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The Dutch swimmer won gold in Beijing, having been diagnosed with cancer seven years earlier.

We hear why he decided to stop competing, and instead turn his attention to charity fundraising - setting up his own foundation and raising millions for cancer research through long-distance swimming endurance events.

And how he ensures that the money raised is correctly managed.

Produced and presented by Matthew Kenyon.

(Image: Maarten van der Weijden setting a new world record in 2021, by swimming continuously against a current for more than 31 hours and 7 minutes. Credit: Getty Images)