Regular physical activity is consistently associated with a lower risk of 13 or more types of cancer, with risk reductions of 5 to 42%. The effect already occurs at 7,000 to 9,000 steps per day. Note: exercise is associated with a higher risk of melanoma, likely due to greater sun exposure.
Yes, exercise lowers the risk of most types of cancer, and the evidence for this is robust. In a pooled analysis of 1.44 million adults (PMID 27183032), a high level of leisure-time physical activity was associated with a lower risk of no fewer than 13 types of cancer. The risk reduction ranged from 10% (breast cancer) to 42% (oesophageal cancer), depending on the type. A separate systematic analysis of 98 studies involving more than 16 million participants (PMID 37743572) confirms this: for breast, colorectal, lung, stomach and liver cancer, greater physical activity was consistently associated with 5 to 17% lower risk. For six cancer types (bladder, breast, colon, uterine, oesophageal and gastric cardia cancer), the evidence is even strong and consistent across multiple large studies (PMID 32741068, 31626056).
You do not need to do an extreme amount to see an effect. A study of 85,394 participants from the UK Biobank, in which physical activity was measured objectively using a wrist accelerometer (PMID 40139674), showed that 7,000 to 9,000 steps per day was already associated with 11 to 16% lower cancer risk compared with 5,000 steps. Beyond that point the effect levelled off: even more steps did not lead to clearly additional benefit. Light activity, such as gentle walking or sitting less, was also already associated with 6% lower risk. You therefore do not need to exercise intensively to gain some benefit.
An important safety note: the same large study of 1.44 million participants (PMID 27183032) also found that a high level of physical activity was associated with a HIGHER risk of melanoma (a serious form of skin cancer), specifically 27% higher, and of prostate cancer, 5% higher. The increased melanoma risk is likely explained by greater sun exposure during outdoor sports. People who exercise a great deal outdoors would be wise to use adequate sun protection.
Prolonged sitting represents a separate risk, independent of how much you exercise. Even if you are physically active on a regular basis but spend most of the rest of the day sitting, the risk of colon, uterine and lung cancer may remain elevated (PMID 32741068, 31626056, 28759385). In other words, exercising does not fully compensate for prolonged sitting. Regularly getting up and moving throughout the day therefore appears worthwhile in addition to regular sporting activity.
From a biological standpoint, there are plausible explanations for the protective effect of exercise. Physical activity lowers sex hormones such as oestrogen, improves the body's response to insulin, reduces chronic low-grade inflammation and may influence the immune system, DNA methylation and the gut microbiome (PMID 32741068, 28718417, 28759385). These mechanisms are biologically credible, but the evidence from human studies is not yet fully conclusive. Moreover, all available data are observational: a direct causal relationship is plausible but has not yet been definitively proven through large-scale randomised trials.
All claims are based on large observational studies and systematic analyses (up to 16 million participants). No randomised controlled trials are available that test cancer as an endpoint. Causality is plausible but not proven. Meta-analyses used as a direct source: 1 (PMID 37743572, systematic analysis of 98 studies).