Restricting calories may help extend life by reducing age-related damage to cells, say researchers who studied what happens to the body during extreme diets.

Calorie-restricted diets provide all nutrients needed for a healthy life but minimize calories. Studies on mice and other animals show that cutting back calories by about 30 per cent improves metabolism and lowers the risk of age-related diseases such as cancer, heart disease and diabetes.

Anthony Civitarese of the Pennington Biomedical Research Center in Baton Rouge, La., and his colleagues focused on "oxidative damage" to the body from the diets.

Oxidants, such as damaging free radicals, are produced when food is converted to energy by a cell's mitochondria.

It's thought that calorie restriction may slow aging by lowering production of free radicals that damage the body's proteins and DNA.

Mitochondria, gene effects observed

The researchers tested the idea in 36 healthy, overweight young people who were not obese.

About one-third of the participants took in all of their energy needs from their diet, another group had their calorie intake cut by 25 per cent, and the rest had their calories restricted 12.5 per cent and exercised more to increase the number of calories burned by the same percentage.

"This study provides, to our knowledge, the first evidence that a 25-per-cent caloric deficit decreased 24-hour energy expenditure and improved mitochondrial function," the researchers wrote in Tuesday's issue of the online journal Public Library of Science Medicine.

The people taking in the fewest calories also showed an increase in the activity of the SIRT1 gene in their skeletal muscle, the researchers found.

The animal version of the gene regulates cell metabolism and has been linked to increased lifespan.

"The observed increase in muscle mitochondrial DNA in association with a decrease in whole body oxygen consumption and DNA damage suggests that caloric restriction improves mitochondrial function in young non-obese adults," the team concluded.

Longer-term studies are needed to see whether the benefits of calorie restriction can be sustained.