A large study found that vegetarian diets were associated with lower risks of multiple cancer types in a large North American cohort, according to findings from the Adventist Health Study-2.
Researchers followed 79,468 Seventh-day Adventists who were cancer-free at enrollment, monitoring their health for nearly eight years, totaling 628,989 person-years of follow-up.
Participants were categorized by dietary patterns: vegans, lacto-ovo vegetarians, pesco-vegetarians, and non-vegetarians. Vegetarian diets were linked to a reduced risk of all cancers combined (hazard ratio [HR] = 0.88, 95% confidence interval [CI] = 0.83–0.93) compared to non-vegetarians. Medium-frequency cancers, which include lymphoma, stomach, ovarian, and pancreatic cancers, showed a stronger association (HR = 0.82, CI = 0.76–0.89).
Site-specific analysis revealed lower incidence for several cancers among vegetarians. Colorectal cancer was lower (HR = 0.79, CI = 0.66–0.95), as was stomach cancer risk (HR = 0.55, CI = 0.32–0.93). Lymphoproliferative cancers, including lymphoma and certain leukemias, had an HR of 0.75. Lymphoma alone showed a similar reduction.
Differences emerged when examining vegetarian subtypes. Among vegans, the risk of all cancers was lower (HR = 0.76, CI = 0.65–0.89). Lacto-ovo vegetarians had an HR of 0.91 (CI = 0.85–0.98), and pesco-vegetarians had an HR of 0.89 (CI = 0.81–0.98). For medium-frequency cancers, vegans (HR = 0.77, CI = 0.63–0.95), lacto-ovo vegetarians (HR 0.82), and pesco-vegetarians (HR = 0.87, CI = 0.75–1.01) each demonstrated lower risks.
Further breakdown by age showed that younger vegans had lower risks for prostate cancer (HR 0.57, CI = 0.41-0.80)) and breast cancer (HR 0.69, CI = 0.53-0.91)). Lymphoma risk was reduced among older vegans and among both younger and older lacto-ovo vegetarians.
The study adjusted for multiple non-dietary factors known to influence cancer risk, including age, sex, race, education, smoking status, alcohol use, family cancer history, physical activity, and cancer screening. Reproductive history and hormone use were also considered where relevant.
Body mass index (BMI), which was generally lower among vegetarians, was examined as a potential mediator. After adjusting for BMI, many associations remained, though hazard ratios moved closer to neutral, suggesting that BMI may partially influence risk, but diet may also have an independent effect.
Participants had long-term adherence to their dietary patterns. On average, non-vegetarians followed their diet for 36 years, lacto-ovo vegetarians for 30 years, pesco-vegetarians for 19 years, and vegans for 14 years. The large sample and extended follow-up allowed researchers to evaluate risks even for less common cancer types.
While the findings suggest protective associations between vegetarian diets and certain cancers, the authors noted that observational studies cannot establish causation. No increased cancer risks were observed for any vegetarian subgroup.
The study was funded by the National Cancer Institute and the World Cancer Research Fund.
Disclosures are available in the original publication.
