A new comprehensive study has shown that reducing caloric intake slows down metabolism. Researchers believe the findings indicate that a low-calorie diet could extend lifespan and prolong health in old age.

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Previous studies on animals with short lifespans — such as worms, mice, and flies — have shown that reducing calorie intake might slow down metabolism and prolong life. However, demonstrating this effect on humans and other animals with long lifespans has proven quite difficult.

Researchers studied some of the people who participated in the multi-center trial CALERIE (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy), sponsored by the US National Institutes of Health. Scientists observed the effects of restricting calories for 2 years on metabolism in over 200 healthy, non-fat adult participants.

“The CALERIE trial has been important in addressing the question of whether the pace of aging can be altered in humans,” says Rozalyn Anderson, who studies aging at the University of Wisconsin–Madison. She leads one of two large, independent studies on calorie restriction in rhesus monkeys. “This new report provides the most robust evidence to date that everything we have learnt in other animals can be applied to ourselves.”

The latest paper, which was published on March 22nd in the journal Cell Metabolism, monitored 53 CALERIE participants recruited at the Pennington Biomedical Research Center in Baton Rouge, Louisiana. Researchers were able to track how the participants used energy with unprecedented precision thanks to state-of-the-art metabolic chambers —  small, hotel-like sealed rooms that measure oxygen and carbon dioxide concentrations every 60 seconds. Researchers calculated the ratio between the two gases and then analyzed occupants’ urinary nitrogen, indicating whether the occupant is burning fat, carbohydrate or protein.

Participants, with ages between 21 and 50, were randomly separated into two groups: the 34 people in the experimental group reduced their calorie consumption by an average of 15%, while the 19 people in the control group ate as usual. Next, researchers tested the participants annually to record overall metabolism and biological markers of aging, including damage associated with oxygen free radicals released during metabolism. At the end of the trial, participants spent 24 hours in the metabolic chamber.

Researchers discovered that the people who had dieted used energy much more efficiently while sleeping than the control group. Their base metabolism had essentially slowed down. In consequence, people in the experimental group lost an average of 9 kilos individually. All other measurements showed a reduced metabolic rate and fewer signs of aging.

“The Rolls-Royce of a human longevity study would carry on for many decades to see if people do actually live longer,” says Pennington physiologist Leanne Redman, the lead author of the latest study.

Low-calorie diets have previously been shown to extend life in different species, such as the short lifespan worm Caenorhabditis elegans, and in the fly Drosophila melanogaster. Following studies also revealed that mice with restricted diets can live up to 65% longer than mice allowed to eat freely. In addition, studies on monkeys suggest longer survival and reduced signs of aging.

Redman wants to repeat the study combining moderate calorie restriction with a diet rich in antioxidants to monitor oxidative stress, or with a drug such as resveratrol, which mimics key aspects of calorie restriction.

If researchers demonstrate the causality between caloric reduction and longer lives in humans, could you stick to such a diet?