Thursday, January 21, 2010

Rate of living theory of biological aging

History:
Perhaps one of the oldest theories of aging is called the rate of living theory. The theory originated in 1908 when a physiologist, named Max Rubner, discovered a relationship between metabolic rate, body size, and longevity. It states that living organisms possess a certain amount of a "vital substance" and when all of that substance is used up, they die.
So people (and other creatures), based on this theory, have a finite number of breaths, heartbeats or other measures.

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In ancient times, people also believed that just as a machine will begin to deteriorate after a certain number of uses, the human body deteriorates in direct proportion to its use.

This theory was initially developed to explain why most of the large animals live longer than the most of the smaller animals. Animals with the most rapid metabolism tend to have the shortest life spans, while animals with slower metabolic rates tend to have longer life spans. This has to do with the belief that all organisms are born with a certain amount of energy. If we use this energy slowly then our rate of aging is slowed. If the energy is consumed quickly aging is hastened. Long-lived animal species are on average bigger and spend fewer calories per gram of body mass than smaller, short-lived species. Although this is true among many species in the animal kingdom, it does not apply universally, particularly amongst mammals.

Modern Version:
The modern version of this theory recognizes that the number of heartbeats does not predict lifespan. Instead, researchers examine the speed at which an organism processes oxygen. There is some evidence, when comparing species, that creature with faster oxygen metabolisms dies younger. Tiny mammals with rapid heartbeats metabolize oxygen quickly and have short lifespans. Tortoises, on the other hand, metabolize oxygen very slowly and have long lifespans.

So, in the 20th century, scientists proposed a new twist on this old theory: energy consumption limits longevity. In other words, an organism’s metabolic rate determines its lifespan. This idea was supported by the discovery that reactive oxygen species (free radicals), a byproduct of normal metabolism, can damage cells and contribute to aging. Experiments in cold-blooded organisms showed that their life span was inversely related to the temperature they lived in or how active they were. More recent work with C. elegans, a roundworm, showed that changing just one gene related to metabolism could significantly extend the worm’s lifespan.

On the other hand, some experimental evidence has shown no clear relationship between temperature and longevity. Experiments in fruit flies have shown that temperature either has no effect, or the opposite effect. For example, a 1997 experiment showed that briefly exposing fruit flies to elevated temperature could actually slow aging for several weeks. Scientists now believe that although metabolic rate can affect aging, that doesn’t mean that it always does so. Caloric restriction, the only intervention known to extend life in mammals, does so without reducing the animal’s metabolic rate. In addition, experimentally boosting an animal’s metabolic rate does not always reduce longevity.

And even though there is a rough correlation among species between body size, metabolic rate, and longevity, there are many exceptions to this rule. For example, birds typically have a metabolic rate 1.5 2.0 times as high as similar-sized mammals, yet they live on average about three times as long—a pattern inconsistent with the rate of living theory.

In another investigation, researchers genetically engineered mice with a defect in the hypothalamus to cause the mice to overexert. Because the hypothalamus in mice is near the temperature control center, the mice's brain thought the body was overheating and lowered the core temperature. The results show that a drop of .6 degrees Celsius extended the life of the mice by 12 to 20 percent. The lower temperature may slow the rate of oxygen metabolism. The problem is that the lower temperature may also change a number of other systems and processes in the body. We don't know why the mice lived longer, only that they did.

One more study, led by Lobke Vaanholt of the University of Groningen in the Netherlands and now at the University of Aberdeen, was designed to test the rate-of-living theory among mice as well.

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Two groups of mice were tracked through their entire lifespans; one group’s environment was 71 degrees Fahrenheit, the other 50 degrees. According to the rate-of-living theory, because the colder group had to expend more energy to maintain body temperature, the chillier mice should have died sooner.

 “Despite a 48 percent increase in overall daily energy expenditure and a 64 percent increase in mass-specific energy expenditure throughout adult life, mice in the cold lived just as long on average as mice in warm temperatures,” the authors write. “These results strengthen existing doubts about the rate-or-living theory.”

The finding correlates with another Vaanholt’s experiment, in which exercise - rather than temperature - was used to manipulate metabolic rates. In this study the researchers divided the mice into three groups of 100 mice each. Two groups were "runner" mice, that is, mice that loved to run on the running wheels placed in their cages. One group of runner mice had access to running wheels, but the second group of runner mice did not. The third group consisted of regular laboratory mice that had a running wheel.

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Vaanholt's team followed 60 mice from each of the three groups throughout their natural lives, nearly three years. They measured wheel running activity and took periodic measurements of body mass.

They found:
  • Runner mice that had access to a wheel expended 25% more energy over the course of their lives compared to both the runner group that did not have a wheel and the regular mice
  • Both groups of mice bred for running, one group with the wheel and one without, lived about 90 days less than the regular mice
  • The regular (non-runner) mice lived longest, 826 days, compared to the runners with a wheel, 735 days and runners without a wheel, 725 days
The rate of living theory would have predicted that the running group that expended more energy would die earlier than the two groups that did not, Vaanholt said. This was not the case. There was no difference in life span between the two runner groups, even though one expended more energy.

In addition, the rate of living theory would have predicted that the runner mice without the wheel and the normal mice would live approximately the same life span because there was no difference in energy expenditure between the two. This was not the case. The runner mice without the wheel died sooner.

"The shorter life span cannot, therefore, be explained by a difference in metabolism," Vaanholt concluded. "There must be something else going on that causes these animals to age and die."

Summary:
Don't go into hibernation yet. There really is no data that slowing the metabolism extends human life. In fact, a slower metabolism would put someone at risk for obesity and other nutritional-related illnesses. Your best bet is still a healthy lifestyle with plenty of exercise, a diet with lots of plants, and a positive, relaxed attitude.

Sources and Additional Information:
http://www.eurekalert.org/pub_releases/2006-10/aps-ed100206.php

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