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Can a restricted diet slow aging?

A diet is a constant series of inner negotiations and brokered deals. You agree to give up that second slice of pizza, for example, in hopes of fitting into your skinny jeans. You live without bacon so that you don't have to spend more time on the treadmill later. You'll pass on the french fries if you can have a scoop of ice cream instead. We give up something in hopes of getting something better. But what if by giving up that slice of pizza, you could live 10 minutes longer -- would you make that deal?

That's no hypothetical deal with the devil; it's part of a bargain struck by people who've adopted calorie restriction as a way of life. Calorie restriction is a diet that involves cutting up to 30 percent of the calories that you consume on a daily basis, while still consuming all necessary nutrients. That percentage of calories is nothing to sneeze at. Let's say you need about 2,000 calories a day to maintain your weight, which is a rough average for most people. If you adopted a diet of caloric restriction, you'd be shooting for 1,400 calories. So you'd be giving up more than just that slice of pizza -- you'd be giving up entire meals. The diet is frequently described as austere, drastic and draconian, and even proponents admit they're hungry a good deal of the time.

So what's the payoff? Scientists believe that caloric restriction helps mammals live longer. In 1935, a nutritionist at Cornell University found that a diet of 30 percent fewer calories caused a 40 percent increase in lifespan -- in mice [source: Mason]. This kind of increase is equivalent to a human living to the ripe old age of 160 [source: Dibbell]. Since then, mice have been on a vast array of diets as scientists try to determine which is optimal for longevity. So, too, have yeast cells, worms, fruit flies, monkeys and Labrador retrievers, all of which have shown increases in lifespan on fewer calories. And it's not just a matter of years, either. These species not only live longer, they live significantly more active lives than their non-dieting counterparts, and they also have fewer instances of diseases associated with aging, such as cancer.

Does that mean humans should adopt a calorie restricted diet? What does calorie restriction look like in practice?

The recipe for longevity?

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What's a Calorie Restricted Diet Like?

There have been no formal studies of calorie restriction in humans. We know caloric restriction works for mice and fruit flies because they have such short life spans; a study that aimed to duplicate the results in humans would take decades. It's very hard to imagine a person volunteering to go hungry for much of their life when long-term benefits are still unknown.

Still, we have some basic anecdotal evidence that such a regimen might have benefits. In Okinawa, Japan, for example, which counts many centenarians in its population, people have traditionally consumed fewer calories. After World Wars I and II, when food was in short supply, fewer people died from aging-related diseases such as coronary artery disease, Type 2 diabetes and cancer [source: Hochman]. When researchers in the self-sustained Biosphere II found themselves running low on food in the 1990s, they adopted a calorie restricted diet; one of the researchers, Roy Walford, became one of the leading proponents of calorie restriction, writing books such as "The 120-Year Diet" and "The Anti-Aging Plan." Walford died in 2004 from amyotrophic lateral sclerosis (ALS, more commonly known as Lou Gehrig's disease).

Even without scientific evidence of the longevity benefits, plenty of people have adopted calorie restriction in the hopes of slowing the aging process. One calorie restriction advocate shared his daily diet with the New York Times in 2003:

  • Breakfast: a megamuffin, a homemade concoction consisting of about 30 ingredients including raw wheat germ, rice bran, brewer's yeast, one carrot, strawberries and psyllium husk (the active ingredient in Metamucil)
  • Lunch: a protein bar or a roast beef sandwich, hold the bread (simple sugars and flours are the first to go in a calorie restricted diet)
  • Dinner: a serving of broccoli, zucchini and canned pink salmon, totaling exactly 300 calories, followed by a dessert of fruit salad topped with whey protein

[source: Hochman]

It should be noted that this diet wasn't shared by the man's wife or children, which must only add to the work involved in this routine. Calorie restriction can seem like a full-time job that includes measuring and weighing food, learning about which foods pack the nutrients without the caloric punch and tracking consumed calories while maintaining a balance of carbohydrates, proteins and fats. It can also be expensive, since it involves lots of fresh vegetables and obscure grains available only from high-end brands.

Again, there have been no formal studies of how caloric restriction works in humans. Still, some scientists think they have an idea of how many more years a lifetime of megamuffins will buy you.

Veggies are a staple of a calorie restricted diet.

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Is Calorie Restriction the Future?

Scientists believe that by consuming significantly fewer calories, you could increase your lifespan by about four or five years [source: Britt]. Though a few extra years might not seem impressive considering how much longer, percentage-wise, the lab mice lived, it's worth considering that there may be some additional health benefits. In some studies, caloric restriction has been shown to lower blood pressure, improve heart health and boost the immune system in people. Some people also demonstrated lower body temperatures and lower insulin levels, which are believed to be tied to longevity [source: Mason].

That's the problem with longevity research and caloric restriction: Scientists have many beliefs and theories on how to measure results, but they aren't exactly sure what they're looking for. We're not even sure exactly how calorie restriction works. There are two basic schools of thought, one being that calorie restriction works because the stress of starving shifts the body into a self-preservation response that helps it survive longer on less. Others believe that by simply putting less into our body, certain systems don't have to work as hard, thus experiencing less wear and tear. But getting any more specific involves different hypotheses about whether genes, hormones, free radicals or cell divisions make the ultimate difference.

Even those trusty lab creatures are no help on sorting it all out, and many studies only demonstrate how little we know about this diet. Calorie restriction extends life in fruit flies, but not houseflies [source: Wanjek]. It benefits fat mice, but not already-lean mice, and calorie restriction works for mice that were bred specifically for laboratory studies, but not mice found in the wild [sources: Britt, Wanjek]. When the caloric restriction is too extreme, at 60 percent reduction, the mice starve to death [source: Wanjek].

This diet would stunt children's growth, but scientists aren't sure when a person should begin calorie restriction to obtain the maximum benefit. Calorie restriction also wouldn't be prudent for the elderly or the ill, and even perfectly healthy folks may not be able to sustain such a sparse diet. There are, of course, some side effects to eating less, including loss of libido, cessation of menstrual periods, loss of memory and muscle mass, and dizziness. You may look gaunt or emaciated, and there can be other weird cosmetic effects -- a New York magazine article featured a man who had orange hands from consuming so many carotenoids [source: Dibbell].

Those interested in calorie restriction are urged to consult a doctor before beginning, though don't be surprised if you're met with disdain. Many simply don't believe this extreme diet is feasible, which is why some researchers focus on finding a pill that could mimic the effects of caloric restriction. Until then, consider this: Will it be fun to live forever if you can't order popcorn at the movies, travel to Italy for a heaping bowl of pasta or have a slice of cake at a loved one's wedding?

Lots More Information

Related HowStuffWorks ArticlesSources
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  • "Calorie restriction: Is this anti-aging diet worth a try?" Mayo Clinic. (April 27, 2009)http://www.mayoclinic.com/health/anti-aging/HQ00233
  • Cohen, Philip. "Eat more, weigh less, live longer?" New Scientist. Jan. 23, 2003. (April 27, 2009)http://www.newscientist.com/article/dn3303-eat-more-weigh-less-live-longer.html
  • Dibbell, Julian. "The Fast Supper." New York. Oct. 23, 2006. (April 27, 2009)http://nymag.com/news/features/23169/
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