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Most people diet to trim waistlines a belt notch or two.
Bob Cavanaugh and other members of the US-based Calorie Restriction Society are more ambitious.
Inspired by animal experiments showing that underfeeding enhances vitality and prolongs life by 30 percent or more, they are slashing calorie intake in a bid to beat back the clock and halt the ageing process.
"Some people are doing it strictly to enhance longevity," Cavanaugh said by phone from his home near Moorehead City in North Carolina.
"Others do it to avoid age-related disease, or because they already have diabetes, high cholesterol or clogged arteries and want to clean up their bodies by using diet."
A worldwide epidemic of obesity-related diseases has put a spotlight over the last decade on the link between food and health.
"In rich countries, 90 percent of the population probably eats, on average, about 50 percent too much," noted Christiaan Leeuwenburgh, head of the biology of aeging division at the University of Florida's College of Medicine.
"Even if they were to reduce their calorie intake by half, they would still only be at baseline," the optimal balance between energy input and output, he told AFP.
A wealth of scientific evidence has confirmed that maintaining that balance helps prevent type-2 diabetes, cardiovascular disease and cancer.
But experiments with both animals and humans have also shown that pushing one's calorie intake 10 to 20 percent below that baseline threshold -- without lowering nutrients -- may provide additional health advantages.
Luigi Fontana, a professor in the Division of Geriatrics and Nutritional Science of Washington University in St. Louis, Missouri, has led or co-authored more than a dozen studies on reduced calorie intake in humans.
He is also one of a handful of researchers studying longterm impacts by monitoring a group of nearly 50 adults who have been on calorie restriction diets for at least a decade.
"Most are middle-aged, but they have the cardiovascular profile of a teenager," he said by phone.
Blood pressure, cholesterol, blood sugar and insulin levels are all low while so-called "good" cholesterol remains high, he said. Diabetes and cancer rates are down too.
Studies published earlier this year point to other, specifically age-related, benefits as well.
One shows that cutting calorie intake 20 percent cut damage in DNA and RNA caused by oxidation in half compared to control groups.
Oxidative damage to DNA, proteins and other cellular building blocks accumulate over time and are thought to be a major driver of ageing.
A second study by Fontana, published in July in Aeging Cell, shows that a combination of calorie reduction and limiting protein intake lowers levels of insulin-like growth factor, commonly known as IFG-1.
IFG-1 is a high-risk marker for prostate, breast and colon cancer, and plays a key role in regulating cell growth linked to the ageing process.
Cavanaugh, 61, an ex-marine, started the diet eight years ago after a 15-year history of high cholesterol and blood pressure.
At first he improvised. "I designed a diet I thought was very nutritious, but I had a problem with hunger and would sometimes go on candy binges," he said.
Not until he began to keep track not just of calories but vitamins, minerals and amino acids did the diet really work.
"My level of vitality soared," he said, insisting he has more energy today than 20 years ago.
Consuming less calories does not necessarily mean eating less food, he said. While he only takes in two meals a day, he tucks away large quantities of fruits and vegetables, along with smaller portions of lean meats and fish.
Refined, processed foods high in sugar, fat or salt -- junk food, in other words -- is off the menu.
The average calorie intake for men is about 1,800, and for women between 1,200 and 1,600, depending on height.
Despite the proven health benefits, the jury is still out on whether counting calories enhances longevity, which some scientists think has a genetically-imposed ceiling.
"It may be unlikely that it will extend human lifespan significantly," said Jan Vijg, a scientist at the Buck Institute for Age Research who recently co-authored an overview article on ageing in the London-based journal Nature.
The very fact humans live so long makes it difficult to conduct controlled experiments, he said.
Tests with monkeys underway for two decades give no indication that life in primates can be extended by the 30 or 40 percent seen in rats and mice.
"Will this add 10 years to your life? Nobody knows," said Leeuwenburgh, adding that reducing calories late in life could make it difficult to maintain needed muscle mass.
"But one thing is sure -- calorie restriction will help you reach your maximum lifespan potential, which is different for all of us depending on our genetic profile," he said.
The Calories Restriction Society has about 3,500 dues-paying members, and its website gets about 4,500 hits a day.
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