A hormone that connects the body's metabolism and immune response system may explain why COVID-19 is so dangerous for people with obesity.

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"The problem for people with obesity is that their leptin levels are always high, and that can affect the response to a COVID-19 infection," said Candida Rebello, Ph.D., RD, lead author of a new paper that traces the link between obesity and the virus.

The hormone leptin regulates appetite and metabolism. Leptin also regulates the cells that fight infection. Leptin is produced by fat cells, and to a lesser extent by tissues in the lungs. The more fat a person has, the more leptin circulates in their body.