Overweight and obese people may be more at risk from swine flu

Overweight and obese people may be more at risk from swine flu.

AS IF people struggling with obesity did not have enough to worry about, they now face a new health hazard. According to statistics from the US, overweight people appear more likely to die of swine flu.

Most of the people who have died from H1N1 swine flu have had an underlying health problem that weakened their ability to fight off the virus. Among the conditions recognised as increasing the risk from flu are hypertension, diabetes, chronic lung obstruction and coronary disease. Now it may be time to add obesity to the list.

Unpublished figures reported at a recent meeting of the US Centers for Disease Control and Prevention in Atlanta, Georgia, show that

of 99 people who died in the early stages of the pandemic in the US, 45 per cent were obese. As only 26 per cent of US adults are obese, this suggests that obesity doubles the risk of getting seriously ill with swine flu (see chart).

The figures surprised most flu researchers. "In 40 years of studying flu, I have never heard anything about obesity," says virologist John Oxford of Barts and The London School of Medicine and Dentistry, University of London. Obesity specialists, however, say it fits with what they have learned in recent years.

The only study looking directly at flu and obesity was done in 2007 by Melinda Beck and colleagues at the University of North Carolina, Chapel Hill. It was already known that abdominal fat releases a continuous stream of chemicals that trigger inflammation, an immune response normally aimed at killing invading pathogens and infected cells. So Beck's team wondered what effect this had on flu. They were especially interested, she says, because runaway inflammation, known as a "cytokine storm", is what kills most flu victims.

Beck and her team found that overfed, obese mice are nearly seven times as likely to die of ordinary flu as genetically identical lean mice (The Journal of Nutrition, vol 137, p 1236).

The researchers also measured immune chemicals in the mice's blood. Prior to infection, the obese mice had much higher levels of a hormone called leptin than the normal mice. During the initial stages of infection, they had fewer virus-killing cells and chemicals.

Leptin is released by fat cells and, among other things, triggers immune reactions. Beck thinks that obese mice become desensitised to leptin, making their immune system slow to react. "Our experiments suggest the problem is the fat itself."

As their flu worsened, the obese mice did mount an immune response, but it was "too little too late", says Beck. It failed to get rid of the virus and eventually triggered a runaway immune response that escalated until it killed the mice - much as the cytokine storm does in people.

We don't know if the same series of events happen in obese people with swine flu, Beck warns. But it is possible that, as in mice, obesity dampens our ability to fight flu by disrupting the immune response, says Jesse Roth, a diabetes specialist at Albert Einstein College of Medicine in New York. "The resting level of inflammation goes up in obesity," he says. He suspects that this disrupts the body's immune response to viruses, making a lethal runaway reaction more likely.

During a flu pandemic, it is more important than ever to tackle obesity, Roth says. "It's amazing how much obesity-related inflammation you can reverse with just a little diet and exercise." He says a daily half-hour walk and losing about 5 per cent of body weight if you are overweight is enough to reduce inflammation.

David Fedson, a former flu researcher at the University of Virginia in Charlottesville, has long proposed using drugs that damp down inflammation, such as statins, fibrates and glitazones, as an additional way of cutting deaths from flu. These drugs are normally prescribed for obesity-related disorders such as high cholesterol and insulin insensitivity.

Drugs that damp down inflammation could offer an additional way of cutting deaths from flu

The new figures on obesity and swine flu strengthen the case for stockpiling the drugs, given that shortages of vaccine and antiviral drugs are likely, Fedson says. "These drugs are safe and cheap, but they are being ignored by pandemic planners."