A low-protein high-carbohydrate diet like those practiced in parts of the Mediterranean and Japan may be the best for longevity and healthy brain ageing, according to a study on mice conducted by the University of Sydney released on Wednesday.

The study showed that mice fed an unrestricted low-protein high-carbohydrate diet displayed improvements in overall health and brain health, as well as learning and memory.

"There are currently no effective pharmaceutical treatments for dementia. We can slow these diseases, but we can't stop them. So it's exciting that we are starting to identify diets that are impacting how the brain ages," lead author Devin Wahl said.

According to the latest results, similar benefits to that of a low-calorie diet, which is widely known for its longevity benefits, are largely unsustainable in most cases.

"We have close to 100 years of quality research extolling the benefits of calorie restriction as the most powerful diet to improve brain health and delay the onset of neurodegenerative disease in rodents," the lead author said.

"However, the majority of people have a hard time restricting calories, especially in Western societies where food is so freely available," Devin said.

"It shows a lot of promise that we have been able to replicate the same kind of gene changes in the part of the brain responsible for memory that we also see when we severely restrict calories," he added.

Meanwhile, some parts of the world have already been enjoying the benefits of a low-protein high-carbohydrate diet as a function in their daily life.

Many cultures around the world, including parts of the Mediterranean and Japan have been consuming a diet similar to that of the study for centuries, senior author Professor David Le Couteur said.

On average, Japanese people have the second-highest life expectancy in the world, a statistic which has been directly attributed to their diet.

"The traditional diet of Japan's Okinawa is around nine percent protein, which is similar to our study, with sources including lean fish, soy and plants, with very little beef," David said.