Here’s a fun fact: human beings and Rhesus monkeys share 93 percent of the same DNA. Here’s another: Rhesus monkeys age in the same way human beings do—the hair grays, the skin sags, the joints stiffen. And as these primates age, rates of cancer and heart disease skyrocket, just like in humans. So when it comes to testing how scientific discoveries will affect humans, the Rhesus monkey is—to borrow from elsewhere in the animal kingdom—as good a Guinea pig as we will get, and scientists are uncovering how these primate cousins of ours may hold the key to longevity. And it all comes down to your calories.
Some brief backstory: In the late 1980s, the University of Wisconsin and the National Institute on Aging (NIA) started two long-term studies on the aging of Rhesus monkeys. Both studies featured bespoke diets for each Rhesus monkey—76 at the University of Wisconsin and 121 at the NIA. Every primate received all of the essential nutrients to maintain a healthy lifestyle. But half of the monkeys received a diet with 30 percent fewer calories. Not only are the undertakers fine fine, they’re thriving.
One monkey named Sherman just turned 43, which is practically ancient in Rhesus monkey years. (The average age for Rhesus monkey years in captivity is about 25.) And while that is indeed remarkable, Sherman isn’t a standalone case. Of all the monkeys on a calorie-restricted diet, 13 percent died of age-related causes. Of those who could eat to their heart’s content, stuffing themselves with calories? That figure is 37 percent. As Rozalyn Anderson, a gerontologist from the University of Wisconsin, told the BBC, “We have demonstrated the aging can be manipulated in primates.”
Anderson says the implications are enormous. “Going after each disease one at a time isn’t going to significantly extend lifespan for people, because they’ll die of something else,” she said. “If you cured all cancers, you wouldn’t offset death to do cardiovascular disease, or dementia, or diabetes-associated disorders. Whereas if you go after aging, you can offset the lot in one go.”
In fact, the tests on Rhesus monkeys have proven so effective that the science community has picked up and started to apply the thinking to human beings. Susan Roberts, a dietary scientist at Tufts, told the BBC that she maintains a caloric intake of about 80 percent. As a result, her BMI is a 22. If she didn’t slash those calories, she says, it would be at a 30.
Roberts is also a leading scientist on a study called the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy. (CALERIE, get it?) For all intents and purposes, it’s a replication of the dual Rhesus monkey studies, just with human beings. The study only lasted for two years—as opposed to two decades—but already showed outsize effects. The folks on the calorie-restricted diets showed a 40 percent decrease in levels of insulin resistance, which causes diabetes, and a 25 percent decrease in molecules called tumor necrosis factors. Those are the evil cells that cause cancer.
In short, the researchers indicate that reducing your calorie intake by 20 or 30 percent can significantly reduce your risk of developing a life-ending disease.
Of course, a reduction in calories isn’t all great news; there are some negative consequences. Among the nearly 220 human volunteers for the CALERIE study, none of them reported additional hunger, mood swings, or decreased sex drive. However, they were put at risk for a slight decrease in bone density. And if you dramatically cut calories—as contestants of The Biggest Loser have shown—you’re at risk of seriously hobbling your metabolism and wreaking havoc on your endocrine system.
So consider that a fair warning, which is nothing to monkey around about.
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