The rate of living theory postulates that the faster an organism's

metabolism Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...

, the shorter its lifespan. First proposed by

Max Rubner Max Rubner (2 June 1854, Munich27 April 1932, Berlin) was a German physiologist and hygienist. Academic career He studied at the University of Munich and worked as an assistant under Adolf von Baeyer and Carl von Voit (doctorate 1878). Later ...

in 1908, the theory was based on his observation that smaller animals had faster metabolisms and shorter lifespans compared to larger animals with slower metabolisms. The theory gained further credibility through the work of

Raymond Pearl Raymond Pearl (June 3, 1879 – November 17, 1940) was an American biologist, regarded as one of the founders of biogerontology. He spent most of his career at Johns Hopkins University in Baltimore. Pearl was a prolific writer of academic books, ...

, who conducted experiments on

drosophila ''Drosophila'' (), from Ancient Greek δρόσος (''drósos''), meaning "dew", and φίλος (''phílos''), meaning "loving", is a genus of fly, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or p ...

and cantaloupe seeds, which supported Rubner's initial observation. Pearl's findings were later published in his book, ''The Rate of Living'', in 1928, in which he expounded upon Rubner's theory and demonstrated a causal relationship between the slowing of metabolism and an increase in lifespan. The theory gained additional credibility with the discovery of Max Kleiber's law in 1932. Kleiber found that an organism's

basal metabolic rate Basal metabolic rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest.. In other words it is the energy required by body organs to perform normal It is reported in energy units per unit time ranging from watt ( ...

could be predicted by taking 3/4 the power of the organism's body weight. This finding was noteworthy because the inversion of the scaling exponent, between 0.2 and 0.33, also demonstrated the scaling for both lifespan and metabolic rate, and was colloquially called the "mouse-to-elephant" curve.

Mechanism

Mechanistic evidence was provided by Denham Harman's

free radical theory of aging Free may refer to: Concept * Freedom, the ability to act or change without constraint or restriction * Emancipate, attaining civil and political rights or equality * Free (''gratis''), free of charge * Gratis versus libre, the difference bet ...

, created in the 1950s. This theory stated that organisms age over time due to the accumulation of damage from free radicals in the body. It also showed that metabolic processes, specifically the

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

, are prominent producers of free radicals. This provided a mechanistic link between Rubner's initial observations of decreased lifespan in conjunction with increased metabolism.

Current state of theory

Support for this theory has been bolstered by studies linking a lower

(evident with a lowered heartbeat) to increased life expectancy. This has been proposed by some to be the key to why animals like the

giant tortoise Giant tortoises are any of several species of various large land tortoises, which include a number of extinct species, as well as two extant species with multiple subspecies formerly common on the islands of the western Indian Ocean and on the ...

can live over 150 years. However, the ratio of resting metabolic rate to total daily

energy Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

expenditure can vary between 1.6 and 8.0 between species of

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s. Animals also vary in the degree of coupling between oxidative phosphorylation and ATP production, the amount of

saturated fat A saturated fat is a type of fat in which the fatty acid chains have all single bonds between the carbon atoms. A fat known as a glyceride is made of two kinds of smaller molecules: a short glycerol backbone, and fatty acids that each cont ...

in mitochondrial

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...

s, the amount of

DNA repair DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...

, and many other factors that affect maximum life span. Furthermore, a number of species with high metabolic rate, like bats and birds, are long-lived. In a 2007 analysis it was shown that, when modern statistical methods for correcting for the effects of body size and phylogeny are employed, metabolic rate does not correlate with longevity in mammals or birds.

References

* Rubner, M. (1908). Das Problem der Lebensdauer und seiner beziehungen zum Wachstum und Ernährung. Munich: Oldenberg. * Raymond Pearl. The Rate of Living. 1928 * * * * {{cite journal , author1=Holloszy J. O. , author2=Smith E. K. , year = 1986 , title = Longevity of cold-exposed rats: A reevaluation of the "rate-of-living theory , journal = Journal of Applied Physiology , volume = 61 , issue = Suppl 2 , pages = 1656–1660 , doi=10.1152/jappl.1986.61.5.1656 , pmid=3781978 Theories of biological ageing Metabolism

Mechanism

Current state of theory

See also

References