Most of us aren’t fond of our flab but perhaps it’s time to see it in a new light. Fat cells may be among our first line of defence against pathogens.
“Fat has an additional role we didn’t suspect,” says Jay Kolls of the University of Pittsburgh.
Beyond physical barriers such as skin, our main protection against marauding bacteria and viruses is the immune system. This is a complex network of cells with sophisticated weapons such as antibodies, which recognise and destroy foreign cells and proteins. But although many microbes multiply rapidly, it can take days for antibody production to ramp up. It now seems fat cells also play a defensive role – and they respond more speedily than many parts of the immune system.
A team led by Richard Gallo of the University of California, San Diego, injected mice with Staphylococcus aureus bacteria under the skin. Within hours, the subcutaneous fat cells had released a chemical called cathelicidin. This is thought to disrupt bacterial cell membranes and is also known to harm viruses, although it is unclear exactly how.
Compared with these animals, another set of mice that had been genetically modified to have hardly any fat cells developed much worse infections when exposed to the bacteria. This suggests the cathelicidin-producing fat cells had a protective effect.
Gallo’s team was also able to show that human fat cells grown in the lab secrete cathelicidin, so something similar may happen in people.
But more fat does not equal more protection. In fact, people who are obese have a higher rate of soft-tissue infections. Kolls thinks this might be because obese people are more likely to have type 2 diabetes, in which fat cells are resistant to insulin. This may also reduce cathelicidin production. “A little bit of fat is good – too much is not good,” he says.