Cutting calories can
lead to a number of health benefits, from better metabolism to reduced diabetes
and cancer rates, and stronger immune function, according to a new study in
monkeys.
The long-term study
did not, however, support the notion that calorie restriction is a fountain of
youth.
Over the past 70
years, researchers have described how eating fewer calories means living longer
lives, at least for animals with shorter life spans to start with, such as mice
and rats.
In the new study on
calorie restriction in primates, which are more closely related to humans,
researchers compared the health and longevity of more than 100 monkeys on diets
that either supplied all the recommended daily calories (the
"control" group) or about 25 percent fewer calories.
The animals that had
their calories cut did not survive any longer, although they were more likely
to stave off diabetes and have improved metabolism. Monkeys that started the
calorie-restricted diet when they were juveniles or adolescents also gained
protection from cancer and a boost in their immune response.
"I don't think we
are contradicting the dogma of calorie restriction. Our study is not the direct
opposite, saying 'No, it doesn't work.' It shows that it works
differently," said Julie Mattison, a staff scientist at the Laboratory of
Experimental Gerontology at the U.S. National Institute on Aging (NIA) in
Dickerson, Md., and lead author of the study.
The study was
published Aug. 29 in the online edition of Nature.
Previous studies have
reported that calorie-restricted monkeys live longer, although differences in the diets used in
different studies could be important, Mattison noted.
For example, a study
done at the Wisconsin National Primate Research Center gave both its
low-calorie and "control" monkeys a diet that was higher in sugar than
the one used in the current NIA study. (Both studies supplemented the
calorie-restricted monkeys' diets to meet the recommended daily allowance of
vitamins and minerals.)
While the control
monkeys in the Wisconsin study had all-you-can-eat meals, the NIA study gave
its control group set portions.
These differences in
diet plan "give us two different answers that are pretty valuable to
have," Mattison said. She added that her study could capture the effects
of calorie restriction in monkeys — and perhaps humans, too — that are already
eating a balanced and moderate diet, while the Wisconsin study suggests the
possible outcome of scaling back calorie intake if you overeat or have a poorer
diet.
In the current study,
Mattison and her colleagues began calorie restriction in a group of 86 young
monkeys, between 1 and 14 years of age, and in 35 older monkeys between 16 and
23 years. The average lifespan of this type of monkey, called rhesus macaque,
in captivity is about 27 years.
Although all of the
calorie-restricted monkeys had lower rates of diabetes and weighed less than
their control counterparts, some health outcomes varied by age.
While the young
calorie-restricted monkeys had lower rates of cancer and seemed to have a more
robust immune system, calorie restriction in older monkeys led to improvements
in metabolic health. These animals had lower levels of triglycerides, or fat in their blood, and at least for the old
calorie-restricted males, lower levels of cholesterol and glucose. (In the
young animals, these markers were low to begin with, Mattison noted.)
However these health
strides in calorie-restricted monkeys were not enough to affect rates of death.
Common causes of death were cardiovascular disease, endometriosis in females
and cancer, Mattison said.
The rate of heart
disease appeared to be slightly higher in young calorie-restricted animals,
although the rate is low and it is too soon to know what it means, Mattison
said.
Commenting on the
research, Leonard Guarente, director of the Glenn Laboratory for the Science of
Aging at the Massachusetts Institute of Technology, said: "The evidence
all told that calorie restriction improves health even in primates is a pretty good
case."
However, Guarente does
not advocate calorie restriction for people beyond just eating a healthy diet
and avoiding overeating. "I am told it is not fun and if you don't do it
properly, you could do harm."
There is still a lot
to learn about how to safely reap the benefits of calorie restriction, Mattison
said. "At what level of calorie restriction and at what age are you
benefiting one system and sacrificing another?"
Half of the animals in
the young group are still alive, and it is possible that the team will end up
seeing an improvement in longevity associated with a calorie-restricted regimen
in these survivors, Mattison said. The Wisconsin study only tested calorie
restriction in young monkeys, between 7 and 14 years of age.