Model organisms have potential value as ‘canaries in the coalmines’ or ‘sentinels’ informing us about environmental factors potentially impacting humans . In this light, we compiled data to assess time trends in body weight in mammalian species that live with or around humans in industrialized societies. Such observations might help identify environmental influences that might otherwise go undetected.
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.
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. As if research wasn’t complicated enough as it was 🙂
Hat tip to our good friend, colleague, and former labmate Dr. Jen Kuk for passing along the study.
Klimentidis, Y., Beasley, T., Lin, H., Murati, G., Glass, G., Guyton, M., Newton, W., Jorgensen, M., Heymsfield, S., Kemnitz, J., Fairbanks, L., & Allison, D. (2010). Canaries in the coal mine: a cross-species analysis of the plurality of obesity epidemics Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.1890