Infectious Concept of Obesity: Review Article
Obesity is currently experiencing an epidemic as declared by the World Health Organization. Obesity has multiple etiologies, one of them an infection. Many microbes are linked to obesity both in animals and human, but conclusive evidence for its role in human is lacking. At least, six pathogens are reported to cause obesity in animals. Canine distemper virus was discovered as the first virus reported to cause obesity in mice, followed by Rous-associated virus-7, which has been shown to cause obesity and hyperlipidemia in chickens. The obesity-promoting effect of Borna disease virus was demonstrated in rats. Scrapie agent was reported to increase the risk of obesity in mice and hamsters. The last two reports were of SMAM-1 and Ad-36, a human adenovirus that caused obesity in animals. An association with human obesity is the unique feature of SMAM-1 and Ad-36. Human studies are less convincing; but two adenoviruses, Ad-36 and SMAM-1, did show adipogenic properties. It is clear that in response to certain infections, adipose tissue expands similar to the expansion of cells of the immune system. In vitro studies with 3T3-L1 cells stated the activation of the enzymatic pathway that leads to fatty tissue accumulation; also higher levels of antibodies against such viruses in obese subjects were detected in vivo studies. Also, recent studies showed that C. pneumoniae and H. pylori are also associated with human obesity. This review discusses the related published data as well as the characteristics of pathogens that may be implicated in obesity.