24 hour fasting can weaken the immune system

Mice that fasted for 24 hours had more inflammation and were more likely to die from a bacterial infection than mice on a regular eating schedule


February 23, 2023

Fasting for a whole day can impair the immune system

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Fasting has previously been suggested as a useful approach to preventing or treating chronic diseases like cancer, obesity and heart disease – but results from mouse studies show that prolonged fasting can impair the immune system.

Filip Swirski of the Icahn School of Medicine at Mount Sinai in New York and his colleagues analyzed blood and tissue samples from five mice without access to food for 24 hours. Blood tests comparing these mice to mice fed a normal diet showed that the fasted mice had, on average, less than 10 percent the number of monocytes — a type of white blood cell that help destroy germs and other immune cells used to treat injuries to recruit and fight infection.

“These are the cells that are really important foot soldiers of the immune system,” says Swirski. The researchers found that the decline was due to monocytes retreating from the blood into the bone marrow, where they essentially hibernated, he says.

When the fasted mice were fed again, monocytes flooded back into their bloodstream. “Due to this excess of monocytes, which overwinter and live in the bone marrow, they survive longer than usual,” says Swirski. “So when we start feeding again, we see a surge of monocytes.” Compared to mice that continued to fast or never fasted, these mice had, on average, almost four times as many monocytes in their blood.

To understand how this immunity worked, he and his team injected 45 mice with a strain of bacteria that infects the lungs. 23 of the mice fasted 24 hours prior to injection. The researchers then allowed them access to food.

After 72 hours, nearly 90 percent of the mice that fasted died, while about 60 percent of the mice that never fasted died. The mice on the restricted diet also had greater levels of inflammation, suggesting that long periods of fasting impair the immune response.

However, Satchidananda Panda of the Salk Institute for Biological Studies in California says that most common human fasts last less than 24 hours. In fact, his own research showed that fasting for 15 hours boosted immunity in mice.

Still, Swirski says these results are important for how we think about the duration and effects of fasting. “Like so many things in life, balance is important. What can be beneficial on the one hand can have unexpected negative effects on the other,” he says.

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