The Adventist Health Study found that patients who were vegetarians had a lower risk of medium-frequency cancers.

For patients who observe a vegetarian lifestyle, a lower risk of all cancers combined and medium-frequency cancers as a group was noted, according to results from The Adventist Health Study published in The American Journal of Clinical Nutrition.1

Comparing vegetarians and non-vegetarians, the HR for total cancers was 0.88 (95% CI, 0.83-0.93; P <.001), and the total for all medium frequency cancers had an HR of 0.82 (95% CI, 0.76-0.89; P <.001). Significance was achieved among specific cancers between vegetarians and non-vegetarians, including carcinoma of the stomach (HR, 0.55; 95% CI, 0.32-0.93; P = .025), lymphoma (HR, 0.75; 95% CI, 0.59-0.97; P = .028), lymphoproliferative disorder (HR, 0.75; 95% CI, 0.60-0.93; P = .010), and colorectal cancers (HR, 0.79; 95% CI, 0.66-0.95; P = .011).

A null hypothesis was tested across dietary subtypes, that the dietary variables did not contribute additional information to the models. The hypothesis was rejected for breast cancer (P = .012), lymphoma (P = .031), all lymphoproliferative cancers (P = .004), prostate cancer (P = .030), colorectal cancers (P = .023), and possibly pancreatic cancer (P = .054). The hypotheses were also rejected for medium-frequency (P <.001) and all cancers (P <.001).

Dietary patterns were also assessed, and vegetarians continued to show lower estimates. Other patterns included vegans (HR, 0.76; 95% CI, 0.68-0.85) assessed in 365 cancers, lacto-ovo-vegetarians (HR, 0.91; 95% CI, 0.85-0.97) assessed in 1675 cancers, and pesco-vegetarians (HR, 0.89; 95% CI, 0.82-0.98) assessed in 560 cancers. Total medium-frequency cancers was also studied in vegans (HR, 0.77; 95% CI, 0.66-0.89) in 192 cancers, lacto-ovo-vegetarians (HR, 0.82; 95% CI, 0.75-0.90) in 805 cancers, and pesco-vegetarians (HR, 0.87; 95% CI, 0.76-0.99) in 273 cancers.

Cancer site and diet patterns combined were assessed and found that the 95% CI excluded the null hypothesis in multiple cancers and ages. These included prostate cancer in younger vegans (HR, 0.57; 95% CI, 0.41-0.80) but not older vegans (HR, 1.08; 95% CI, 0.71-1.65); breast cancer in younger vegans (HR, 0.69; 95% CI, 0.51-0.92) and older vegans (HR, 0.74; 95% CI, 0.46-1.19); lymphoma in younger vegans (HR, 0.90; 95% CI, 0.56-1.46) and older vegans (HR, 0.44; 95% CI, 0.20-0.98); lymphoma in lacto-ovo-vegetarians (HR, 0.64; 95% CI, 0.46-0.91) and older lacto-ovo-vegetarians (HR, 0.61; 95% CI, 0.40-0.95).

Of note, colorectal cancers were found to be less common in patients who were pesco-vegetarians (HR, 0.61; 95% CI, 0.44-0.86) and possibly lacto-ovo-vegetarians (HR, 0.83; 95% CI, 0.68-1.02).

"This is all relatively unique information and possibly the most robust that’s out there concerning cancers such as stomach and lymphoma. It may also be pointing the finger at several other cancers — such as lung, ovary, and pancreas — where the evidence from this study was suggestive of lower risk in vegetarians, but did not quite reach the necessary standard to say more,” Gary E. Fraser, MBCHB, PhD, distinguished professor of Preventive Medicine, distinguished professor in the School of Public Health, and distinguished professor of medicine at Loma Linda University, said in a press release on the findings.2

The study analyzed 79,468 patients from the US and Canada between 2002 and 2007. The study population was made up of available volunteer members of the Seventh-day Adventist church in North America. Patients were required to complete an extensive questionnaire sent by mail, which included a validated food frequency questionnaire, where patients could report dietary habits focusing on the previous year.

A total of 26% of patients were Black, and 65% were female.

“As compared with Adventist nonvegetarians, modest reductions in risk of both total and medium frequency cancers among vegetarians were evident, and this was true for vegans, lacto-ovo-vegetarians, and pesco-vegetarians,” the study authors concluded. “Across individual cancer sites, however, results were much more variable. For the more common cancers, there was evidence of lower rates in vegans for breast and prostate cancers at younger ages. Pesco-vegetarians had lower rates of colorectal cancer and breast cancer at older ages. Among medium-frequency cancers, lymphomas (also when combined with lymphoid leukemias as lymphoproliferative) were less frequent in vegans and lacto-ovo-vegetarians. Lower rates of carcinoma of the stomach were seen among all vegetarians combined and in lacto-ovo-vegetarians. Other sites among the medium frequency cancers may also be reduced among vegetarians, but these data could not identify them. Pancreatic, lung, and ovarian cancers appear as possibilities but must await more robust information.”

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