Monkeys fed a severely low-calorie diet didn’t live longer than their normal-diet peers, a 23-year study showed, contradicting research that suggests living thin on greatly curtailed food intake extends lifespans.
The study by researchers from the National Institute on Aging sought to address whether diet restrictions had health benefits in rhesus monkeys, long-lived primates like people, thus rendering clues about human aging. Though the thin monkeys seemed healthier by some measures, calorie restriction failed to alter either cause of death or survival, the research showed.
Since the 1930s, reducing food intake has been shown to boost the lifespan of yeast, worms, rats, flies and some mice. Research published in the journal Science in 2009 from the Wisconsin National Primate Center showed that rhesus monkeys on the restricted diet lived longer and had fewer diseases than normal monkeys.
“It will be valuable to continue to compare findings from ongoing monkey calorie restriction studies to dissect the mechanisms behind the improvement in health that occurred with and without significant effects on survival,” researchers wrote in the paper published today in the journal Nature.
Studies on low-calorie diets have allowed scientists to explore how the aging process works. Many have hoped that caloric restrictions could extend life and improve health in people, said Steven Austad, a researcher at the University Texas Health Science Center at San Antonio, in an editorial accompanying the study.
The diet, comprised of 30 percent fewer calories than standard, boosted the metabolism of those monkeys who adopted it in old age. Those monkeys who adopted it at younger ages didn’t show any improvements.
The conflicting results from the Wisconsin and NIA research may be due to differences in the studies’ control monkey diets, said Julie Mattison, a staff scientist at the Bethesda, Maryland-based NIA and author of today’s study, in a telephone interview.
“These monkeys were healthier than really healthy control counterparts,” Mattison said. “So it’s quite possible that we’ve maximized our lifespan effect.”
In the Wisconsin study, the animals were allowed to eat freely during the day, whereas the rhesus monkeys in today’s study had a more-controlled diet. The foods in each study were slightly different, said Ricki Colman, a senior scientist at the Wisconsin National Primate Research Center, who was involved in the 2009 research.
“We have shown in the Wisconsin study, in which the control animals can eat as much as they choose during the daytime, more like a ‘normal’ human diet, that there are benefits to caloric restriction,” Colman said in an e-mail. “Perhaps these are harder to detect when compared to animals eating a diet more like an ’ideal’ human diet.”
The source of calories, rather than or in addition to the quantity, may matter, Mattison said.
Also, the genetic makeup of the animals may explain the differences, Colman said. Her group used Indian rhesus monkeys, while the NIA animals are mixed Indian and Chinese. All of the Wisconsin monkeys were born at the lab, which isn’t the case for the NIA monkeys, not all of whom have known birthdates. Any of these differences, alone or together, may explain the differences in findings, researchers said.
The Wisconsin and NIA researchers plan to work together to compare information and understand the differences, Colman and Mattison said.
Additionally, some human studies on the effects of caloric restriction are under way. The Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy, or Calerie, is assessing the effects of restricted-calorie diets in people over the course of two years.