Sydney, Aug 13 - Scientists have discovered a natural defence mechanism against ageing.

They are amino acids working hard to protect our bodies from the ageing process caused by exposure to free radicals and oxygen.

Free radicals -- reactive chemical entities -- and other oxidative species are known to contribute to ageing, and are linked to cancers, heart disease and neuro-degenerative diseases.

'We have found that amino acids are remarkably resistant to damage by free radicals and oxygen,' said Chris Easton, professor at the Free Radical Centre, Australian National University (ANU), who led the team.

'What many people don't realise is that just by breathing in oxygen we are exposing our bodies to free radicals and oxidative stress which leads to ageing. But of course we need to breathe to stay alive, so it is one of the great paradoxes of life.'

The group simulated exposure to hydrogen peroxide and oxygen the amino acids in our bodies would incur simply from continuing to breath and other normal biological functions, and were surprised to find a lower than expected level of free radical damage.