Mice fed on high-protein, low-carbohydrate diets had shorter lifespans and more heart and diabetes problems than those with diets low in animal proteins and high in plant products, a study led by Sydney researchers has found.

The study found that the interaction between the major nutrients - protein, fat and carbohydrates - was critical to whether a diet was beneficial to health and longevity rather than the properties of the individual nutrients themselves.

''People talk about a balanced diet but no one knows what it is, particularly in terms of the balance of macronutrients, the ratio of protein to carbohydrate to fat,'' said David Le Couteur, a co-author of the study from the Charles Perkins Centre at the University of Sydney.

While the study was conducted on mice, the findings, published in the journal Cell Metabolism, are in line with recent epidemiological studies that have found people on low protein or vegetarian diets have better long-term health.

In the most comprehensive study of its kind, the scientists fed almost 900 mice on one of 25 diets, each with a different ratio of protein, carbohydrates and fat or with a reduced total energy content.

While mice on high-protein, low-carbohydrate diets had reduced body fat and consumed less food overall, they had shorter lifespans and poor heart health.

Professor Le Couteur said the negative health effects of high-protein diets appeared to be a high content of branched-chain amino acids, the supplement body builders took to bulk up their muscles, which were associated with ageing and other health problems.

While high-protein diets shortened lifespan, the researchers found the nutrient was the major driver of the animals' total food intake, a finding first established in insects.

''When mice reached a certain protein target they would stop eating,'' Professor Le Couteur said.

''If they didn't have enough protein they would overeat.''

By contrast, mice that consumed a high-carbohydrate, low-protein diet were slightly rounder from an increase in body fat, up to 10 per cent more than high-protein diet mice, but lived longer and had fewer age-related health problems.

A high-carbohydrate intake suppressed mice's protein appetite, which lowered branched-chain amino acids.

These mice lived, on average, 50 weeks longer than mice on high-protein diets.

Mice on low-protein, high-fat diets or those with a restricted calorie intake were the least healthy and died younger.

While it was generally the case that being obese was bad for human health, Professor Le Couteur said the reality was more complex, and it was possible for some people to be overweight but metabolically healthy.

''If you become overweight from eating a low-protein, high-carb diet, you may have better health outcomes compared with becoming overweight on a low-protein, high-fat diet from the cheap, nasty foods our society lives on these days,'' he said.