It's called the obesity paradox. Although obese people are more apt to suffer from inflammatory diseases, such as diabetes, heart disease, and stroke, they are also more likely to survive a major attack caused by one of those conditions.
University of Illinois scientists Gregory Freund and Christina Sherry shed light on the reasons for this phenomenon in a study in this month's issue of Endocrinology.
"Fat is a very complex and active tissue--it has important functions beyond providing energy and insulating us from the cold," said Freund, a professor in the U of I College of Medicine's Department of Pathology and a faculty member in the U of I Division of Nutritional Sciences.
"We now know that leptin, a hormone secreted by fat tissue, plays a key role in regulating the immune system. When we exposed mice to hypoxia (simulating an event, such as a heart attack, in which a part of the body is deprived of oxygen), leptin triggered the immune system to increase production of an anti-inflammatory molecule, interleukin-1 receptor antagonist (IL-1RA)," he said.
"And, when we gave non-obese mice leptin injections, they recovered three times faster. Leptin did not hasten recovery though in IL-1RA knockout mice," Sherry said. That earlier work was published in a recent issue of Brain, Behavior, and Immunity.
In the Endocrinology study, one group of mice was fed a high-fat diet for 12 weeks (Sherry described this group as being in a mildly obese, pre-diabetic state), while another group was fed a normal diet. The obese mice recovered from acute hypoxia five times faster than the mice fed normal diets.
In a second experiment, Freund and Sherry examined macrophages (or immune cells) that were resident in peritoneal fat tissue from both groups of mice. In mice fed the high-fat diet, there was a very significant increase in IL-1RA as compared to mice fed the normal diet (330 pg vs.15 pg).
"Our hypothesis is that the macrophages from animals fed the high-fat diet are making more IL-1RA because they're 'living' in an environment of significantly increased leptin. Obesity can be considered a state of hyperleptinemia," said Sherry.
The scientists then isolated the stromal vascular fraction of the fat tissue--which contains the macrophages--from three obese mice and injected it into the peritoneal cavity of a normal mouse. "Within 3 hours we saw an 836 percent increase in the IL-1RA serum level of the normal mouse and an accelerated recovery from hypoxia," Sherry said.
To confirm that IL-1RA was implicated in this accelerated recovery, normal mice were given injections of IL-1RA, and their recovery matched that of mice fed the high-fat diet.
Finally, Freund and Sherry injected IL-1RA antiserum into the obese animals an hour before exposing them to hypoxia.
"We expected the antibodies to bind all the IL-1RA so it couldn't affect recovery," she said. "And, sure enough, these animals didn't recover until 6 hours after the hypoxic event, basically the same pattern we saw with non-obese animals."
"So we were able to prove through several different mechanisms that in cases of obesity, there's a significant increase in the anti-inflammatory molecule IL-1RA, which helps animals recover from a traumatic loss of oxygen. The obese mice recovered faster because of the leptin-induced increase in IL-1RA," said Sherry.
Sherry repeated that obesity is a predisposing factor in many inflammatory conditions and encouraged people who are at a healthy weight to maintain that weight.
"However, obese persons do have about six times more circulating IL-1RA, which again is the anti-inflammatory molecule that aids recovery in oxygen-starved parts of the body," she noted.
"Once a health problem is established, there are certain conditions--congestive heart failure, rheumatoid arthritis, and other diseases that involve the utilization of oxygen--in which obese persons have been shown to have a better prognosis. And there is a legitimate discussion among physicians about how such patients should deal with their excess weight," said Freund.
Stephanie Kim of the U of I College of Medicine's Department of Pathology co-authored the Endocrinology study. Jason M. Kramer, also of the U of I College of Medicine's Department of Pathology, and Jason M. York of the U of I Department of Animal Sciences, are co-authors of the Brain, Behavior, and Immunity paper.
Funding for both studies was provided by grants from the American Heart Association, the National Institutes of Health, the U of I Agricultural Experiment Station, and in part by the U.S. Department of Homeland Security, Assistance to Firefighters Grants Office, Research and Development Grants.