Calorie restriction confers health benefits to monkeys but doesn't increase their life span, a new study suggests, undermining some people's belief that a sharply restricted diet could help them live longer.
Decades of experiments have shown that the
technique, known as calorie restriction, increases longevity by 30% to
40% in lab-bred mice and rats. The idea got a significant boost in 2009
when rhesus monkeys—which are genetically closer to humans than rodents
and also live a long time—showed a trend towards longer life when fed a
calorie-restricted diet, though that finding wasn't clear cut.
The findings have promoted the alluring idea that it might be
possible to live longer simply by limiting what's on the dinner plate.
To increase their lifespan, thousands of people today cut their food
intake by as much as 30% below a typical diet of 2,200 calories a day.
Pharmaceutical companies are seeking drugs that might mimic the salutary
effects of a skimpy diet without triggering severe hunger pangs.
Scientists speculate the benefits of cutting back the calories may be
linked to an adaptive response: When there's a food shortage, an animal
can't reproduce and its aging process slows down. That buys time until
food becomes plentiful again and it can reproduce.
But the latest data, published online by the journal Nature on
Wednesday, suggests the theory might not easily extend to people. "One
thing that's becoming clear is that calorie restriction is not a Holy
Grail for extending the life span of everything that walks on earth,"
said Rafael de Cabo, an experimental gerontologist at the U.S. National
Institute on Aging in Baltimore, Md., and lead author of the paper.
In the experiment, two sets of monkeys, one group aged 1-14 and
another group aged 16-23, were fed 30% less than their normal diets.
Their outcomes were compared with similar outcomes of two control sets
of monkeys fed a diet that was closer to normal. The treated monkeys, in
both age groups, lived no longer than their untreated counterparts.
The health benefits were mixed. Male animals on a restricted diet had
significantly lower cholesterol levels, but not the females. Cutting
back the calories appeared to have lowered the incidence of cancer, but
it also triggered a slight increase in the incidence of cardiovascular
disease. One promising outcome was that various aging-related diseases
appeared slightly later in the food-restricted animals.
The NIA's monkey experiments began in the late 1980s, when similar
studies also got started at the University of Wisconsin. Because rhesus
monkeys live nearly 30 years on average, scientists must wait a long
time to measure a difference in lifespan.
The Wisconsin study was the first to yield a definitive finding. A
paper published in 2009 showed that calorie restriction extended the
lives of monkeys, as long as deaths from non-aging-related causes were
excluded in the calculation. Some scientists questioned that
methodology. If those deaths were included, the benefit to lifespan
Nonetheless, the Wisconsin data was the first to provide hints that
calorie restriction could have an effect on the lifespan of primates.
The mystery is why the NIA study came to a different conclusion about
One reason may be that the studies were organized differently.
Wisconsin monkeys got a much higher sucrose content in their food than
did the NIA animals. Also, the Wisconsin control animals could eat as
much as they wanted while the NIA controls were given a set amount of
"Given the constraints, the studies were both done fine," says Steven
Austad, a bio-gerontologist at the University of Texas Health Science
Center in San Antonio, Texas, who wasn't involved in the studies but
wrote a commentary on the NIA findings in Nature. "They both raise the
question of how you'd translate the findings in human terms."
People may respond differently to calorie restriction depending on
their genetic make-up and the composition of their diets. Outcomes will
also vary depending on whether people are overweight or already lean
when they start this technique .
When done properly in people, calorie restriction appears to provide
some health benefits, such as lowering the risk of heart disease. In
June, scientists at Washington University in St. Louis, Mo., published
research showing that people on calorie-restricted diets had hearts that
function more like those found in people two decades younger.
On the downside, said Dr. Austad, male subjects on severely
restricted diets can show low testosterone levels and have problems
maintaining bone density.
The latest data on longevity may not deter many from giving up
low-calorie diets. "They see their blood pressure go down, their fasting
glucose go down, their cholesterol go down, and they feel better," says
Brian Delaney, president of Calorie Restriction Society International,
based in Sweden and the U.S., which currently claims 500 paid-up
members. "They believe it will enable them to live youthfully into their
70s, 80s and even 90s."