Exercise is a cornerstone of good health and evidence shows it can even help prevent cancers returning following treatment .

But new findings are raising an unexpected question : could very high-volume endurance training carry its own risks?

At the 2025 American Society of Clinical Oncology meeting, researchers from Inova Schar Cancer Institute reported that a surprising number of dedicated marathon and ultramarathon runners had precancerous growths in their colons.

Among 100 athletes aged 35 to 50, 15% had advanced adenomas – lesions that can develop into bowel cancer – while 41% had at least one adenoma.

The study is small and not yet peer-reviewed, but the signal is strong enough to have captured global attention. Here's what the findings really mean, why experts urg

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