Eat less — a lot less — and live a lot longer. Or so the theory goes.
Proponents of caloric restriction argue that taking in the minimum number of calories your body needs — packed with enough nutrients to keep you firing on all cylinders — will delay the aging process and extend your lifespan beyond that of Jeanne Calment, who lived 122 years and 164 days.
In 1934, researchers discovered that laboratory rats that were fed far less than normal while still getting all the vitamins and minerals they needed, lived far longer than rats on a regular diet. Lifespan increased in proportion to the reduction in calories consumed. Some rats lived twice as long as expected when their caloric intake was cut by 30-40 per cent.
Cutting calories much beyond that level, however, led to starvation and death.
How caloric restriction works in the lab, and whether it works for people, has been the subject of dozens of studies ever since.
Researchers have looked at how caloric restriction affects rats, mice, monkeys, fruit flies and yeast. And they've found strong evidence of longer, healthier lives and delayed signs of aging.
In humans, it's been shown to lower cholesterol and blood pressure. It may improve your memory and lower your insulin levels.
In the 1980s, Dr. Roy Walford and a student, Richard Weindruch, conducted a series of caloric restriction experiments on mice. In 1988, they published their findings in a book, The Retardation of Aging and Disease by Dietary Restriction. They found that not only did mice on restricted diets live longer than mice on normal diets, but they looked younger, acted younger and fended off diseases of the elderly longer than mice on normal diets.
In September 1991, Walford got to put his theories to the test on people for two years. He was one of eight people sealed inside Biosphere 2, a 1.3-hectare self-sustaining ecological system. When the team found they could not grow as much food as they had expected to, Walford persuaded the others to try his calorie-restricted diet.
Walford published Beyond the 120-Year Diet: How to Double Your Vital Years in 2000, in which he brought together his 20 years of research into caloric restriction and life extension for a mass audience. It's seen as the definitive book on how to implement a calorie-restricted diet.
Walford died four years later of respiratory failure, a complication of amyotrophic lateral sclerosis (ALS).
When to implement a restricted calorie diet?
Some research suggests that the effects of caloric restriction are muted the older the animal is. Improvements in lifespan were far less significant when rats were older than 18 months.
A study released in January 2009, suggested that caloric restriction works — but only if you're an obese mouse. That study suggested that for lean mice, and possibly for lean humans, caloric restriction may be a "pointless, frustrating and even dangerous exercise."
"Your energy expenditure and your energy intake should be in balance," lead researcher Raj Sohal of the University of Southern California said. "It's as simple as that. And how do you know that? By gain or loss of weight.
"The whole thing is very commonsensical."
'Your energy expenditure and your energy intake should be in balance'—Raj Sohal
In an earlier study, Sohal reported that caloric restriction begun in older mice actually shortened life span.
Two studies released days apart in July 2009 came to different conclusions on the benefits of caloric restriction. The first, involving rhesus monkeys studied by a team at the University of Wisconsin, found that a nutritious but reduced-calorie diet blunts aging and significantly delays the onset of such age-related disorders as cancer, diabetes, cardiovascular disease and brain atrophy.
The lead researcher was Richard Weindruch, Dr. Roy Walford's former student and now a professor of medicine in the University of Wisconsin School of Medicine and Public Health.
At the end of a 20-year long study, half the rhesus monkeys that were allowed to eat what they wanted were still alive. Of the monkeys on calorie-restricted diets, 80 per cent were still living.
The second study, from the Stanford University School of Medicine, found that fruit flies on restricted calorie diets were less able to fend off some infections than flies on normal diets. The researchers concluded that "diet restriction can have complex effects on the realized immune response of a diet-restricted animal."
The findings echoed those of a study released in November 2008, which found that restricting calories may make it tougher to fend off illness, at least if you're a mouse.
But does it work with people?
Researchers have had difficulty trying to determine whether these studies will translate to gains for humans. They've debated over questions like:
* Do you study the effects of caloric restriction on lean or overweight people?
* Will the results vary depending on the age of people when they begin a restricted diet?
* Is caloric restriction expending more calories than eaten or eating fewer calories?
The Calorie Restriction Society was set up in 1994 by scientists involved in research into the goal of extending lifespans by reducing calories consumed.
The group warns that caloric restriction is not something that an adult can just jump into. It's not a diet. The society notes that early research has shown that caloric restriction doesn't extend lifespan when started in adult animals later in life, and may even shorten lifespan. It suggests implementing caloric restriction gradually over a year or two.
There's still no definitive evidence that drastically cutting caloric intake will allow people to live healthy lives for a longer period of time. The Calorie Restriction Society started collecting data from its members and other volunteers in 1997, as part of its effort to back up its claims with solid research.
While it's easy to control what an animal eats in a lab, it's a different matter when you're dealing with people who have a wide variety of easily available food.
It could take decades for today's research subjects to generate useful information.
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