Researchers who set out to prove that a mega-dose of vitamin C can improve exercise performance in fact showed it has no effect at all, according to a report published on Friday.

The vitamin, given intravenously to younger and older volunteers, did nothing to help them work harder on a treadmill, the team at the University of Colorado found.

But the vitamin, given as ascorbic acid, did reduce oxidative stress - chemical damage done to cells and proteins in the body, the researchers found.

“We did see a decrease in oxidative stress with large doses of vitamin C, but this decrease didn’t improve aerobic abilities either for younger or older subjects,” said Christopher Bell, now at Colorado State University, who led the research.

The experiment was based on the theory that aging and weakness is caused to a large degree by oxidative stress - the same chemical process that leads to rusting. This can affect the heart’s ability to pump, for instance, as cells are damaged by day-to-day living.

The researchers tested 12 healthy but mostly inactive people aged 22 to 24, and 10 people aged 59 to 61. They checked their exercise ability several times - before any treatment, during treatment and after a month of tests.

Part way through they gave the volunteers daily 500 mg vitamin C tablets for a month. At the time of testing the volunteers randomly got salt water or ascorbic acid in a slow-drip infusion.

The researchers hoped to show that the huge doses of the vitamin given directly in the blood would increase breathing capacity and heart output.

But oxygen uptake and heart output were unaffected, they reported in the Journal of Applied Physiology. Volunteers given a saltwater solution did no worse than those given vitamins.

They could clearly show the vitamins were affecting the body - they reduced the oxidation of low density lipoprotein cholesterol, for instance, which in theory should reduce the effects of clogged arteries over time.

It could be that taking supplements only helps people who are lacking normal levels of a vitamin, Bell suggested. Everyone in his study was healthy.

Was the wrong antioxidant vitamin used? The researchers said they did not test other vitamins, such as vitamin E, but noted that other people have, with mixed results.