Flu hits obese people harder: study
Immune-system cells may suffer deficiencies
As flu cases climb, new Canadian research shows that obesity increases the risk of being infected with flu and other potentially serious respiratory-disease causing organisms.
Researchers who combed through the health records of more than 100,000 Ontario residents over 13 influenza seasons found that overweight and obese people were more likely to visit a doctor or to be treated in an emergency department for an acute respiratory infection than normal-weight people — even though they were more likely to be vaccinated.
The finding, published in the International Journal of Obesity, adds to a growing pool of evidence that obesity somehow alters the body’s immune response, leaving heavy people vulnerable to infectious diseases.
Earlier studies have shown that antibodies produced in response to flu shots plunge dramatically in the obese compared to people with healthy weights, and that obese people produce defective killer immune cells that are vital in fighting infections.
During the 2009 H1N1 “swine flu” pandemic, obese people — and particularly the “morbidly” obese, those with a body mass index of 40 or greater — were more likely to be hospitalized, admitted to an intensive care unit or die after being infected with H1N1.
But less clear is whether heavy people are more likely to catch the flu and other respiratory infections to begin with.
In Canada, 62 per cent of the adult population is overweight or obese.
For their new study, researchers tracked 104,665 Ontario residents aged 18 to 64 who responded to population health surveys between 1996 and 2008. Using physicians’ billing claims, the team counted up the number of visits to a doctor’s office or emergency department for acute respiratory infections during 13 flu seasons, and “control” periods when flu wasn’t circulating.
Since many people who see a doctor for flu aren’t swabbed and tested, the team used a cluster of diagnoses, including acute bronchitis and pneumonia, as a measure of flu during influenza season, said senior author Michael Campitelli, an epidemiologist at the Institute for Clinical Evaluative Sciences in Toronto.
Overweight people (those with a body mass index, or BMI, of 25 to 30) and obese people (a BMI of 30 to 35) had higher rates of visits for acute respiratory infections compared with normal-weight people during flu seasons. The increased risk was highest in the biggest weight classes, with severely obese people (a BMI of 35 or higher) having rates 20 per cent higher than normal weight people.
The team saw lower, but similar increased risks of respiratory infections among the obese when flu wasn’t circulating, “suggesting that increased BMI may enhance susceptibility to other viral and bacterial respiratory pathogens,” they write.
One theory is that obese people have “multiple deficiencies” in cytokines and other substances secreted by immune system cells, cells “that we rely on to fight against not just respiratory infection, but any real infection,” Campitelli said. “If your body’s got these deficits, they’re not going to go away when flu isn’t circulating.”
Other studies have shown the obese have more than a two-fold greater risk of developing a bloodstream, urinary tract or respiratory infection after surgery compared to healthy-weight patients.
The study has several caveats. Importantly, the researchers didn’t have access to lab test results to confirm the diagnosis of respiratory infection. However, obesity is known to be a risk factor for an ever-growing list of chronic diseases, such as diabetes, heart disease and certain cancers.
“Interventions designed to reduce the prevalence of obesity and subsequent chronic diseases may have the added benefit of reducing the burden of respiratory infections in the population,” the researchers write.
According to federal health officials, the number of positive influenza tests continued to increase in Canada for the weekending Jan.4, and H1N1 is the dominant strain.