We know that obesity ups the risk for chronic conditions like heart disease, diabetes, stroke, and some cancers, but carrying around a significant amount of extra weight can also weaken the immune system, making the body more susceptible to sneezes, sniffles, and sick days.
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According to a study published in Occupational Medicine, when researchers at King’s College in London analyzed the BMIs and number of sick days for short- and long-term illnesses among 625 people who worked for London’s public transportation system, they found that those who had a BMI of 30 or higher took 4 more sick days, on average, per year compared with their slimmer coworkers. (Video: Stay fit while you work at home. Here’s how to set up a healthy home office)
Evidence also suggests that obese individuals have more respiratory infections and are more prone to viral infections, including the flu, notes Pamela Fraker, PhD, a professor of Biochemistry and Molecular Biology at Michigan State University. In fact, during the 2009 H1N1, or “swine flu,” pandemic, a disproportionate number of obese individuals were hospitalized for their symptoms, according to a report published in the Journal of the American Medical Association.
Among 1,088 swine-flu-related hospitalizations in California during the outbreak, 25% were for those who were classified as morbidly obese, meaning they had a BMI of at least 40, yet these individuals made up less than 5% of the US population at the time the study was conducted. When combined, obese (those with a BMI of 30 or greater) and morbidly obese individuals made up 34% of the US population at the time of the study, yet they represented 58% of swine flu hospitalizations. (Search: How do I know if it’s a cold or the flu?)
A possible explanation for the overweight/under-the-weather connection: Obesity overstimulates the immune system and impairs the body’s normal response to bacterial and viral infections. “Obesity results in a low-grade, chronic state of inflammation,” says Patricia Sheridan, PhD, a research assistant professor at the University of North Carolina at Chapel Hill. While we typically think of inflammation as the swelling that follows a joint injury or a bee sting, the term can be used to describe the body’s defense against anything foreign—including excess fat.
“An increase in fat results in the production of various hormones and cytokines (immune molecules), all of which can affect the immune system,” says Sheridan. While the body requires a certain balance of immune molecules in order to set in motion a complex and coordinated response to germs, viruses, and other pesky invaders, excess body fat fuels the production of so many immune cells that signals get jammed and the body begins to attack itself.
What do hormone levels have to do with an obese person’s immune system? Researchers at Michigan State University found that leptin, a hormone that is produced by fat cells and controls appetite and how quickly we burn off calories, also enhances the body’s immune response. (Related: Are Hormones Sabotaging Your Weight Loss?) The more body fat you have, the more leptin there is floating around in your bloodstream. Problem is, too much of the hormone can lead to leptin resistance, the point at which leptin can no longer do its job. For their study, published in Proceedings of the National Academy of Sciences, MSU researchers provided supplemental leptin to obese mice that had nonfunctional leptin (they were leptin resistant). While it was no surprise that the supplement helped the obese mice eat less, it also doubled their number of B cells, a type of white blood cell that produces antibodies and fights infection.
Similar results have been observed in humans. For a recent study published in Innate Immunity, researchers investigated the effects of obesity on the immune system in 119 female college students in Saudi Arabia. When comparing blood samples, BMI, and waist-to-hip ratios (WHR), the researchers observed that BMI and WHR correlated to concentrations of two of the most common types of disease-fighting white blood cells.
