"Diet and Caloric Restriction for Longevity— The Monkey Trials"
Calorie restriction in primates to extend lifespan.
Will it work?
There's only one way to find out.
There have been four investigations
of calorie restriction and lifespan in nonhuman primates.
The first was published in 2003,
an analysis of the mortality of 117 rhesus monkeys
followed for about 25 years in a lab,
eight of whom had their Purina monkey chow restricted.
The average survival of the restricted monkeys
was to 32 years of age,
compared to 25 years for the control monkeys.
However, it was more of an observational study,
since the monkeys weren't randomly assigned,
and although in the abstract
they talk about the survival advantage
and how the ad libitum monkeys, the eat-all-you-want monkeys,
had more than twice the risk of death,
they acknowledge deeper in the paper
that the difference in death
did not reach statistical significance,
meaning it may very well have been a fluke.
That was all we had, though,
until results started trickling in
from the famous pair of studies that involved randomizing
about 200 rhesus monkeys to caloric restriction
or more normal diets,
one out of the University of Wisconsin-Madison
and another from the National Institute of Aging.
The UW study reported a 30 percent caloric restriction
significantly delayed disease and improved survival,
but the NIA study did not.
In the UW study,
caloric restriction reduced the incidence
of age-related diseases including cancer,
cardiovascular disease, brain atrophy,
and muscle wasting,
preserving muscle mass,
demonstrating caloric restriction can delay aging,
but in the NIA study,
the difference in age-related diseases
did not achieve statistical significance.
In the UW study,
the restricted monkeys lived to an average of about 29 years
compared to the control monkeys,
who lived closer to the average for monkeys in captivity
of about 26 years old.
In contrast, though one of the restricted monkeys
in the NIA study became the longevity record holder
for the species at age 43,
on average, the restricted group didn't live any longer
than the control animals.
Why the disparate results between the two studies?
In the NIA study that found no significant lifespan difference,
the control group was not fed ad libitum,
but rather had food portioned out to prevent
excessive weight gain.
In contrast, in the UW study,
the control monkeys could eat as much as they wanted
throughout the day
so ended up weighing more than the NIA control animals.
Of course caloric restriction would improve
the health and survival of overweight monkeys
just as weight loss would be beneficial
for overweight people,
but the null results in the NIA study suggest
that normal weight people might not benefit
from restricting further.
The NIA monkeys were also fed a healthier diet.
The diet in the UW study
where they saw significant benefit to cutting down
was an ultra-processed concoction of largely
milk protein, corn oil, corn starch, and table sugar,
whereas the NIA diet actually included unprocessed plant foods
such as corn, soybeans, and wheat,
and so actually had some phytonutrients.
About 29 percent of the UW diet was straight sugar,
compared to 4 percent in the NIA diet.
American adults get about 17 teaspoons of sugar a day,
which is about 13 percent of calories.
The contrasting findings suggest that the worse your diet is,
the more important it is to eat less of it.
The fourth study was on grey mouse lemurs,
among the smallest of primates,
standing just three inches tall.
Those randomized to a 30 percent caloric restriction
compared to an ad libitum control group
lived a whopping 50 percent longer.
And not just average lifespan, but maximal lifespan.
The maximum lifespan was boosted by about 20 percent.
However, the "calorie restricted" group
still weighed heavier than their wild counterparts,
so again this may just be an illustration
of the harms of obesity
and another indictment against all-you-can-eat buffets.
Also, those in the calorie restriction group experienced
an acceleration of age-related loss of grey matter
throughout their brains,
though this did not appear to translate into cognitive
or behavioral differences.
Pooling the three rhesus monkey studies together,
there seemed to be lower age-related mortality
but no significant difference in average lifespans overall
between the caloric restriction groups
and the control groups.
Given the time and expense,
there is little chance we are going to see any more
long-term primate studies
on caloric restriction for life extension.
So what can we draw from the primate data to date?
If you're overweight or living off junk food,
eating less is a good idea.