Australian scientists hope they might soon be able to repair the damaged DNA of astronauts and children with cancer using a compound found in fruit and vege.

Scientists are a step closer to unlocking the mystery of how to reverse the ageing process.

They've discovered that a natural compound found in fruit and vegetables including avocado and broccoli can help repair DNA that's been damaged by ageing and radiation exposure.

Their discovery has attracted interest from US space agency NASA and sparked hope that it might one day help improve the long-term health of children with cancer.

So far the scientists from the University of NSW and Harvard Medical School in the US city of Boston have only tested the compound known as NMN in mice.

They plan to start clinical trials to test its safety in 25 people later this year in Boston.

Their work, published in the prestigious journal Science on Friday, builds on previous studies demonstrating the anti-ageing effects of NMN in older mice.

The latest study focused on NMN's ability to repair DNA, which gets damaged every time we go out into the sun or are exposed to radiation.

The ability of our cells to repair the damage decreases as we age.

The scientists found that giving mice a dose of NMN in their drinking water improved the ability of their cells to repair DNA damaged by radiation or old age.

"The cells of the old mice were indistinguishable from the young mice after just one week of treatment," the study's lead author professor David Sinclair, who works at the UNSW and Harvard and shot to fame after identifying anti-ageing qualities in red wine more than a decade ago.

"This is the closest we are to a safe and effective anti-ageing drug that's perhaps only three to five years away from being on the market if trials go well."

NASA is working with Prof Sinclair and his UNSW colleague Dr Lindsay Wu to see if NMN could help protect astronauts from having their body cells age prematurely as a result of being exposed to cosmic radiation.

The scientists won a NASA competition in 2016 designed to help the space agency send astronauts to Mars.

"That trip will take two years in each direction, so that's four years of being exposed to cosmic radiation and this is likely to cause a substantial amount of DNA damage to those astronauts," Dr Wu told AAP.

The scientists believe NMN could also help repair the damaged DNA of aircraft passengers exposed to cosmic radiation as well as children with cancer who have undergone radiotherapy.

Dr Wu said nearly all childhood cancer survivors go on to develop by age 45 chronic illnesses including cardiovascular disease, type 2 diabetes, and Alzheimer's - conditions usually associated with older adults.

The radiotherapy is blamed for causing the children's cells to age prematurely.

"Our primary goal at the moment is treating the side effects of radiotherapy primarily because that's where the biggest need is," Dr Wu said.

"There's nothing else out there to treat those side effects."