How well you respond to stress predicts how long you will live, at least if you are a little worm, U.S. scientists reported on Monday.

Genetically identical worms responded to stress in greatly different ways - and those with more active stress reactions lived much longer than worms with less active stress proteins, the researchers found. More active stress responses suggest the animal is coping with the stress.

The findings will almost certainly apply to humans in some way, they report in this week’s issue of the journal Nature Genetics.

Shane Rea of the University of Colorado at Boulder tested more than 10,000 nematodes known as Caenorhabditis elegans - a worm favored by scientists because it is easy to work with.

Despite its tiny size, C. elegans is genetically complex and has much in common with “higher” animals such as humans.

They genetically engineered the little transparent worms to carry a jellyfish gene called green fluorescent protein, which glows green under certain light. They tagged this gene to a gene called hsp-16.2, a stress protein found in most organisms that is associated with the health of cells.

The more active the hsp-16.2 gene was, the brighter the worms glowed green and, presumably, the better they coped with stress.

In a typical experiment, the worms that glowed the brightest green lived about 16 days, compared to about three days for those that glowed the most weakly - under identical conditions.

“We have shown it’s possible to predict the life span in an organism on the first day of adult life based on how it responds to stress,” said Thomas Johnson, a professor who helped lead the study.

“This is something that has not been done before, and has implications for human longevity and health.”

This gene now might be useful for predicting how robust an animal is. “We have engineered a single gene to monitor the health of an organism, which is a first,” said Johnson.

The stress gene itself probably does not decide how long an animal lives, the researchers wrote in their report, but instead reflects some as-yet unknown trait.

Most scientists say the lifespan of living creatures is affected by a combination of genetic, environmental and chance factors. Studies done in twins suggest that genes are only about 15 percent to 30 percent responsible for how long an otherwise healthy person will live.

“This work starts to address the question of why genetically identical organisms raised in identical environments still age at different rates,” said Rea.

It might be possible to test people for stress compounds such as hsp-16.2 and predict how long they will live.

“They might even be able to tweak each stress-response system and set them for maximum longevity, which is believed to be about 120 years,” Rea said.