Spanning 23 years, the research found monkeys that ate fewer calories than non-dieting counterparts were healthier but did not live any longer.

Rhesus monkeys are a preferred choice for lab study, as they are long-lived primates like humans -- their average lifespan in captivity is 27 years and the usual maximum is 40 years.

The exceptionally long study, launched at the National Institute on Aging (NIA) in Maryland in 1987, saw monkeys of different ages fed a diet 30 percent lower in calories than others that followed a "normal," nutritious diet.

Animals in both groups lived on average longer than wild rhesus monkeys and were heavier too. None was malnourished, and they were given vitamin and mineral supplements, the researchers wrote in the journal Nature.

Those on the calorie-restricted diet had a lower incidence of diabetes, cardiovascular disease and cancer than the rest, and dieting males also had lower cholesterol.

"However, these effects did not translate directly to a beneficial effect in longevity," over the control monkeys, Rafael de Cabo of the NIA's Laboratory of Experimental Gerontology, told AFP.

The study was not designed to explain this phenomenon, and the authors say matching research to measure the impact of calorie restriction on longevity in humans was unlikely.

The findings seem to contradict those of other projects, including an ongoing study at the Wisconsin National Primate Research Centre (WNPRC) which has shown that rhesus monkeys placed on a restricted calorie diet lived longer.

These "mixed results" raised intriguing questions about the benefits of calorie restriction in primates, Steven Austad of the University of Texas' Barshop Institute for Longevity and Aging Studies wrote in a comment on the study, also published by Nature.

WNPRC senior scientist Ricki Colman told AFP there were many differences between the two studies that may explain the conflicting outcomes.

Importantly, the WNPRC control monkeys (those whose calorie intake was not restricted) were allowed free access to food, therefore mimicking a human in charge of his own nutrition intake.

In contrast, the NIA control group were given a limited amount of food, thus resembling an ideal human diet -- which may explain why they lived as long as the monkeys on the low-calorie diet.

They were also given vitamin and mineral supplements and the WNPRC group not.

Comparing the two approaches, a compelling picture emerges, said De Cabo: a healthy diet does improve longevity, and eating less of it may slow the onset of some diseases but will not actually prolong life.

Source-AFP