Exercise triggers chemical changes in the body
Taking vitamins after exercise may undo some of the beneficial effects of the workout, research suggests.
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.
But Dr Michael Ristow, of the University of Jena, and his team have shown free radicals may have a positive effect on the body by increasing its sensitivity to insulin - something that is lost in type 2 diabetes.
And this effect is blocked by antioxidant vitamins.
Too much of a good thing
Men who took vitamin C and E supplements showed no changes in their free radical levels, whereas those who did not take these antioxidants showed increased levels of free radical oxidative stress.
After four weeks of intensive exercise training, insulin sensitivity was restored only in the group of men who did not take antioxidant supplements.
The men who took the vitamin supplements fared worse, metabolically.
Dr Sarah Aldred, a lecturer in exercise biochemistry at the School of Sport and Exercise Sciences at the University of Birmingham, said: "It doesn't mean that antioxidants like vitamin C and vitamin E are bad for us, it just means that sometimes we need to consider whether taking supplements is actually beneficial.
"As this study shows it is not actually always the case."
Dr Elisabeth Weichselbaum of the British Nutrition Foundation said: "Antioxidants protect the cells in your body from damage and therefore help to reduce the risk of certain diseases such as cancer.
"But you should not consume high doses on a regular basis as this can have negative effects on your body.
"If you stick to a healthy and varied diet, you generally get enough of the nutrients you need and you don't run the risk of consuming large amounts that may be harmful for you."