A massive metabolic study has shown counting calories is not the secret to a longer life, but balancing protein and carbohydrates just might be.

A team at the University of Sydney's Charles Perkins Centre, which is focused on the study of obesity, diabetes and cardiovascular disease, has completed what may be the most comprehensive study of macronutrient balance ever undertaken.

Researchers found food intake is regulated primarily by dietary protein and carbohydrate, and not by the number of calories consumed.

Specifically, experiments showed that calorie restriction in mice, through high protein diets or dietary dilution, has no beneficial effects on lifespan. Researchers predict the phenomenon will apply in humans too.

A high protein, low carbohydrate diet reportedly resulted in reduced body fat and food intake, it also led to a shorter lifespan and poor cardiometabolic health.

By contrast, a high carbohydrate, low protein diet resulted in longer lifespan and better cardiometabolic health, despite also increasing body fat.

A low protein, high fat diet provided the worst health outcomes, with fat content showing no negative influence on food intake.

“This research has enormous implications for how much food we eat, our body fat, our heart and metabolic health, and ultimately the duration of our lives,” said Professor Steve Simpson, Academic Director of the Charles Perkins Centre and corresponding author of the study.

“We have shown explicitly why it is that calories aren't all the same - we need to look at where the calories come from and how they interact.”

“We now face a new frontier in nutrition research,” said co-author Professor David Le Couteur.

“This represents an enormous leap in our understanding of the impact of diet quality and diet balance on food intake, health, ageing and longevity.”

By examining mice fed a variety of 25 diets, the research team used an innovative state-space nutritional modelling method to measure the interactive effects of dietary energy, protein, fat and carbohydrate on food intake, cardiometabolic health and longevity.

The results suggest that lifespan could be extended in animals by manipulating the ratio of macronutrients in their diet - the first evidence that pharmacology could be used to extend lifespan in mammals.

“Up until this point, most research has either concentrated on a single nutritional variable, such as fat, carbohydrate or calories, so much of our understanding of energy intake and diet balance is based on one-dimensional single nutrient assessments,” Dr Le Couteur said.

“The advice we are always given is to eat a healthy balanced diet, but what does that mean? We have some idea, but in relation to nutritional composition we don't know terribly well. This research represents an important step in finding out.”

In terms of practical advice, the researchers predict that a diet with moderate amounts of high quality protein (around 15 to 20 percent of total calorie intake), that is relatively low in fat and high in good quality complex carbohydrates will yield the best metabolic health and the longest life.

More details are available in the full report, accessible here.