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Cutting kilojoules 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 kilojoule restriction is a fountain of youth.
Over the past 70 years, researchers have described how eating fewer kilojoules means living longer lives, at least for animals with shorter life spans to start with, such as mice and rats.
How the study was done
In the new study on kilojoule 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 kilojoules (the "control" group) or about 25% fewer kilojoules.
The animals that had their kilojoules cut did not survive any longer, although they were more likely to stave off diabetes and have improved metabolism. Monkeys that started the kilojoule-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 US National Institute on Aging (NIA) in Dickerson, Md., and lead author of the study.
The study was published in the online edition of Nature.
Similar studies tied to longevity
Previous studies have reported that kilojoule-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 kilojoule 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 kilojoule intake if you overeat or have a poorer diet.
In the current study, Mattison and her colleagues began kilojoule 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 kilojoule-restricted monkeys had lower rates of diabetes and weighed less than their control counterparts, some health outcomes varied by age.
What the study showed
While the young kilojoule-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 kilojoule-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 kilojoule-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 kilojoule-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 kilojoule restriction improves health even in primates is a pretty good case."
However, Guarente does not advocate kilojoule 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 kilojoule 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 kilojoule-restricted regimen in these survivors, Mattison said. The Wisconsin study only tested calorie restriction in young monkeys, between 7 and 14 years of age.
Scientists note that research with animals often fails to provide similar results in humans.
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For more about aging and health, visit the U.S. National Institute on Aging.
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