How Does Diet Affect Cancer Risk and Which Foods Are Most Protective?

The short answer

Diet accounts for an estimated 30–35% of cancer risk. The International Agency for Research on Cancer (IARC) classifies processed meat as a Group 1 carcinogen (causes cancer) and red meat as a Group 2A carcinogen (probably causes cancer). High dietary fibre intake reduces colorectal cancer risk by 15–20%. Alcohol is a Group 1 carcinogen for at least 7 cancer types.

What the evidence actually shows

The World Cancer Research Fund's Third Expert Report (2018), the most comprehensive review of diet and cancer, found convincing evidence that: processed meat increases colorectal cancer risk (each 50g/day increases risk by 16%); alcohol increases risk of breast, colorectal, oesophageal, liver, and head and neck cancers; excess body fat increases risk of at least 12 cancers; and dietary fibre reduces colorectal cancer risk. IARC (Bouvard et al., 2015) classified processed meat as Group 1 (carcinogenic to humans) and red meat as Group 2A (probably carcinogenic). Reynolds et al. (2019) in The Lancet found that high fibre intake was associated with a 15–20% lower colorectal cancer risk.

"Each 50g/day of processed meat was associated with a 16% increased risk of colorectal cancer."

— World Cancer Research Fund, Third Expert Report 2018

What the major health authorities say

The NIH National Cancer Institute recommends a diet rich in fruits, vegetables, and whole grains, and low in processed and red meat, as a cancer prevention strategy. MedlinePlus identifies diet as a modifiable cancer risk factor and recommends limiting alcohol, processed meat, and red meat. The NIA recommends a healthy diet as a cornerstone of cancer prevention in older adults.

Practical implications

The most impactful dietary changes for cancer prevention are: limiting processed meat (bacon, sausage, deli meats) to occasional consumption; limiting red meat to no more than 500g (cooked weight) per week; limiting alcohol; maintaining a healthy body weight; and increasing dietary fibre to 25–30g/day through whole grains, legumes, and vegetables. Cruciferous vegetables (broccoli, Brussels sprouts, cabbage) contain glucosinolates that are converted to isothiocyanates — compounds with anti-cancer activity in laboratory studies — but human evidence for specific cancer risk reduction is weaker than for fibre and processed meat.

Where reasonable people still disagree

• The magnitude of cancer risk from red meat versus processed meat — whether unprocessed red meat at moderate consumption levels carries meaningful cancer risk.
• Whether specific 'superfoods' (berries, cruciferous vegetables, green tea) provide meaningful cancer risk reduction beyond the overall dietary pattern.
• The role of ultra-processed food consumption in cancer risk, independent of its effects on obesity and caloric intake.