People who ate a primarily vegetarian diet had a reduced risk for overall cancer and for several specific cancers compared with people who were nonvegetarian, according to a study.

The new findings, published in The American Journal of Clinical Nutrition, lend more credence to the existing evidence base suggesting that a heavily plant-based diet may reduce cancer risk.

A 2012 meta-analysis of seven studies found that vegetarians had an 18% lower risk for cancer than nonvegetarians. Similarly, a 2017 meta-analysis of 10 prospective cohort studies revealed an 8% reduced risk for cancer among vegans and vegetarians compared with omnivores. Another meta-analysis of eight studies with 686,691 participants published in 2023 showed vegetarian diets lowered the risk for gastrointestinal cancers, specifically, by 23%, with greater benefits for men and Asian populations.

More recently, a study published on January 6 in Nature Microbiology compared the microbiomes of vegans, vegetarians, and omnivores across five cohorts totaling 21,561 individuals. Omnivores had more bacteria linked to increased risk for colon cancer, the researchers found, and microbes with favorable cardiometabolic markers were particularly plentiful in vegans’ microbiomes. But those healthy microbes in vegans also appeared in greater amounts in the microbiomes of omnivores who ate more plant-based foods.

In the new study, vegetarians had a 12% lower risk for any type of cancer than non-vegetarians (hazard ratio [HR], 0.88; P < .001) and an 18% lower risk for cancers that occur with medium frequency (HR, 0.82; P < .001). Cancers categorized as occurring with medium frequency included melanoma, lymphoma, myeloid leukemias, and endometrial, renal, urothelial, thyroid, ovarian, central nervous system, lung, rectal, pancreatic, primary liver, stomach, esophageal, mouth-pharyngeal-laryngeal, and lymphoid cancers.

This study focused mainly on those with a pure vegetarian diet, but it included analysis of subcategories of vegans, those who ate fish (pesco-vegetarians) and those who ate dairy and egg products (lacto-ovo-vegetarians), all of which also showed reduced cancer risk. It further found reduced risks in some cancers that are less common, such as stomach cancer in all vegetarians and in lacto-ovo-vegetarians, and in cancers not necessarily thought to be strongly associated with lifestyle factors, such as lymphoproliferative cancers across all vegetarians.

“The results for lymphomas and lymphoproliferative cancers is a new finding as far as I know, and our results were fairly convincing,” lead author Gary Fraser, MBChB, PhD, told Medscape Medical News.

Fraser said the evidence to date that plant foods protect against cancers has been considered suggestive, but it has yet to be proven and that a limitation of past studies in vegetarians has been small populations for each cancer type, a challenge that existed for certain cancers in this study as well.

“But the evidence that vegetarians compared to nonvegetarians have lower rates of colorectal and stomach cancers is not unexpected due to other work that has implicated red meats and, for stomach cancer particularly, processed meats, in increasing risk,” Fraser continued.

“We have previously reported that vegans have lower risks of breast and prostate cancers,” a finding that shows up in these data as well, he said. Because of this study’s large population, “this is really the only study to date that can look with any authority to risks in vegans,” Fraser said. “Interestingly we find in other reports from [this cohort] that this may well be related to long-term relatively low intakes — < 2/3 cups per day — of dairy milk for both of these cancers.”

Nigel Brockton, PhD, the vice president of research at the American Institute for Cancer Research (AICR), told Medscape Medical News that the findings “are consistent with previous research and support AICR’s recommendations for people to eat a diet rich in whole grains, vegetables, fruits, and beans and limit red and processed meats.”

This cohort had a very high prevalence of vegetarians — about 50% compared with 6% in the US as a whole — and yet it still became more difficult to identify statistically significant associations for cancer when breaking down the data based on different types of vegetarians, he explained.

“The evidence becomes more scattered when subcategories are assessed,” Brokton noted.

Study Methods

Fraser, of Loma Linda University in Loma Linda, California, and his colleagues analyzed data from a cohort of 95,863 North American Seventh-Day Adventists who enrolled in the longitudinal Adventist Health Study between 2002 and 2007. Specifically, the authors compared diets and incidence of different cancers in 79,468 participants from the US and Canada who did not have cancer at enrollment and had adequate demographic data available. The population included 26% Black participants and 65% female participants, tracked for an average 7.9 years of follow-up.

The participants had filled out food frequency questionnaires about their diet over the previous year, enabling the researchers to categorize vegetarian types. The investigators validated those answers with six 24-hour dietary recalls by telephone in a subgroup of 1100 participants. The information on participants’ dietary patterns was then cross-referenced with state and Canadian cancer registries data.

Ultimately, semi-vegetarians, those who ate some flesh foods besides fish less than once a week but more than once a month, were excluded because there were so few. The authors did not adjust their findings for covariates that might mediate dietary effects, but they stratified the results by age and adjusted for self-described gender, education, race, and past and present screening habits with mammography, prostate-specific antigen testing, prostate screening, colonoscopy, and fecal blood testing for colorectal cancers.

Additionally, the researchers made adjustments for specific behavioral or reproductive factors — such as lactation history, exercise frequency, alcohol consumption, and smoking history, among others — for the analysis of certain individual cancers known to be affected by those factors. Adjustments for breast cancer analyses, for example, included menopausal status, age at menarche, height, history of menopausal hormone therapy or oral contraceptives, family history of breast cancer, physical activity, and number of months spent breastfeeding.

Study Results

Four groups of specific cancers had significantly lower risks in vegetarians compared with nonvegetarians. Risk in vegetarians was 21% lower for colorectal cancer (HR, 0.79; P = .01), 45% lower for stomach cancer (HR, 0.55; P = .025), and 25% lower for lymphoma and lymphoproliferative cancer (HR, 0.75; P = .028 and P = .01, respectively).

Analysis of subgroups of vegetarians revealed reduced risks for all cancers and for medium-frequency cancers as well as some specific cancers. Vegans had a significant 24% reduced risk for all cancers and a 23% reduced risk for medium-frequency cancers. Lacto-ovo-vegetarians had a significant 9% reduced risk for all cancers and an 18% reduced risk for medium-frequency cancers. Pesco-vegetarians had a significant 11% reduced risk for all cancers and a 13% reduced risk for medium-frequency cancers.

Associations with vegetarian subgroups and other specific cancers were often mediated with age, such as a lower risk for prostate and breast cancers in younger vegans, at age 65, but not older ones, at age 85.

Lymphoma risk, however, was significantly lower in older vegans but not in younger vegans, and the risk was low in both younger and older lacto-ovo-vegetarians. Colorectal cancer risk was only lower in pesco-vegetarians. All of these associations, however, were limited by small numbers of total cases, though statistical adjustment for multiple comparisons, including Bonferroni corrections, appeared to be used when deemed appropriate.

Clinical Implications

“There are some interesting hypothesis-generating associations reported but this study, in itself, would not lead to changes in dietary recommendations for cancer risk reduction,” Brokton noted.

Given that only around 10% of US adults meet current recommendations for the intake of fruits and vegetables, “the most important message for medical practitioners is that eating a diet rich in whole grains, vegetables, fruits, and beans is more important than rigidly excluding certain types of foods,” Brockton said. Further, he underscored that diet is just one aspect of an individual’s lifestyle that can play a role in reducing one’s cancer risk.

“Physical activity, weight management, avoiding alcohol and tobacco use, reducing excess UV [ultraviolet] exposure and following screening and vaccination recommendations, can all additionally help reduce cancer risk,” he said.

The authors said several cancers, including nervous system cancers, uterine cancer, and myeloid hematologic cancers, do not appear associated at all with plant-based diets. These cancers were included in the analysis of the outcome for all medium-frequency cancers that showed an overall reduced risk, but individual analyses for each cancer did not show any associations for these specific ones.

“Plant-based diets — and vegetarian diets may be considered more extreme examples — are well-known to be associated with lower rates of cardiovascular disease, and certainly in our data and others, also with greater life expectancy,” Fraser said. “While they are not associated with lower risk of every cancer, there were [no cancers] where they were associated convincingly with increased risk, and risk of several common cancers was significantly lower. So now, the evidence for probable benefits of vegetarian and plant-based diets still holds and is arguably strengthened.” 

The study’s generalizability may be limited by the fact that the total incidence of cancers in US Adventists is 30% lower than in the general population, they said.

The research was funded by the National Cancer Institute, the World Cancer Research Fund, and Loma Linda University. The authors reported no conflicts of interest or industry disclosures.

Tara Haelle is a science and health journalist based in Dallas.