A calorie-restricted diet helps monkeys live longer, healthier lives, a new study suggests.
Previously, research teams from the University of Wisconsin-Madison and the US National Institute on Ageing (NIA) conducted separate calorie-restricted diet studies on rhesus monkeys. But the two teams had different findings.
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In 2009, the UW-Madison study team concluded that a calorie-restricted diet led to significant benefits in survival and reductions in cancer, heart disease and insulin resistance for monkeys.
Meanwhile, a 2012 NIA study found that restricting calories in the diet didn't significantly improve survival. But the NIA study said that calorie restriction did improve health.
The conflicting outcomes "cast a shadow of doubt" on how calorie restriction might help to understand ageing, said study co-corresponding author Rozalyn Anderson, an associate professor of medicine at UW-Madison.
The two teams decided to work together, and reviewed the findings from both studies.
Content of diet important
The researchers concluded that a calorie-restricted diet does help monkeys live longer, healthier lives. But age, diet and sex must all be factored in to realise the full benefits of lower caloric intake, the investigators found.
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For example, calorie restriction began at different ages in each study. The collaborative research team saw that calorie restriction was only beneficial for older monkeys.
The content of the diet made a difference, too, the study authors explained in a university news release. NIA monkeys were fed a "natural" diet. Monkeys at UW-Madison had a diet that included processed foods with a higher sugar content. The UW-Madison monkeys were heavier than the NIA monkeys.
The researchers also noted that female monkeys seemed to be less vulnerable to the bad effects of excess fat than males. The study teams believe this finding is likely similar in humans. However, studies in animals often fail to produce similar results in humans.
The study was published in the journal Nature Communications.
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