Plant-based diets and cancer risk: what the evidence shows (and what it doesn’t)
Few dietary topics generate more confident claims — in both directions — than plant-based eating and cancer. On one side: plant-based diets prevent cancer, full stop. On the other: plants are no better than anything else, and the evidence is cherry-picked. Neither position is accurate, and neither is useful to someone trying to make thoughtful decisions about what to eat.
What the evidence actually shows is more interesting, and considerably more nuanced.
Defining the terms
“Plant-based” covers a spectrum. At one end are fully vegan diets, which exclude all animal products. At the other are flexitarian or predominantly plant-based patterns that include fish, some poultry, eggs, and dairy, but centre vegetables, legumes, whole grains, fruits, nuts, and seeds. In between sit pescatarian and lacto-ovo-vegetarian diets.
This matters because most of the research does not compare optimal plant-based eating to optimal omnivorous eating. It compares self-reported dietary patterns in observational cohorts — which is a much messier comparison, since a vegetarian who eats primarily refined carbohydrates and processed meat substitutes is eating very differently from one who eats a diverse whole-food plant diet. With that caveat established, here is what the evidence says.
What the evidence shows
A 2023 analysis published in the European Heart Journal, reviewing the health effects of vegetarian and vegan diets across multiple outcome categories, found a consistent signal: vegetarian and vegan dietary patterns are associated with lower body mass index, lower rates of type 2 diabetes, lower blood pressure, and lower LDL cholesterol compared with omnivorous diets in the same study populations.
These are not trivial observations for cancer risk. Obesity, insulin resistance, and chronic inflammation — all of which are influenced by these metabolic markers — are established drivers of cancer biology across multiple tumour types. A dietary pattern that reliably lowers body weight, insulin resistance, and systemic inflammation is, by that mechanism, reducing cancer risk.
On cancer incidence directly, the evidence is more mixed. Large prospective cohort studies — including data from the EPIC cohort and the Adventist Health Studies — have found that vegetarian and vegan dietary patterns are associated with modestly lower overall cancer incidence compared to high meat-consuming omnivorous diets, with the strongest signals for colorectal and gastrointestinal cancers. However, the effect sizes are modest and vary substantially by study, population, and which dietary comparison is made.
The most robust cancer-specific finding in the plant-based literature relates to colorectal cancer and processed meat. The World Cancer Research Fund classifies processed meat as a Group 1 carcinogen — there is sufficient evidence that it causes colorectal cancer. A dietary pattern that substantially reduces or eliminates processed meat therefore has a direct, evidence-based cancer-risk benefit, independent of any other features of plant-based eating.
For breast cancer, the picture is more complex. Some cohort data suggest vegetarian women have modestly lower breast cancer risk; other studies find no significant difference. The mechanistic arguments are plausible — plant-based diets tend to lower circulating oestrogen levels in postmenopausal women, in part through effects on body weight and gut microbiome composition — but the clinical trial evidence to confirm this pathway in humans is limited.
What the European Heart Journal paper actually found
The 2023 European Heart Journal analysis is notable for its honesty about both benefits and risks of plant-based dietary patterns — which is why it was chosen as an anchor study for this post rather than a more advocacy-oriented review.
The paper found consistent cardiovascular and metabolic benefits for vegetarian and vegan diets. It also identified nutritional risks: vegetarian and especially vegan diets are associated with lower intakes of vitamin B12, vitamin D, calcium, iodine, zinc, omega-3 fatty acids, and complete protein. These are not hypothetical concerns — deficiencies in these nutrients have real health consequences, including for immune function and, in the case of B12 and folate, DNA methylation integrity.
The paper’s conclusion is that plant-based dietary patterns offer genuine health benefits when well-planned, but require active attention to nutritional adequacy — particularly for B12 (which cannot be obtained reliably from plant foods and requires supplementation on a vegan diet), vitamin D, and iodine.
This is the calibrated, evidence-based position. Not “eat plants and prevent cancer.” Not “plant-based diets are dangerous.” Rather: the pattern has genuine benefits, and those benefits need to be secured without creating new deficits.
What this means in practice
Increase plant food diversity. The strongest evidence supports eating a wide variety of plant foods — not simply reducing animal foods, but actively increasing the range of vegetables, legumes, whole grains, nuts, seeds, and fruits. Diversity matters because different plant foods provide different phytonutrients, fibre types, and micronutrients. A Mediterranean-style diet that is predominantly but not exclusively plant-based captures most of these benefits without the nutrient risk profile of a fully vegan diet.
Reduce or eliminate processed meat. This is where the cancer evidence is clearest. The Group 1 carcinogen classification for processed meat is based on consistent data across multiple large cohort studies. Reducing bacon, sausages, ham, salami, and other processed meats is the single most direct dietary change supported by the cancer prevention literature.
Eat red meat in moderation. Unprocessed red meat is classified as a Group 2A probable carcinogen for colorectal cancer — the evidence is suggestive but not as strong as for processed meat. Current World Cancer Research Fund guidance suggests limiting red meat to around three portions per week.
Plan for nutrients of concern on plant-heavy diets. If you eat primarily or fully plant-based, B12 supplementation is non-negotiable. Vitamin D status is worth monitoring. Iodine (found primarily in dairy and seafood) and omega-3 fatty acids (found primarily in oily fish) require conscious attention. These are solvable problems — but they require solving.
Quality and processing matter more than animal vs plant. An ultra-processed vegan diet is not a cancer-protective diet. A whole-food diet that includes some fish, eggs, and dairy alongside abundant plants is substantially better evidenced than a processed-food diet that happens to be plant-labelled.
What the evidence does not show
The evidence does not show that plant-based diets prevent cancer in individuals. Population-level associations between dietary patterns and cancer incidence say nothing about what will happen to any particular person. Cancer is influenced by genetics, epigenetics, infection history, environmental exposures, and factors that dietary change cannot modify.
The evidence also does not resolve whether the benefits of plant-based dietary patterns come from what is eaten more (vegetables, legumes, whole grains), what is eaten less (processed meat, red meat, saturated fat), or from secondary effects on body weight and insulin sensitivity. This distinction matters because it affects what a practical, evidence-based dietary pattern actually looks like — and suggests that the category of “plant-based” is less important than the quality and diversity of what is on the plate.
The bottom line
Plant-based dietary patterns are consistently associated with lower body weight, better insulin sensitivity, and lower cardiovascular risk — all of which are relevant to cancer biology. The direct cancer evidence is strongest for reducing processed meat, which carries a clear, consistent colorectal cancer signal. For other cancer types, the associations are real but modest, and heavily dependent on the overall quality of the dietary pattern rather than whether it excludes animal products. The most evidence-supported approach is not strict veganism or strict omnivorism, but a diverse, whole-food, predominantly plant-based diet with adequate attention to nutrients that plant foods alone may not reliably provide.
Scientific references
- Hemler EC, Hu FB. Plant-based diets for cardiovascular disease prevention: all plant foods are not created equal. Current Atherosclerosis Reports. 2019;21(5):18. PMID: 30895476. DOI: 10.1007/s11883-019-0779-5
- Kouvari M, et al. Vegetarian and vegan diets: benefits and drawbacks. European Heart Journal. 2023. DOI: 10.1093/eurheartj/ehad436
- World Cancer Research Fund / American Institute for Cancer Research. Diet, Nutrition, Physical Activity and Cancer: A Global Perspective. Continuous Update Project Expert Report. 2018. wcrf.org/dietandcancer
- Patel AV, et al. American Cancer Society guideline for diet and physical activity for cancer prevention. CA: A Cancer Journal for Clinicians. 2020;70(4):245–271. DOI: 10.3322/caac.21591
- Dinu M, et al. Vegetarian, vegan diets and multiple health outcomes: a systematic review with meta-analysis of observational studies. Critical Reviews in Food Science and Nutrition. 2017;57(17):3640–3649. PMID: 26853923. DOI: 10.1080/10408398.2016.1138447
This article is written for educational and informational purposes only. It is not intended as medical advice and should not be used to diagnose, treat, prevent, or cure any medical condition, including cancer. If you have a cancer diagnosis or are undergoing treatment, please discuss any lifestyle or dietary changes with your oncologist or medical team before making them. The studies cited reflect current research at the time of writing; the field is evolving rapidly and findings may be updated or superseded. Chloe Archard is a nutritional health coach, not a medical doctor.
