Oops, caloric restriction may not increase lifespan

Caloric restriction (CR) is one of my favorite cocktail party conversation topics. Whenever someone starts talking about antioxidants or resveratrol or whatever other compound they’ve read will extend their lifespan, I chime in with the observation that there isn’t good, consistent data on those things. If you’re serious about living longer, I explain, you should consider following a CR diet, which has been experimentally demonstrated to improve longevity in yeast, worms, mice and – as of a 2009 report on rhesus monkeys – even primates (you know, like us).

The punchline is that the diet sounds somewhere between excruciating and impossible. It’s not one of these easy breezy plans where you trade in sweets for bacon. CR is essentially controlled starvation, requiring about a 30% reduction in daily calorie intake. That means that an adult male would have need to make do with a meager 1750 nutritionally dense calories (i.e., don’t blow your whole day’s allowance on a giant piece of cheesecake) per day instead of the usual 2500.^* For the majority of humans, the only way that would happen is if they found themselves locked in Biosphere 2 with CRON diet (calorie restriction with optimal nutrition) pioneer Dr. Roy Walford. And even those folks went a little crazy on the reduced food regimen. Nevertheless it’s a fascinating idea and elegant in its simplicity: eat less, live longer. No pills necessary.

Unfortunately a newly published caloric restriction study in primates (rhesus monkeys again) largely contradicts the previous one. It appears I’ll have to find a new subject with which to alienate people at social functions.

The upheaval in CR certainty comes from a National Institute of Aging (NIA) study that began in 1987. The researchers started with over one hundred monkeys of varying ages, divided them into CR and control groups and monitored their health while patiently waiting to tally when each died and assess what killed them. The expectation was to see more and earlier age-related deaths in the control group. But the skinny monkeys expired at about the same rate as the heavier controls. The data on the younger monkeys is still preliminary (rhesus monkeys can live over 25 years in captivity, so not all the research animals have kicked it yet), but the authors don’t anticipate a major reversal of the trend. Still, it wouldn’t hurt to check back in, oh, about ten years. Mark your calendar.

So that’s not the best of news for the adventurous individuals currently trying their luck with CR diets.^† But caloric restriction isn’t just about extending lifespan, it’s about extending healthy years, so the authors also examined several measures of health. And the results were… ugh, kind of a mixed bag I’m afraid. Some measures improved, but the overall pattern wasn’t consistent. For instance, in monkeys that entered the experiment at older ages, cholesterol levels were lower in CR males, but not so much for females. In monkeys recruited at younger ages, those on the CR diet fared much better than the control group in avoiding cancer, but they weren’t spared from diabetes or cardiovascular disease.

So what happened? Does caloric restriction work or doesn’t it? The authors of the NIA paper point out some potentially important differences between their study and the 2009 report, conducted by the Wisconsin National Primate Research Center (WNPRC). For one thing, the food provided to the monkeys wasn’t the same. Compared to the WNPRC monkeys, the NIA animals ate a pretty high-quality, gourmet diet (some just got smaller portions than others).  Where WNPRC monkeys were fed ingredients like corn oil, cornstarch, and sucrose (table sugar), NIA monkeys got their fats and carbohydrates from ground wheat and corn, and omega-3 rich fish oil. Effectively, the NIA chefs shopped at Whole Foods, while those cooking for WNPRC just went to a convenience store down the street.

Additionally, in the NIA study food was carefully portioned out to both CR and control groups (with CR monkeys getting 30% smaller doses). Meanwhile over at WNPRC, control monkeys had sky’s-the-limit food allowances. And, as is often the case with animals in captivity, they probably overdid it on the tasty corn and sugar (the WNPRC control monkeys were heavier than their NIA counterparts). So while the WNPRC study initially seemed to support the belief that CR increases longevity, it may have simply demonstrated the rather well established observation that overconsumption of low quality calories is bad for your health.

But we shouldn’t conclude that the NIA study means CR in primates is a complete fail. The problem is that we don’t really know how caloric restriction works, when it works. When science first began subjecting lab animals to extreme diets in the early 20th century it wasn’t because they expected to find benefits. After all, improved access to nutritious food seemed to be extending human life expectancy in industrialized nations. But somewhere in their experiments, they noticed that starved animals sometimes outlived the well-fed ones. But why? The best current explanation (though certainly not the only one) for how CR might affect lifespan is that when an organism has access to ample food the body’s priorities are growth and reproduction, but in leaner times the focus shifts to repairing cells and basically just staying alive long enough to ride out the famine. So if the perceived famine is indefinite, perhaps the organism can live longer.

However the mechanisms by which the body senses and responds to food scarcity are less clear. Scientists are exploring possible metabolic pathways involved, but they may differ from one life form to the next. The degree of improvements to lifespan are also varies. According to a 2010 review in Science, hungry yeast can expect to live three times their normal lifespan, while food-deprived mice will only increase theirs by 30-50%. Perhaps caloric restriction has more to offer to microorganisms than to mammals like ourselves? Until we have a better understanding of the metabolic pathways involved, it’s hard to say. In the meantime, don’t worry about it. Given that over one third of Americans are currently obese, we may want to address more immediate health issues before attempting to calorie count our way to a 120-year lifespan.

^* That’s an approximate RDA value, as caloric needs vary with body size and activity level.

^† Walford founded the Caloric Restriction Society in 1994 and it still exists today.