Skip lunch every day for the rest of your life, and you could live to be one hundred and twenty years old. At least, that’s the claim by some scientists and physicians who state  that markedly decreasing your caloric intake could dramatically increase your lifespan. They’re not talking about just skipping dessert, but skipping entire meals, maybe even entire days’ worth of food.  Known as caloric restriction with optimal nutrition (CRON), this extreme diet made news again this month with the publication of a twenty-year study in rhesus monkeys.  The study, in the July 10 issue of Science, states that monkeys fed 30 percent fewer calories — in essence, skipping lunch every day — are less likely to die from old age than monkeys fed a standard diet.  Is it time to put down that sandwich?

CRONies certainly think so.  Members of the Calorie Restriction Society advocate a CRON diet to maintain body weight 10-to-25 percent lighter than you were during your (presumably healthy and lean) teenage years.  A six-foot tall man who weighed 165 pounds in high school should have a goal as low as 125 pounds, and a 5 foot 8 inches woman with a lean weight of 140 pounds should aim as low as 105 pounds. Lisa Walford, one prominent member of the group, has been reported to weigh 80 pounds and stand about 5 feet tall.

CRONies point to studies that severely restrict calories in yeast, worms, fruit flies, and mice and demonstrate substantial increases in lifespan and improved health in old age.  Mice that were fed 30 to 60 percent less calories lived 30 to 50 percent longer. The theory behind these remarkable results is that decreasing caloric intake decreases metabolic rate, which decreases aging.  This led a physician behind the studies, Dr. Roy Walford (the father of Lisa Walford, mentioned above), to publish a book titled “The 120-Year Diet,” advocating caloric restriction to increase human lifespan.

If it works in mice, does it work in humans?  A study in monkeys is an important step to answer this question.  Scientists at the University of Wisconsin separated the monkeys into a control group, fed a standard diet of monkey chow and apples, and a study group, fed a diet composed of the same food but only two-thirds as much.  In the photo, Owen, on the right, is 29 years old and on the regular diet, while Canto, on the left, is 27 years old and on the calorie-restricted diet.  After twenty years, researchers found that calorie-restricted monkeys like Canto had lower rates of death from cancer, heart disease, and diabetes.  Plus their brains were bigger and they were more muscular.  Canto is not only healthier, but smarter and sexier than Owen.  Those are impressive findings, except for one important, missing detail.

Here’s what the researchers didn’t find: a difference in death rates.  There was no statistically significant difference in life span between the calorie-restricted and regular diet monkeys.  There were some statistically significant differences in deaths from the specific diseases mentioned above, but overall the monkeys died at the same rate.  While the skinny monkeys were dying at lower rates from certain diseases linked to old age, they were dying at higher rates from other causes, so in the end it was a wash.  It doesn’t matter if you die of old age, or if you die of something else.  You’re still dead.

I’m going to repeat myself, as I think this message has not been clearly stated in the media: this study found no difference in overall life span between calorie-restricted monkeys and regular diet monkeys.

Furthermore, it’s unclear that this study truly compared the effects of a calorie-restricted diet versus a regular diet.  Its design might have actually compared the effect of normal caloric intake versus overeating.  Here’s why: to calculate how much to give the calorie-restricted group, researchers placed the monkeys in cages with a few plastic toys and access to more food than they could possibly eat.  After observing for several months how much monkeys with unlimited food would eat, the researchers called this the baseline for the regular diet monkeys.  Then they subtracted 30 percent from that total, and placed the restricted-calorie monkeys on this diet for the next two decades.

I don’t know much about monkey psychology, but I do know a little about humans.  Imagine a vacation on the world’s dullest cruise ship, with no shows, no shuffleboard, not even any bingo.  The only distractions are a few plastic toys and a tempting buffet.  How much would you eat?  I wouldn’t eat three balanced meals a day, I’d eat until I was stuffed, and then go back for dessert twice.

Similarly, perhaps the caloric intake of the monkeys on a regular diet in the lab is actually higher than what their bodies are designed for in the wild.  The “regular” calorie group could be consuming the same excess amount you would on the Cruise Ship Boredom, while the “calorie-restricted” group could actually be consuming the standard amount of calories they would forage in the wild.  Maybe what this study actually shows us is the impact of a modern, sedentary lifestyle with easy access to calories: monkeys eat more than their bodies are designed for, leading to higher rates of heart disease, cancer, and diabetes.

So what’s the bottom line?  I emailed Dr. Ricki Colman, the lead author the study, to ask if she practiced caloric restriction.  “I look at calorie restriction as a tool to better understand the aging process,” she replied.  “While I personally believe a healthy diet is important (along with regular exercise), I do not practice calorie restriction.”

Sounds like good advice.  Studies in humans have demonstrated what you already know.  Being too skinny (BMI less than 18.5) or too fat (BMI greater than 24.9) will shorten your life.  This study should not convince you to put down that sandwich if you’re already eating a healthy diet.  Few of us will live to be 120 years old, and those of us who make it anywhere close to that will be grateful we didn’t sacrifice more than a century of good meals.

Photo by Jeff Miller at the University of Wisconsin-Madison.  More photos from the study.