More may be making us fat than simply too much food and too little exercise, according to a new study led by UAB researchers.
The study, published late Tuesday in the biological journal of The Royal Society, looked at eight kinds of mammals and found that all of them, from monkeys to mice, have gotten heavier in the past few decades.
The researchers suggest that there could be a number of things responsible, from chemical pollution to infections to even the temperature we keep our buildings -- all less-considered factors for the obesity epidemic.
"We can't say the marmosets are getting fatter because they have less physical education in school," said one of the authors, David Allison, a professor of biostatistics at the University of Alabama at Birmingham, head of the section on statistical genetics and director of UAB's Nutrition Obesity Research Center. "There's no TV; they don't drive cars; they don't get to order from restaurants. We're not saying those things don't matter, but they can't be explained here."
Allison's not offering free rein to pig out on Thanksgiving, however. Instead, he hopes the report spurs more studies into the possible causes of human obesity.
"I don't think our story should affect what the individual member of the public does at this time at all," he said. "But I think it should affect other researchers and policy makers to be more open-minded, to recognize what we know and don't know and to consider alternatives."
The report, called "Canaries in the coal mine: A cross-species analysis of the plurality of obesity epidemics," looked at data collected over many decades on 24 populations of eight kinds of animals. The populations included lab macaques in Wisconsin, research chimpanzees in Georgia, pet dogs and cats, and wild rats gathered from the alleys of Baltimore. Some of the information dates as far back as 1948.
Researchers picked a base age for each species -- something roughly equivalent to the 30s in humans -- and charted weights over time. Without exception, the animals have gotten fatter, Allison said.
But why the tendency to pack on the pounds? In some cases, the study points out, there may be easy answers. Domestic dogs, for example, have been anecdotally reported as being heavier because they get less exercise than they once did. City rats may have access to more calories as people have started throwing away more food; alternately, cats tend to kill off the smaller rats over time, leaving the more robust rodents behind.
But when it comes to lab animals, who generally eat a rigid diet, the question is harder to answer, Allison said.
He figures there may be individual factors for some species plus some overriding reason, possibly a pollutant or infection, that explains why all the animals got heavier. He said that while most pathogens cause weight loss, some actually cause weight gain.
Another culprit could be endocrine disruptors, a class of chemicals found in industrial waste and some plastics that affect how hormones work and that have been linked to early sexual development and weight gain.
Allison said research is also looking into how temperatures might affect weight. There have been studies that show some wild animals getting heavier because global warming makes food more available. And indoor climate control -- that is, the way we keep our houses cool in summer and warm in winter -- can affect the way we eat and burn energy, which could also have an impact on animals housed indoors.
"There's a whole emerging line of research on this that's very exciting," Allison said.
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