In a study from Mount Sinai, scientists found fasting may be detrimental to fighting off infection and could lead to an increased risk of heart disease.
They found that skipping meals triggers a response in the brain that negatively affects immune cells. The results could lead to a better understanding of how chronic fasting may affect the body long term.
In the study, researchers aimed to better understand how fasting—from a relatively short fast of only a few hours to a more severe fast of 24 hours—affects the immune system.
They analyzed two groups of mice. One group ate breakfast right after waking up (breakfast is their largest meal of the day), and the other group had no breakfast.
The team collected blood samples in both groups when mice woke up (baseline), then four hours later, and eight hours later.
When examining the blood work, researchers noticed a distinct difference in the fasting group.
Specifically, the researchers saw a difference in the number of monocytes, which are white blood cells that are made in the bone marrow and travel through the body, where they play many critical roles, from fighting infections, to heart disease, to cancer.
At baseline, all mice had the same amount of monocytes. But after four hours, monocytes in mice from the fasting group were dramatically affected.
The team found 90% of these cells disappeared from the bloodstream, and the number further declined at eight hours. Meanwhile, monocytes in the non-fasting group were unaffected.
In fasting mice, the researchers found the monocytes traveled back to the bone marrow to hibernate. Concurrently, the production of new cells in the bone marrow diminished.
The monocytes in the bone marrow—which typically have a short lifespan—strongly changed.
They survived longer as a consequence of staying in the bone marrow, and aged differently than the monocytes that stayed in the blood.
The researchers continued to fast mice for up to 24 hours and then reintroduced food. The cells hiding in the bone marrow surged back into the bloodstream within a few hours.
This surge led to a heightened level of inflammation. Instead of protecting against infection, these altered monocytes were more inflammatory, making the body less resistant to fighting infection.
This study is among the first to make the connection between the brain and these immune cells during fasting.
The findings suggest that, on the one hand, fasting reduces the number of circulating monocytes, which one might think is a good thing, as these cells are important components of inflammation.
On the other hand, the reintroduction of food creates a surge of monocytes flooding back into the blood, which can be problematic.
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The study was conducted by Filip Swirski et al and published in Immunity.
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