Study finds vegetarians less likely to develop several common and rare cancers

A long-running study of nearly 80,000 people shows that plant-based eating offers broad cancer protection, with benefits spanning from the digestive tract to hormone-related cancers, without raising risk for any type.

Study: Longitudinal associations between vegetarian dietary habits and site-specific cancers in the Adventist Health Study-2 North American cohort. Image Credit: Adri Yadam Nasir / Shutterstock

A recent study in The American Journal of Clinical Nutrition investigated the associations between different types of vegetarian diets and the risk of cancer at various bodily sites.

Diet and cancer

A vegetarian diet is a dietary pattern that generally focuses on higher intake of vegetables, fruit, nuts, and legumes while limiting or excluding meat. In contrast, the Western diet is characterized by high intake of refined grains, processed food, red meat, and sugary drinks.

A high intake of fruits and vegetables leads to increased exposure to phytoconstituents that possess anticancer effects. Furthermore, the exclusion of meat intake in a vegetarian diet decreases the risk of obesity, which has been linked to many cancer types. Although the association between vegetarian dietary habits and cancers at various bodily sites has remained controversial, a few robust links have been established. For instance, both processed and unprocessed red meat consumption has been linked with increased risk of colorectal cancers.

Previous publications from the group behind this study indicated a protective association between a vegetarian diet and common cancers, such as colorectal, breast, and prostate. However, more research is needed to link a vegetarian diet with less common cancer types.

About the study

The current study examined the Adventist Health Study-2 (AHS-2) cohort to investigate the association between a vegetarian dietary pattern and less common cancers. This study focuses on building on the knowledge regarding the benefits of vegetarian diets across a broader range of cancer types.

The AHS-2 cohort recruited participants from the United States and Canada between 2002 and 2007. Out of the 95,863 enrolled participants, approximately 26,000 were Black, which enabled scientists to focus on this minority group as well. Participants were screened based on eligibility criteria and the availability of complete data relevant to this study.

All participants completed a comprehensive questionnaire about their dietary patterns, and were divided into different groups based on their answers. Participants were assigned to one of the following groups: vegans, lacto-ovo-vegetarians, pesco-vegetarians, semi-vegetarians, or non-vegetarians.

Participants who avoided all animal products were included in the vegan group, while those who avoided all flesh (meat or fish) foods but consumed eggs and dairy were assigned to the lacto-ovo-vegetarian group. Similarly, pesco-vegetarians were defined as lacto-ovo-vegetarians who also consumed fish at least once a month, while semi-vegetarians ate non-fish flesh foods infrequently (at least once a month but less than once a week).

The authors matched AHS-2 data to all United States state cancer registries except Maine and three Canadian provincial registries to identify new cancer diagnoses. For each cancer site, participants were stratified for age, gender, education, and past and present cancer screening habits.

Study findings

A total of 79,468 participants met the eligibility criteria, comprising 26% Black and 65% female participants. The current study enabled an average of 7.9 years of follow-up per participant. The marginal standardization method was used to compare vegetarians to non-vegetarian participants, after adjusting for age, sex, and race.

Except for the family history of breast cancer, the study observed considerable differences within groups. For instance, a greater proportion of vegetarians were older, were less likely to use contraceptives, were less likely to undergo postmenopausal hormone replacement therapy, had breastfed infants a little longer, and were more educated.

The current study indicated differences between vegetarians and non-vegetarians were near-null or favored vegetarians for all cancer types, except perhaps myeloma. In comparison to Adventist non-vegetarians, all vegetarian diets were associated with a reduced risk of both total (Hazard Ratio [HR]: 0.88) and medium-frequency cancers (HR: 0.82).

The authors noted that adjusting for Body Mass Index (BMI) slightly weakened these protective associations, suggesting that lower body weight among vegetarians partially mediates these benefits. It is notable that even the non-vegetarian Adventists in the study are a health-conscious group, consuming less meat and alcohol than the general population, which may mean the protective effects of vegetarian diets could be even more pronounced when compared to a typical Western diet.

Different vegetarian diets have been shown to independently influence specific types of cancer. For example, a vegan diet was linked to a lower incidence of breast and prostate cancers at younger ages, though the protective association for prostate cancer was not observed in older men. Pesco-vegetarians were at a lower risk of colorectal cancer (a finding also significant for all vegetarians combined, HR: 0.79) and breast cancer at older ages.

A significantly lower risk for lymphoproliferative cancers (which include lymphomas) was observed in vegetarians (HR: 0.75), with lacto-ovo-vegetarians and, in some age groups, vegans showing a lower risk.

The risk of stomach cancer was significantly lower among all vegetarians combined (HR: 0.55), with the paper noting this was also seen in lacto-ovo-vegetarians. Some of these site-specific findings were based on relatively small case numbers and should be interpreted with caution. Further research is needed to determine whether a vegetarian diet affects the risk of pancreatic, lung, and ovarian cancers.

Conclusions

The current study highlighted the benefits of a vegetarian diet in lowering the risk of several cancers. No evidence has indicated that a vegetarian dietary pattern increases the risk of any cancer.

The authors indicated several limitations of the current study. For instance, relatively small numbers of less common cancers could be linked with vegans and pesco-vegetarians. The authors also noted that dietary information was only collected at the beginning of the study and not during the follow-up period. Future studies must interpret the findings of the current study, taking into consideration the particular racial mix of the AHS-2 cohort.