It’s been a frequently debated controversy within scientific circles. Does restricting calories actually help living things live longer? A new study has found that in rhesus monkeys at least, less caloric intake helps them live longer, healthier lives.
Two competing teams collaborate for new results
A remarkable collaboration between two competing research teams — one from the University of Wisconsin–Madison and one from the National Institute on Aging — is the first time the groups worked together to resolve one of the most controversial stories in aging research.
The findings by the collaboration — including Senior Scientist Ricki Colman of the Wisconsin National Primate Research Center and UW–Madison Associate Professor of Medicine Rozalyn Anderson; and NIA Staff Scientist and Nonhuman Primate Core Facility Head Julie Mattison and Senior Investigator and Chief of the Translational Gerontology Branch Rafael de Cabo — were publishe in the journal Nature Communications.
In 2009, the UW–Madison study team reported significant benefits in survival and reductions in cancer, cardiovascular disease, and insulin resistance for monkeys that ate less than their peers. In 2012, however, the NIA study team reported no significant improvement in survival, but did find a trend toward improved health.
“These conflicting outcomes had cast a shadow of doubt on the translatability of the caloric-restriction paradigm as a means to understand aging and what creates age-related disease vulnerability,” says Anderson, one of the report’s corresponding authors.
Working together, the competing laboratories analyzed data gathered over many years and including data from almost 200 monkeys from both studies. Now, scientists think they know why the studies showed different results.
Different diets fed to two separate groups
The University of Wisconsin (UW) study found that rhesus monkeys that were fed a calorie-restricted diet, which contained 30 percent fewer calories than a control group’s diet, survived to about 28 years for males and about 30 years for females—above average for such primates in captivity.
In contrast, the National Institute on Aging (NIA) study found no significant effect of calorie restriction on survival.
As they age, rhesus monkeys are vulnerable to many of the same diseases as humans, including cancer, heart disease, and diabetes. Data from both studies showed fewer age-related health conditions in the calorie-restricted groups compared to controls.
First, the animals in the two studies had their diets restricted at different ages. Comparative analysis reveals that eating less is beneficial in adult and older primates but is not beneficial for younger animals. This is a major departure from prior studies in rodents, where starting at an earlier age is better in achieving the benefits of a low-calorie diet.
Second, in the old-onset group of monkeys at NIA, the control monkeys ate less than the Wisconsin control group. This lower food intake was associated with improved survival compared to the Wisconsin controls. The previously reported lack of difference in survival between control and restricted groups for older-onset monkeys within NIA emerges as beneficial differences when compared to the UW–Madison data. In this way, it seems that small differences in food intake in primates could meaningfully affect aging and health.
Third, diet composition was substantially different between studies. The NIA monkeys ate naturally sourced foods and the UW–Madison monkeys, part of the colony at the Wisconsin National Primate Research Center, ate processed food with higher sugar content. The UW–Madison control animals were fatter than the control monkeys at NIA, indicating that at nonrestricted levels of food intake, what is eaten can make a big difference for fat mass and body composition.
Finally, the team identified key sex differences in the relationship between diet, adiposity (fat), and insulin sensitivity, where females seem to be less vulnerable to adverse effects of adiposity than males. This new insight appears to be particularly important in primates and likely is translatable to humans.
Results show “dieting” affects aging
The upshot of the report is that caloric restriction does indeed seem to be a means to affect aging. However, for primates, age, diet and sex must all be factored in to realize the full benefits of lower caloric intake.
In the study, the researchers reported that any animal that died underwent a complete necropsy by a board-certified pathologist.
A gross description of the pathology related to each organ was provided along with the probable cause of death and any contributing factors. An exact cause of death was not always evident and, in some cases, more than one contributing factor was noted.
In these cases, the predominant factor listed on the gross pathology report was listed as the cause of death. Survival data were analyzed in two ways: all-cause mortality and age-related deaths.
In the new study, researchers compared data from the two previous studies and provided updated longitudinal comparisons. They presented several factors that likely contributed to the different outcomes, including diet composition, feeding regimens, age of onset, and genetic background.
NIA fed their monkeys a naturally sourced diet comprised of varied protein sources, while the UW diet was purified with limited ingredients and contained a significantly higher amount of sucrose compared to the NIA diet. Additionally, the different timing of feeding and access to food may have also contributed to different results.
The UW cohort was more homogenous in age and genetics than the NIA cohort, complicating some comparisons. Finally, the UW monkeys were adults of Indian origin, while NIA’s included both young and old monkeys of Indian and Chinese origin.
Given the similarities between rhesus monkeys and humans, the beneficial effects of calorie restriction on health and life span could also be observed in humans, researchers concluded.
However, more research is needed to study how a diet with fewer calories impacts humans as they age.