Obesity could increase the severity of flu

A new study has found that people who are overweight obese are more susceptible to severe bouts of the flu than those who are a healthy weight. This could be due to changes in the immune system.

Obesity is a global health issue. It has a major impact on national economies and on human capital by reducing productivity and life expectancy, along with increasing disability and health care costs.

According to the World Health Organisation, obesity has tripled globally since 1975. Over two billion people across the world are classified as overweight or obese. 

The National Institute of Diabetes, Digestive and Kidney Diseases state that excess weight increases the risk of a range of health issues including: type 2 diabetes, high blood pressure, heart disease, stroke, sleep apnea, musculoskeletal disorders, fatty liver disease and some cancers.

A preclinical study conducted on mice, published in the open-access journal mBio of the American Society for Microbiology (ASM) states it could shed new light on the links between obesity in humans and flu risk. 

Findings indicate that obesity increases the severity of the influenza (flu) virus, most likely due to an impaired immune response in overweight individuals.

Researchers infected lean and obese mice with influenza for 3 days, allowing time for the virus to replicate. Viral samples were obtained from both the lean and obese mice and reintroduced in other obese and lean mice, respectively. Leader researcher Dr. Stacey Schultz-Cherry explained: "Basically, we wanted to mimic what would happen during an epidemic where the virus goes from one person to the next. What happens if a virus goes from a lean person to a lean person to a lean person versus an obese person to an obese person."

Due to the way that cells in an obese body respond to flu, this research suggests that overweight people do not have good antiviral responses.

When cells interact with the flu, the body usually prepares an immune response to stop the virus from replicating and spreading. It appears that the emergency immune response in obese mice is weakened.

Dr. Schultz-Cherry, who is also a Chair of ASM’s Public and Scientific Affairs Committee (PSAC), explained, "Obesity allows the virus to get in, replicate faster and make more mistakes. Some of those mistakes are potentially beneficial for the virus."

This is not the first time a link has been made between obesity and the influenza virus. Previous research has shown that people who are obese or overweight have higher influenza viral loads on their exhaled breath and take longer to fight off the flu. 

Other animal studies demonstrated that the influenza virus can spread deeper into the lungs for longer periods of time when obesity is present. Each year a new influenza vaccine is created because this virus continues to drift and change. Lead researcher Dr. Schultz-Cherry and colleagues hypothesised that the obese microenvironment may allow the influenza virus to change more rapidly.

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The use of animal models can be useful in the study of certain viruses - due the biological similarities between humans and other mammals. This does not mean that animal models are identical to humans, so conclusions must be taken with caution.

In future research, the scientists would like to focus on what is happening at the population level in humans. Studies like this give clues as to whether obesity is one of the reasons why there is so much viral drift each year, which requires vaccines to be continually updated.

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