Study Finds That Prolonged Fasting May Impair The Immune System

Researchers from the Icahn School of Medicine at Mount Sinai in New York analyzed both tissue and blood samples from mice that did not have access to food for 24 hours and compared these samples with other samples taken from control mice who were given a regular diet. Results showed that on average the fasting mice had less than 10% the number of monocytes than the controls. This is important because monocytes are a type of white blood cell that helps to fight off infections as well as recruit other immune cells to help treat injury. 

“These are the cells that are really critical foot soldiers of the immune system,” says Swirski. The researchers found the decrease was due to monocytes retreating from the blood to the bone marrow, where they essentially hibernated, says Filip Swirski, Ph.D., at the Icahn School of Medicine at Mount Sinai. 

“Because there is this excess of monocytes that are hibernating and living in the bone marrow, they survive longer than they would otherwise,” says Swirski. “So, upon re-feeding, what we see is a surge of monocytes.” When compared with other mice that continued to fast or those that never fasted, these mice had nearly four times as many monocytes in their blood, on average.

To investigate how these findings may affect immunity, 45 mice were injected with a strain of bacteria that infects the lungs, 23 of which fasted for 24 hours before receiving the injections, then they were allowed access to food after the injections. Results showed that after 72 hours, close to 90% of the fasting mice has died, and nearly 60% of the control mice that never fasted died. Additionally, the fasting mice were also found to have had greater levels of inflammation which suggests that long periods of fasting impair immune response. 

According to Satchidananda Panda at the Salk Institute for Biological Studies in California, most of the common forms of fasting regimens for humans do not last for 24 hours, and that his own research has shown that a 15-hour fast helps to improve the immunity in mice. 

Despite Panda’s comments, Swirski says that the finding of this study are important to how we think about the duration and implications of fasting. “Like so many things in life, balance is important. So, what may be beneficial in one way could have an unanticipated negative impact in another,” he says.