Sci. Aging Knowl. Environ., 17 September 2003
Barking up the Tree of Long Life
Dogs give rate-of-living hypothesis the cold nose
Key Words: lean body mass fat mass endotherm exotherm ectotherm
The dog days of an old theory might be upon us. New research shows that the more energy a dog uses in its lifetime, the longer it lives. The results contradict a 1928 hypothesis that the faster an organism's metabolic rate, the more quickly it ages. The data also challenge the idea that leanness enhances longevity.
The rate-of-living hypothesis states that all animals burn the same amount of energy per unit body weight in their lifetimes. Because large organisms generally use energy more slowly than small ones do, their allocation lasts longer: Mice live 2 years, humans about 80, and whales more than 200. Studies on individual species of flies, whose body temperature--and metabolic rate--can be manipulated by changing the temperature of the surroundings, have generated conflicting data. Studies on birds, warm-blooded animals that are more closely related to mammals, have flown in the face of the theory. Investigations into the rate-of-living hypothesis in mammals have compared animals of different species and have also produced conflicting results. The original idea, however, applied only to animals of the same species. To determine whether the rate-of-living hypothesis applies in a mammalian species, Speakman and colleagues at the University of Aberdeen, U.K., focused on different-sized dogs.
The researchers enlisted members of three breeds: papillons, which weighed about 3.0 kg; Labrador retrievers, which averaged about 30 kg; and Great Danes, which came in at just under 63 kg. The scientists measured the weight of the canines' muscle and fat, as well as the rate at which the dogs consumed oxygen and panted carbon dioxide while they sat still. The dogs in the small breed, which lives the longest of the three, used almost three times as much energy per kg of body weight over their lifetimes as the largest did, contradicting the hypothesis.
In addition, the pooches with the lowest percent of body fat lived shortest, contradicting previous work indicating that less fat might prolong life (see "Lasting Without Fasting"). Diabetes researcher C. Ronald Kahn of Harvard Medical School's Joslin Diabetes Center in Boston says the fat results are difficult to interpret because the animals vary so much genetically; the study didn't address potentially confounding factors such as fewer insulin-like molecules--a possible longevity booster--in the petite animals.
"A lot of people thought that higher metabolism meant shorter life," says gerontologist Arlan Richardson of the University of Texas Health Science Center in San Antonio. This work supports the notion that "the simplest explanation [for aging] is out the window." Not everyone agrees, however. Biochemist Rajindar Sohal of the University of Southern California in Los Angeles says that "results like this do not negate the rate-of-living [hypothesis] if you accept that the rate of metabolism is just one factor [in the hypothesis]. Other factors, such as the genetic makeup of the organism, often drown out the effect of metabolism on life span," he says. However, evolutionary biologist Steven Austad of the University of Idaho in Moscow points out that the original hypothesis considered only metabolism. The rate-of-living hypothesis, it seems, might be sleeping in the doghouse tonight.
September 17, 2003
Science of Aging Knowledge Environment. ISSN 1539-6150