Animals are getting fatter, too
Is something in the environment making everyone -- animals and humans -- gain weight?
Obesity levels have risen dramatically in research animals and others living close to humans, suggesting environmental factors are encouraging everyone to gain weight, according to new findings in the linkurl:Proceedings of the Royal Society B.;http://rspb.royalsocietypublishing.org/content/early/2010/11/19/rspb.2010.1890.abstract
|Obese mouse, linkurl:ornl.gov;http://www.csm.ornl.gov/SC99/GENwall.html |
"These results show that the obesity epidemic is not as simple as people might think," said linkurl:Jennifer Kuk,;http://www.yorku.ca/health/people/index.php?dept=K&mid=645785 a biologist who studies obesity at York University in Canada and was not involved with the study.
It's no secret that obesity has become an epidemic in humans -- among American adults, nearly one in three is obese, defined as having a Body Mass Index (BMI) greater than 30. Researchers have pointed their fingers at everything from a lack of physical activity to the highly processed foods that so many of us eat.
But what if something in the environment was at least partly to blame, as well? To investigate, linkurl:David Allison,;http://www.soph.uab.edu/ssg/people/davidallison a statistical geneticist at the University of Alabama, Birmingham, and his colleagues gathered data on body weights of more than 20,000 adult animals from 24 populations of 8 different species from around North America. The authors included only those mammals for which there were two weight measurements in the past 50 years, and whose weight was not deliberately manipulated as part of research or a livestock feeding program.
All 24 populations of animals, which ranged from primates housed in research facilities to feral rats living in the greater Baltimore area, showed significant increases in body weight. Average body weights of captive chimpanzees increased at a rate of 33 percent each decade, and 9 percent per decade in captive marmosets. Laboratory mice got 12 percent fatter every ten years, laboratory rats did by 3 percent; the average body mass of Baltimore's feral rats increased by almost 7 percent each decade. And house pets were no exception, either. The average weight of cats increased by almost 10 percent each decade, while dogs' weights increased by 3 percent every decade.
Not only did body weight increase significantly, but so did the chances than an animal would be obese. In 23 out of the 24 populations, animals were more likely to be obese -- defined as weight above the 85th percentile at the initial time-point -- at the second time-point than at the first.
What's more, the increased body weights and increased likelihood of obesity were found even in animals whose diets and physical activity levels were known to be the same throughout the study period. So if dietary changes and energy imbalance weren't responsible for the rise in obesity, said Allison, it may be some environmental factor.
"If we're seeing these trends in other mammals, it shows that there must be another explanation" besides the main culprits of inertia and poor diet, Kuk said.
Of course, some animals -- such as research animals -- could be eating more and exercising less just as we are. Allison acknowledged that this is likely possible, but noted that scientists have records of exactly what research animals were fed, and housing conditions haven't changed much in the past 50 years.
Environmental toxins and viruses are at the top of Allison's list of potential suspects. Several studies have linked endocrine disruptors such as bisphenol A (BPA) and some tin-containing compounds to increased body mass. Infections by viruses, specifically a type of the common cold-causing adenovirus, have also been linked to significantly increased body mass.
"We've got to really open our minds to thinking about some other things," Allison said. He emphasized that diet and exercise are still key factors in why people and animals gain weight, but "we clearly now have evidence that things outside this realm can shift the body weight distributions of an entire population."
Klimentidis Y, et al. "Canaries in the coal mine: a cross-species analysis of the plurality of obesity epidemics." Proc R Soc B. linkurl:doi: 10.1098/rspb.2010.1890;http://rspb.royalsocietypublishing.org/content/early/2010/11/19/rspb.2010.1890.abstract
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