By Alla Katsnelson

It's not just people that are getting fatter.

A statistical analysis of more than 20,000 animals suggests that the obesity epidemic is spreading to family pets, wild animals living in close proximity to humans, and animals housed in research centers--perhaps indicating that environmental factors beyond diet and exercise are at least partly to blame for expanding waistlines.

David Allison, a biostatistician at the University of Alabama at Birmingham and lead author on a study published online November 24 in the Proceedings of the Royal Society B, stumbled across the trend while looking for a relationship between body weight and longevity in a population of marmosets housed at the Wisconsin National Primate Research Center in Madison. He decided to take a closer look. He and his colleagues examined changes in weight in a total of 24 populations (12 male and 12 female), drawn from 8 different species, including primates and rodents used for research, domestic cats and dogs, and urban feral rats. About half of the data in the analysis comes from rodents that had been used between 1982 and 2005 in the control arms of studies run by the National Toxicology Program, which assesses safe exposure levels for various chemicals.

The researchers tracked the animals' percentage weight gain per decade, as well as animals' odds of being obese. Because there were no clear guidelines for what animals should weigh, the authors defined obesity as the weight above the 85th percentile in each group at the earliest time point for which they had data. Both the percentage increase in body weight and the odds of an animal being overweight in a given population showed a strong trend upwards, and although the shift was statistically significant in fewer than half of the groups when they were analyzed individually, it was highly significant when all of the groups' figures were lumped together.

"Now, we don't know why these increases occurred, but it invites some very interesting speculation," Allison says.

The surge in human obesity is generally attributed to an increasing consumption of calories and a decrease in physical activity. "But maybe there are other things that are important -- because those things can't be acting on the marmosets, or the rats and mice in the National Toxicology Program," he says.

In some cases, the explanation might be obvious: the more than 40 percent jump in body weight in feral rats scavenging on the streets of Baltimore may reflect the increasing richness of their diet as they feed on our more calorie-dense refuse.

In other cases, hidden factors might be at play. For example, toxins that disrupt the endocrine system could be leaching into the water supply, or particular pathogens might be having a widespread effect on mammalian metabolism, Allison says.

Fat virus?

Nikhil Dhurandhar, an obesity researcher at the Pennington Biomedical Research Center in Baton Rouge, La., has shown that a human adenovirus called AD36 raises body-fat levels in animal models such as chickens and rodents. What's more, he found that obese humans were three times more likely to be infected with the virus than non-obese people--and heavier individuals in both groups tended to be infected. Dhurandhar reported these findings in a 2005 paper on which Allison was a co-author.

The current study is conceptually important, Dhurandhar says, because "it draws attention to having to look at the environment and how that is changing, instead of only focusing on lifestyle in a person".

However, Jaap Seidell, a nutrition and health researcher at the VU University Medical Center in Amsterdam who has studied the link between weight gain in pets and their owners, contends that the data presented in the study could be explained by lifestyle factors.

"This is an interesting collection of data, but it's very difficult to interpret them," Seidell says. Pets and feral animals might very well be subject to changes in our eating patterns, and there isn't enough information to conclude that the captive animals are exempt from such influences, he adds. Other factors may also have changed. For example, over the past 30 years the number of rodents housed in each cage may have altered -- which could very well affect the amount of exercise they get.

"I think they are trying to deflect the attention from restriction of physical activity and high-energy foods," Seidell says.

The paper includes the statement that Allison "has received grants, honoraria, consulting fees and donations from numerous food and pharmaceutical companies, litigators, and non-profit and government entities with interests in obesity-related matters". However, Allison stresses that this particular study was not financed by any company.

And Seidell does acknowledge that certain environmental factors could be affecting body-weight cycles in humans and other animals. In the wake of climate change, for example, some animals have stopped hibernating and others have shortened their seasonal migration routes.

The current study, however, is not rigorous enough to pinpoint whether such factors were responsible, he says. "I think it's worthwhile to do such a study in a systematic way," he says.