Diet is one of the few modifiable factors.
Cancer is many diseases, with many causes. Diet alone does not cause or cure cancer, but the evidence linking specific foods to specific cancers is now strong enough that the World Health Organization, World Cancer Research Fund and most national cancer authorities recommend reducing red and processed meat and increasing plants.
In 2015, the IARC, the WHO's cancer-research arm, classified processed meat (bacon, ham, sausages, salami, cured meats) as Group 1 — 'carcinogenic to humans' — for colorectal cancer, the same category as tobacco and asbestos. Red meat (beef, pork, lamb) was classified as Group 2A — 'probably carcinogenic.' Neither classification means meat is as harmful as smoking; it means the evidence of an association is similarly strong.
Fibre intake, fruit and vegetable consumption, legumes, whole grains, and the broader 'Mediterranean / plant-forward' eating pattern are consistently associated with lower rates of colorectal, breast, prostate, oesophageal and stomach cancers. The Adventist Health Study-2 found vegans had 16% lower overall cancer risk than non-vegetarians, with particular reductions in female-specific cancers.
High dairy intake (particularly milk) is associated with increased prostate cancer risk in multiple large cohort studies, including EPIC, the Physicians' Health Study and the Nurses' Health Study. The mechanism is debated (calcium, IGF-1, hormones), the association is robust. Plant-based men have notably lower prostate cancer rates.
"Processed meat sits in the same evidence category as tobacco — Group 1. The category is about strength of evidence, not magnitude of harm."
Build the daily diet around vegetables (especially cruciferous: broccoli, kale, cabbage, cauliflower), legumes (beans, lentils, chickpeas, soya), whole fruits, whole grains, nuts and seeds. Berries, garlic, onions, tomatoes, turmeric and green tea each have plausible anti-cancer mechanisms, though no single food prevents cancer.
The single biggest dietary factor is dwarfed by smoking. If you only do one thing, don't smoke.
Alcohol is a Group 1 carcinogen for breast, oral, oesophageal, liver and colorectal cancer. There is no safe level for cancer risk, though small amounts have small effects.
Physical activity lowers colon, breast and endometrial cancer risk independently of weight.
Obesity is linked to thirteen cancers. Plant-based diets help with weight management.
Pooled cohort meta-analyses. 1.00 = average risk; higher is worse.
Bouvard et al., IARC Monograph 114, 2015
No. Cancer has many causes. Diet shifts probabilities; it does not provide immunity. Don't smoke, limit alcohol, screen on schedule, move daily.
Decades of studies now show soya is protective, not harmful. Women who ate soya in childhood and adolescence have lower breast cancer rates as adults.
Same evidence category, very different magnitude. Smoking raises lung cancer risk roughly 20-fold; daily processed meat raises colon cancer risk roughly 1.18-fold per 50 g/day. Both real, very different scale.
It can worsen risk by producing heterocyclic amines and polycyclic aromatic hydrocarbons. Plant foods do not produce these compounds in significant amounts at normal cooking temperatures.
The same plant-based pattern that reduces cancer risk also helps with heart disease, diabetes and blood pressure. One change, many benefits.