The factors with the strongest evidence

Major cancer bodies estimate that a substantial proportion of cancers could be prevented through modifiable factors. The list is short and consistent.

  • Tobacco — the largest preventable cause, linked to lung, mouth, throat, oesophageal, bladder, kidney, pancreatic, stomach, cervical, and colorectal cancers, among others.
  • Excess body weight — associated with at least a dozen cancers including postmenopausal breast, colorectal, endometrial, kidney, liver, and pancreatic.
  • Alcohol — a recognised cause of cancers of the mouth, throat, oesophagus, larynx, liver, colorectum, and breast, with risk rising from relatively low intakes.
  • Ultraviolet radiation — from sun and tanning beds, the main driver of skin cancer.
  • Infections — HPV, hepatitis B and C, and Helicobacter pylori all cause specific cancers, and all are preventable or treatable.
  • Physical inactivity — associated with higher risk of colon, breast, and endometrial cancers.
  • Processed meat — classified as carcinogenic to humans, with the clearest link to colorectal cancer.

Diet, weight, and activity

Cancer prevention guidance on diet converges with general healthy eating advice, which is reassuring rather than disappointing.

  • Fibre and whole grains — higher intake is associated with lower colorectal cancer risk.
  • Vegetables, fruit, and legumes — plant-forward patterns are consistently associated with lower risk across several cancers.
  • Limit processed meat, and moderate red meat.
  • Limit sugary drinks and highly processed foods, largely through their effect on body weight.
  • Physical activity — meeting the 150 to 300 minute weekly target is associated with lower risk of several cancers.

Weight is the mechanism behind a good deal of this. Excess adipose tissue raises circulating oestrogen after menopause, increases insulin and related growth factors, and maintains low-grade inflammation — all plausible routes to increased cancer risk.

Some cancers are caused by infections, which makes them directly preventable — an unusual and underappreciated opportunity.

  • HPV vaccination prevents infection with the types that cause nearly all cervical cancers and a large share of anal, penile, vaginal, vulvar, and oropharyngeal cancers. It is routinely recommended in adolescence, with catch-up vaccination for some adults.
  • Hepatitis B vaccination prevents infection that can lead to liver cancer.
  • Hepatitis C is now curable with treatment, which reduces liver cancer risk. One-time screening is recommended for adults.
  • Helicobacter pylori infection raises stomach cancer risk and can be treated when identified.

Vaccination is prevention in the most literal sense here: it prevents the infection that causes the cancer.

Environmental and occupational exposures

Some exposures matter and are worth knowing about, without treating every chemical headline as equivalent.

  • Radon — a naturally occurring radioactive gas and a leading cause of lung cancer in people who do not smoke. Home testing kits are inexpensive and mitigation is straightforward.
  • Asbestos — still present in older buildings; disturbing it during renovation is the risk.
  • Air pollution — outdoor particulate pollution is classified as carcinogenic.
  • Occupational exposures — certain industries carry defined risks; workplace safety programmes exist for good reason.
  • Secondhand smoke — a recognised cause of lung cancer in people who have never smoked.

Hazard classifications describe whether something can cause cancer under some conditions, not how much risk a given exposure carries. That distinction is routinely lost in headlines.

Evaluating prevention claims

Cancer attracts more unsupported claims than almost any other health topic. A few principles help.

  • No single food, spice, juice, or supplement prevents or cures cancer. Claims that one does are not supported.
  • Detoxes and cleanses do not remove carcinogens; your liver and kidneys handle clearance.
  • Antioxidant supplements have not reduced cancer risk in trials, and some have shown increased risk in specific groups.
  • Alkaline diets do not change blood pH, which is tightly regulated.
  • Be cautious with anyone selling the product they are citing evidence for.
  • Never delay or replace conventional treatment with an alternative approach; that is associated with worse survival.

Some cancers occur without any identifiable modifiable cause. Reducing risk shifts odds; it does not confer immunity, and a cancer diagnosis is not a personal failure.