Scientists found that fasting mice had reduced levels of circulating immune cells, leaving them vulnerable to infections.
Time-restricted eating and other intermittent fasting diets are gaining popularity as a way to lose weight and improve health. While some reports indicate that periodically restricting food intake may improve metabolism and offer other health benefits, other studies suggest that eating less overall might be the better option for weight loss.
Now, a study published on February 23 in the journal Immunity suggests that skipping meals or fasting reduces the number of immune cells circulating in the blood — which could hamper the body’s ability to fight infections.
To examine the impact fasting had on the immune system, scientists from the Icahn School of Medicine at Mount Sinai divided mice into two groups. The team fed one group free choice and restricted access to food in another group after the mice became awake and active. The researchers also collected blood samples when the mice woke up, four hours later, and eight hours after waking to measure the number of circulating monocytes — an immune cell made in the bone marrow.
After four hours, the fasting rodent’s bloodwork showed a 90% reduction of monocytes circulating in the blood. What’s more, the monocyte numbers continued to decline after eight hours of fasting.
Meanwhile, the mice fed free of choice did not show a reduction in monocytes.
The scientists discovered that monocytes in the fasting mice traveled back to the bone marrow but survived longer than they usually would when circulating in the bloodstream. Still, the production of new monocytes in the bone marrow declined.
After 24 hours, the research team reintroduced food to the fasting mice. Within a few hours, the surviving monocytes hiding in the bone marrow migrated rapidly back into the blood. However, this sudden monocyte surge led to increased inflammation instead of boosting the immune system — making the mice less resistant to infection.
In a news release, lead author Filip Swirski, Ph.D., the director of the Cardiovascular Research Institute at Icahn Mount Sinai, explains, "the study shows 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, reintroduction of food creates a surge of monocytes flooding back to the blood, which can be problematic.”
The study authors also say that while some evidence suggests fasting may have health benefits, these results show that restricting food intake may hamper the body’s ability to fight infection. However, because this study used mice, more research is needed to understand how fasting impacts the immune system in humans.
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