Animal obesity: canary in the coal mine?

There are a number of factors, both behavioural and environmental, which are thought to play important roles in the current epidemic of obesity.  These range from things like increased soft-drink consumption and decreased physical activity, which are at least nominally under our personal control, to more external factors like viruses, light pollution, and environmental contaminants, over which we have little or no control. 

How much of a role do these external factors play in the obesity epidemic?  No one knows.  But if these external factors are playing a role in the human obesity epidemic, then we would also expect to see similar increases in body weight and obesity rates among animals who live with or near humans, since they would be exposed to many of these same factors.  To this end, a fascinating new paper by Yann Klimentidis and colleagues examines the body weight and risk of obesity in 8 different species, and the results suggest that external factors may be playing a larger role in obesity rates that previously thought.
From the paper:

From 24 distinct populations (12 subdivided into separate male and female populations), representing eight species, over 20000 animals were studied. Time trends for mean per cent weight change and the odds of obesity were tested for the samples from each population at an age period that corresponded roughly to early-middle adulthood (35 years) in human development.

So, what did they find?  Out of 24 different animal populations ranging from rodents (both feral and domestic) to several monkey species, every single population showed trends for increasing body weight, a result which is exceedingly unlikely to be due to chance (e.g. p=0.00000012). In general most animal populations, whether they live in the lab or roam our streets, have seen their average body weight increase by 5-10% per decade, although lab animals did tend to gain more than their free-living peers.  The clear losers of this survey are chimpanzees, which have seen more than 30% increase in body weight per decade, which is pretty astonishing.

As the authors point out in their discussion, these animal populations have all gained significant amounts of body weight in recent decades, despite little change to their diet or physical activity levels (for those in lab-controlled conditions, at least).  There are some arguments that feral animals may be under selective pressure to increase in size, and that both feral and domestic animals may have more food available due increasing amounts of food waste in our cities.  But when the authors looked at weight gain in non-lab (feral or domestic) vs lab animals, the non-lab animals actually gained less weight, not more.  In other words, animals populations living in the most strictly controlled conditions were the same ones who saw the greatest increase in body weight in recent years.  The authors also note that there may have been improvements in the housing conditions of lab animals in recent decades, but again it is interesting that the body weight of every single animal population in this study increased over time, despite wildly different living conditions, with no one factor able to explain the increases in these disparate groups.

Assuming that the issues discussed above are unlikely to account for all of the increase in body weight in these animal populations, what options are we left with? All sorts of things!  Everything from the AD36 virus, to endocrine distrupting chemicals, to light pollution and climate change.  Seriously.  There is an excellent paper on all of these putative "non-traditional" causes of the obesity epidemic which I hope to discuss on the blog soon, and which is available to all here.

Now I'm not yet ready to believe that diet and physical activity play no role in the development of obesity, as there is plenty of evidence that they do.  This paper also used some pretty intense statistical procedures that are above my pay-scale (not surprisingly, this paper was written by the same group as the Obesity Paradox paper that I discussed last week).  But there is enough evidence to suggest that these environmental factors play a role of some kind, and are definitely worth further study.  One other interesting point noted by the authors of the present study is that, assuming all of our lab animals are growing heavier over time, there might be unintended consequences for studies using animal models.

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