"Rather, it helps to explain how some dietary patterns that are already very well established to promote health and longevity work," he said.
"Examples are the traditional Okinawan diet, which is associated with the longest lifespans of any human population, and also the Mediterranean diet, which is also among the healthiest diets and associated with long lifespans. Both diets have notably low protein levels.
"Our study shows that the health benefits of such diets - with low protein in relation to carbohydrate - involve the signalling hormone FGF21.
"The work also shows why some popular diets that are designed to avoid carb intake by increasing protein intake might in the long term be doing tremendous harm - for example the paleo diet."
But New Zealand experts cautioned against jumping to conclusions. Auckland University Professor Wayne Cutfield said the Sydney study was based on mice, not humans, and looked at only one hormone called FGF21 out of a large number of hormones and other factors which interacted with each other.
"FGF21 is one of many factors involved in regulating our lifespan. To say that FGF21 is 'the one' is just not true," he said.
The hormone, which is produced in the liver, was dubbed the "fountain of youth" by American scientists in January because it was found to extend life in mice.
The Sydney study, using older mice, found that the mice produced more FGF21, and lived longer, when they were low in protein generally, and especially when they were fed diets with low protein and high carbohydrates.
However there was a feedback effect - the FGF21 stimulated the mice to eat more protein and less carbohydrate.
Solon-Biet said her team had previously found that a low-protein, high-carb diet also increased FGF21 levels in humans - but they had to be the right kind of carbohydrates.
"It's a diet low-to-moderate in protein and high in complex carbohydrates that promote gut health, such as wholegrains, barley, brown rice and oats - as opposed to high in sugars," she said.
"[Also] these mice were old animals. That's important, because a high-protein diet is better for reproduction - testosterone levels and a healthy menstrual cycle. But a low-protein, high-carbohydrate diet is better in late life."
Herald health and fitness commentator Lee-Anne Wann said people should not rush out and eat more carbohydrates to increase their FGF21.
"It's great that they are looking at it. We've looked at other things like leptin which help manage our appetite, we are always finding new things," she said. "But it's too soon to be saying, 'Oh my god, we should be increasing carbohydrates."
She said people were all different.
"Some of us need a high level of good natural carbohydrates. Some of us work better with very limited carbohydrates," she said.
"One diet does not fit all, so let's not jump on this boat too quickly."
