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Calorie Restriction is a term used by "life-extensionists", people trying to extend their lifespan past what is "normal" now, to refer to a special type of diet. This is not like any regular "diet" one goes on to lose weight, where they just eat less food. Instead, this is a carefully planned and permanent dietary change with the intention of extending one's life for significant amounts of time.

The idea is that by seriously reducing the number of calories taken in by food, one can reduce the rate of metabolism in the body. That reduction in metabolism will also reduce the chemical processes that cause aging, either ones that are side effects of metabolism, or ones that are part of the metabolism process. Vitamin and mineral supplements are also part of the regimen, to guarantee that one takes in sufficient amounts to help keep the body running in healtly shape - the point is to be undernourished, not malnourished.

A person following a calorie restriction plan is almost always hungry, as their goal is provide as few calories as possible to their body, just enough to prevent starvation. The body weight of someone on the plan is usually between 15 and 25% less than what is considered the "ideal" weight for their height and body type. This is not universally true - there have been test animals on restriction that were also "overweight", yet lived just as long. (This does say a lot about how much genetics can affect one's weight) However, except for the constant hunger, people on calorie restriction almost universally claim they feel better all around, after some tweaking of when and what to eat to make sure that blood pressure and blood sugar are controlled.

Experiments have been done on animals to determine the effects of calorie restriction, and on the amount of life extension by various types of calorie restriction. Experiemnts have supposedly shown that animals with a restricted calorie diet can live as much as 40% longer. This isn't average lifespan, but maximum lifespan - most health improvment methods work on average lifespan, not really extending how long the oldest live. Calorie restriction, on the other hand, seems to do just that - allow even the oldest in the experimental groups to live longer. Also, it appears that all animals on calorie restriction appear more youthful than control animals of the same age.

Fertility is affected in an interesting way, however. During the periods that are normally considered the "most fertile" of a lifespan of the test animals, the restricted animals seemed to show a much reduced fertility rate. However, the control animals had their fertility drop off, while there was little change in the restricted animals. In fact, restricted animals were able to conceive at an age where ALL of the control animals were dead.

Calorie restriction also has one significant difference from other diets aimed at improving health. Exercise is considered a cornerstone of any weight loss or healthy living program. Keep the body working, raise the metabolism, and the body becomes more robust. Calorie restriction is just the opposite - it argues in favor of reducing physical exertion, because it raises metabolism.

There also seems to be a reduction in disease from the calorie restriction. First, many of the common problems that occur due to lifestyle, such as heart disease, are highly reduced, as the diet more or less completely eliminates foods that can cause those diseases, such as fatty foods. (Cholesterol levels, for example, drop by incredible amounts when on such a diet. The Biosphere II experiment ended with all of the participants having had their level drop by much further than any drug has ever accomplished.) It also seems to cause a reduction in diseases normally associated with aging, such as cancer - rates of occurence seem to drop for the restricted animals, even in their later years, beyond the normal lifespan.

The general diet one follows with calorie restriction is to stick to low calorie, nutrient dense foods, and avoid "empty" ones, such as sugar, chocolate, white flour - and especially fats, as they have more calories per gram than proteins and carbohydrates. Many foods considered healthy are also avoided - fruit juice, for example, contains 100-200 calories per serving. Foods high in fiber are good choices - and foods that the body can quickly turn to sugar, such as bread, are to be avoided, as they can INCREASE your hunger.

There are no definite explanations as to the mechanism by which aging is slowed and lifespan increased. But that is to be expected, as aging itself is not clearly explained. There is not a clear picture of how much is caused by genetics, how much by ordinary body processes, and how much by lifestyle - and this likely changes from person to person. Many of the theories of aging, though, are addressed by this type of diet. The levels of glucose circulating in the blood is much lower, for example, addressing one theory that glucose in the blood can form damaging chemical bonds with essential body molecules like DNA.

There are dangers to calorie restriction that one must be aware of.

Attempting calorie restriction at an age with the body is still growing will likely permanently stunt growth, and thus should not be practiced by young people. Pregnancy is even more of an issue - one should not attempt such a diet while pregnant because of the dangers to the developing fetus. Take a break from the diet if you're already doing it.

Anyone who is subject to eating disorders should probably not practice calorie restriction, as it could lead back into the disorders, and any eating disorder will cause a reduction in one's lifespan, instead of an increase.

And always, while adjusting to any significant change in diet, see a physician to check your health before starting the changes, and should there be any problems.

Note: This writeup is meant as just a description and introduction, and anyone considering starting calorie restriction should go look at many other sources before attempting any changes to their lifestyle or diet.

Sources:
Calorie Restriction FAQ - http://www.infinitefaculty.org/sci/cr/cr.htm
CR "How To" - http://www.aleph.se/Trans/Individual/Life/cr-how-to.html

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