Before you worry too much about putting on those holiday pounds, consider this: We're not the only species with weight issues.
- Mammals that live near or with humans, from lab rats to pet pooches, have gotten fatter over recent decades.
- Fatter lab animals are the hardest to explain, since their diet is tightly controlled and well documented.
- Endocrine disruptors, pathogens or temperature changes might play a role and could also factor into human obesity.
There's no question that humans have been getting bigger and bigger, and now it seems that animals living near us are coming along for the ride. A new study of 12 distinct populations of eight different mammals -- including feral rats, lab animals and domestic pets -- shows that they, too, have been gaining weight over the last several decades.
"It could be that for every one of these populations, there's a different reason why their weights are going up," said study lead author David Allison of the University of Alabama at Birmingham, who published the findings today in the Proceedings of the Royal Society B. "It could also be that within each, there are different reasons. What's exciting to speculate about is, could there be some factor that is affecting all of these?"
Indeed, some of the findings appear straightforward. Rats living on the streets of Baltimore were 40 percent heavier in 2006 than in 1948 probably at least partly because the scraps they are finding to eat are more plentiful and richer or because cats tend to prefer smaller rats. Likewise, we may be more prone to overfeed our house cats and dogs these days.
But other findings are more mysterious. Rats, mice and primates (four types were analyzed in this study) in laboratories are fed a highly controlled, known diet that has remained relatively constant over time. Why are these animals getting fatter?
Perhaps for some reason they're choosing to eat more of what they are offered or are somehow changing how they metabolize it, he said.
Allison pointed out at least three potential contributions to this and the other observations: endocrine disrupting chemicals, pathogens such as a virus, and/or changes in temperature where the animals are kept.
There is evidence to support a role for each of these in obesity. The endocrine-disrupting chemical tributyltin, for instance, which is added to marine paints to prevent growth of aquatic life on ship hulls and other places, has been shown to make mice fatter, Allison said.
Meanwhile, several types of animals have been shown to gain weight when injected with a virus known as adenovirus-36, indicating that pathogens may play a role in some cases of obesity.
As for temperature, Allison pointed out that hog farmers know that it's easier to fatten pigs if you keep the temperature just right: not so cold that the pigs need to expend energy to keep warm, and not so hot that they reduce their appetite. Homes are probably kept at more neutral temperatures today than in the past, and labs may also have better temperature control, he said, which may be part of the story.
Allison came upon the findings by accident. He was looking at the data for a population of lab marmosets and noticed that the weight of the marmosets appeared to be increasing. Intrigued, he looked for similar data available for other mammals, eventually gathering information on the 12 populations used in the study, which he divided further into males and females to make 24 separate analyses.
In all 24 groups, Allison's team found that animals' weight increased over periods that spanned at least ten years and often more.
"It fits perfectly well with the problem that we have to fully explain why people are becoming so obese today," said Arne Astrup of the University of Copenhagen, who was not part of the study. "Why are people so hungry that they eat 20 percent more than they should do to maintain a healthy body weight? Too much fat, soft drinks, refined carbs, etc cannot fully explain why some people are getting so dramatically obese."
"I am very open to these findings because I strongly do not believe that we understand the cause of obesity," said Barbara Corkey of the Boston University School of Medicine. "You can't get obese without overeating, but why do people overeat?"