Life expectancy for humans is unlikely to reach 100 years or beyond for quite some time to come, if ever, a US scientist believes.

Jay Olshansky, Public Health Professor at the University of Illinois, at Chicago, said on current trends life expectancy rates would not reach 100 until at least the 22nd Century in France and the 26th Century in the United States.

This pessimistic outlook comes as research suggests the lifespan of a mouse can be greatly increased by anti-oxidant treatment.

The US team behind the research has already doubled the lifespan of a worm. The Buck Institute team believes that, one day, a similar approach might help humans live longer.

Slowing down

Professor Olshansky's assessment, to be published in the journal Science this week, is based on an analysis of data on death rates and life expectancy for men and women of all ages in Japan, France, and the United States.

Professor Olshansky said the assessment showed that while many people were living longer, the rise in life expectancy was slowing down.

The professor announced the results of his study early at the annual meeting of the American Association for the Advancement of Science in San Francisco.

The oldest person ever to live, according to documented records, was Frenchwoman Jeanne Calment, who died aged 122 in 1997.

Next leap

But Professor Jay Olshansky said the majority of people were unlikely to live beyond 85 because their bodies would simply wear out.

"The human body was not designed for long-term use," he said. "It was designed for short-term use and in effect what we're doing is pushing these bodies beyond the end of the warranty period for living machines.

"So when we survive into old age, just as with automobiles and race cars, things start to go wrong, and unless we can change the structure of the body itself or the rate at which ageing occurs, then inevitably things will go wrong as we push out the envelope of human survival," he said.

Professor Olshansky was optimistic that people would continue to live longer but the next quantum leap in life expectancy could only occur, he said, if "biomedical researchers can discover how to modify the ageing process and make such a discovery widely available to the entire population".

Evolving age

Also speaking to the AAAS meeting, Professor George Martin said that the body was the major flaw in boosting longevity as the evolutionary system had no particular interest in helping people live past their peak productive years.

Professor Martin, associate director of the Alzheimer Disease Research Centre at the University School of Medicine in Seattle, said there were thousands of genes in the human body which could go wrong in different ways in each person.

He said the human body could evolve its own systems for increasing life span, but that this was unlikely to be the same for everyone.

"The bad news is that there are so many different things that can go wrong as we age. These can be affected by an enormous number of potential inborn genetic variations that can modulate how we age," he said.

Dr Mark Eshoo, director of genomics at the Buck Institute, which is carrying out the mouse research, told BBC News: "I think all humans and all animals have a certain natural lifespan, but that lifespan can be often determined by their environment.

"Can we do the same thing in humans. Yes, we absolutely can."