuming alcoholic drinks was found to increase the risk of both pre- and post-menopausal breast cancer.(2)

Additionally, a significant amount of research has been carried out to determine the relationship between soya foods consumption and breast cancer.(11) There is some evidence that soya food and soya isoflavones consumption reduces risk of breast cancer mortality.(2,12) For example, in the Shanghai Breast Cancer Survival Study, women who consumed the highest amounts of soya had a significantly decreased risk of overall mortality and recurrence, compared to those who consumed the least soya foods.(13)

Read more about soya foods and breast cancer.

Conclusions

Although the factors which affect cancer development are complex, lifestyle factors including diet certainly play a role. Experts state that adopting a plant-based diet rich in wholegrains, vegetables, fruit and legumes and limiting consumption of red and processed meat, processed foods high in sugar, starches and fat, sugar sweetened beverages and alcohol is the best way to reduce the risk of developing cancer.

Save article as PDF

Save article as PDF