Why are even the animals getting fat?

The World Health Organization predicts that, in a few years, obesity-related diseases—diabetes, heart disease, strokes and kidney failure—will be the world’s major causes of death.

The reason most commonly given for the increase in obesity, especially in the United States, is that people don’t get enough exercise or eat healthy food.

Fat CatBut a research team headed by a bio-statistician named David B. Allison has found that American animals also are getting fatter—pet dogs and cats, alley rats, laboratory mice, and marmosets, chimpanzees, macaques and vervet monkeys in primate research centers.

Now it could be the case that pet dogs and cats are not getting enough exercise, and alley rats are eating more junk food.  But what about the primates and the laboratory mice?  They live in controlled environments in which their diet and activities don’t change from year to year.

Obviously people who exercise regularly and eat fresh fruit and vegetables will on average be healthier than those who don’t.  But scientists are coming to doubt that exercise and diet alone are the keys to the obesity epidemic.   They look for biochemical factors that cause the body to store more fat.

Allison’s team studied 12 different animal populations, subdivided into male and female.

The biggest weight gainers were the chimpanzees, whose average weight increased 33.6 percent every 10 years.  Marmosets gained 9.3 percent and vervet monkeys gained 8.8 percent.  Macaques gained an average of 11.5 percent per decade in a California primate research center, 9.6 percent in an Oregon center and 5.3 percent in a Wisconsin center.

Cats gained 9.7 percent and dogs gained 2.8 percent

Laboratory mice gained 12.5 percent and laboratory rats gained 3.4 percent.  Feral city rats gained 6.9 percent and feral country rats gained 4.8 percent.

The only sub-group to show no appreciable weight gain over the decades were female laboratory rats, with an average increase of only 0.2 percent every 10 years, barely enough to measure.  Male rats gained an average of 6 percent per decade.

David Berreby, writing in Aeon magazine, reviewed some possible factors.  Stress and sleeplessness, for example, are linked to disruptions in leptin, the hormone that tells the body when it has had enough to eat.   The prevalence of artificial light, interfering with natural sleep and biological rhythms may be a factor.

Other possibilities are a virus, bacteria or industrial chemical.  Candidates include Bisphenol-A (BSA), a chemical found in most household plastics, and the Ad-36 virus, an endocrine disrupter. If there is a common factor affecting humans and animals, my guess is that these are the most likely.

But at this point we just don’t know.

Click on The Animals Are Also Getting Fat for a short article by Alex Tabarrok for Marginal Revolution.

Click on The Obesity Era: As Americans got fatter, so did marmosets, vervet monkeys and mice for a broader survey of the subject by David Berreby for Aeon magazine.

Click on Canaries in the Coal Mine: a cross-species analysis of the plurality of obesity epidemics for the original scientific paper.

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