Skipping breakfast may be bad for the immune system, making it more difficult for the body to fight off infection, according to a new study.
Fasting could also lead to an increased risk of heart disease, the research suggests.
Researchers say the study, carried out in mice, is among the first to show that skipping meals triggers a response in the brain that negatively affects immune cells.
The findings could lead to a better understanding of how chronic fasting may affect the body in the long term.
Director of the Cardiovascular Research Institute at Icahn School of Medicine at Mount Sinai, in New York, Filip Swirski, said: “There is a growing awareness that fasting is healthy, and there is indeed abundant evidence for the benefits of fasting.
“Our study provides a word of caution as it suggests that there may also be a cost to fasting that carries a health risk.
“This is a mechanistic study delving into some of the fundamental biology relevant to fasting.
“The study shows that there is a conversation between the nervous and immune systems.”
Researchers aimed to better understand how fasting for anything from a few hours to a more severe fast of 24 hours affects the immune system.
They analysed two groups of mice – one ate breakfast right after waking up, and the other group had no breakfast.
Blood samples were collected when they woke up, four hours later, and then eight hours later.
When looking at the fasting group, the scientists saw a difference in the number of the white blood cells called monocytes that are made in the bone marrow and travel through the body, where they play many critical roles, from fighting infections, to heart disease and cancer.
Researchers found 90 per cent of these cells disappeared from the bloodstream of the fasting mice, and the number further declined at eight hours. Meanwhile, monocytes in the non-fasting group were unaffected.
In the fasting mice, the researchers found that the cells travelled back to the bone marrow to hibernate, and production of new cells in the bone marrow diminished.
The cells survived longer as a consequence of staying in the bone marrow, and aged differently from the monocytes that stayed in the blood.
The researchers continued to fast mice for up to 24 hours, and then reintroduced food, according to the study published in the Immunity journal.
The cells hiding in the bone marrow surged back into the bloodstream within a few hours, leading 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, the scientists say.
Dr Swirski said: “Because these cells are so important to other diseases like heart disease or cancer, understanding how their function is controlled is critical.”
The scientists also found that specific regions in the brain controlled the monocyte response during fasting.
According to the researchers, the findings demonstrated that fasting elicits a stress response in the brain – potentially what makes people “hangry” (feeling hungry and angry).
This instantly triggers a large-scale migration of these white blood cells from the blood to the bone marrow, and then back to the bloodstream shortly after food is reintroduced.
The researchers say further research is needed to understand their findings.

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