Some advocate taking antioxidants like vitamin C and E to help protect the body from harmful chemical by-products it creates in breaking into a sweat.
But German scientists now believe these "free radicals" may actually be good for us and even buffer against diabetes, PNAS reports.
And mopping them up with antioxidants may do more harm than good.
It is thought that antioxidant vitamins may be able to prevent damage to the body's tissues called "oxidative stress" by eliminating the free radicals which are said to cause it.
This damage has been implicated in several major diseases including cancer and heart disease as well as normal ageing.