During a fast, is a person more vulnerable to disease?

Question

Studies have shown that extended fasting (>~72 hours) can result in a 30% turnover in the immune system, creating healthy new cells in the refeeding period. However, during a fast, is a person more vulnerable to disease? At what point in the fast do the old immune cells die?

Answer 1

The widely cited "30% turnover" in the immune system largely stems from the groundbreaking research by Dr. Valter Longo and his team at the University of Southern California.

Are You More Vulnerable to Disease During a Fast?

Yes, temporarily. During an active, extended fast (>48–72 hours), your body enters an acutely immunosuppressed state.

While you are fasting, your body is conserving energy. It does this by drastically reducing circulating white blood cells (particularly lymphocytes and monocytes) in the bloodstream.

1. Higher infection risk while fasting: Studies (such as a notable Icahn School of Medicine study published in Immunity) show that during a fast, the brain signals immune cells to retreat into the bone marrow or undergo programmed death. If you are exposed to a severe bacterial or viral pathogen during the peak of an extended fast, your body may react more slowly or less aggressively, making you temporarily more susceptible to catching an infection.
2. The "Magic" Happens During Refeeding: The famous immune renewal does not happen while you are starving; it happens when you eat again. Breaking the fast triggers hematopoietic stem cells (HSCs) in your bone marrow to wake up and rapidly produce a fresh batch of young, highly functioning immune cells to replace the ones that were destroyed.

At What Point Do Old Immune Cells Die?

The process is continuous, but the most dramatic decline occurs between 24 and 72 hours:

1. 24 to 48 Hours (The Trigger Phase):

As liver glycogen drops to zero and the body switches entirely to ketosis, nutrient-sensing pathways like PKA (Protein Kinase A) and mTOR shut down. This serves as the biological signal that resources are scarce.

1. 48 to 72 Hours (The Clearance Phase):

Between hours 48 and 72, the body accelerates autophagy (cellular self-eating) and apoptosis (programmed cell death). It selectively targets damaged, old, or inefficient white blood cells to recycle their component amino acids for survival energy. White blood cell counts in the blood drop significantly during this exact window.

1. Beyond 72 Hours (Stem Cell Activation):

By 72 hours, the severe depletion of circulating white blood cells sends an urgent signal to bone marrow stem cells. The moment nutrients enter the body during refeeding, these stem cells activate to rebuild the immune system from scratch.