Sandy Szwarc shares the story we didn’t hear about monkeys on a calorie-restricted diet:
The long-awaited research on the effects of calorie restriction on aging in rhesus monkeys from the University of Wisconsin and Wisconsin National Primate Research Center have just been released. It found no statistically significant difference in the number of deaths among the monkeys who’ve been eating a calorie-restrictive diet for more than 20 years compared to the monkeys who’ve been allowed to eat ad lib all day as much as 20% over their normal calories.
As she points out, definitions are everything when it comes to health statistics:
The lower mortality claimed among the monkeys on the calorie restricted diet were achieved only after eliminating 37% of the monkey deaths. They defined mortality as “age-associated deaths” and eliminated any cause of death they didn’t believe was associated with aging. As the supplemental data explains, 16 deaths from “non-age-associated causes were censored and their age of death used as the time variable in the regression.”
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The non-aging-related causes of death included monkeys who died while taking blood samples under anesthesia, from injuries or from infections, such as gastritis and endometriosis. These causes may not be aging-related as defined by the researchers, but they could realistically be adverse effects of prolonged calorie restrictions on the animals’ health, their immune system, ability to handle stress, physical agility, cognition or behavior.
Humans already live longer than one might expect from other, similar organisms, but comparing them to lab animals is especially problematic. Professor João Pedro de Magalhães at the Integrative Genomics of Ageing Group at the University of Liverpool explains:
One problem is that all models organisms are considerably shorter-lived than humans and were developed for laboratory research based on their high fertility. Not only this means that there are different evolutionary processes acting on these organisms and on humans, but selection for fertility may have also selected for short lifespans in laboratory strains that generate bias in aging studies. In other words, the life-extending alleles found in these organisms may actually be simply restoring lifespan to what is normally found in the wild. The fact that wild-derived mouse strains take longer to reach sexual maturity and live significantly longer than common laboratory strains supports this view. Moreover, laboratory strains are often genetically homogeneous, which provides more consistent results, but also gives rise to discrepancies between strains on the effects of genes or interventions.