FULL CLAIM: the act of fasting for as little as three days can regenerate the entire immune system
The article that contains this claim was originally published by the Daily Health Post in December 2014 and its contents were reposted in a separate article in June 2019. The article has circulated widely, receiving more than 800,000 shares and began trending again on Facebook as of September 2019. It claims that “the act of fasting for as little as three days can regenerate the entire immune system” based on the findings of a 2014 study. Similar claims on the immune-boosting effects of fasting have also been made in outlets such as Medical News Today, The Source and Gaia.
Scientists who evaluated the claim for this review include the senior author of the 2014 study, Professor Valter Longo, who explained that the claim is misleading and unsupported. They highlighted that the study was performed in mice, not humans, and so the effects cannot be directly extrapolated to people.
The study was performed with the goal of determining whether fasting cycles could help protect chemotherapy-treated mice from immunosuppression. Immunosuppression is a problematic side effect of chemotherapy in cancer patients because it can increase the risk of microbial infections. The results of the study therefore hint at a mechanism that could potentially protect chemotherapy-treated patients from immunosuppression, but this remains to be explored in future studies. The effects of fasting on immunosuppression have also not been thoroughly studied in healthy people who are not undergoing chemotherapy treatment – it is therefore premature to claim that fasting would “regenerate” the immune system for everyone.
Several other studies have demonstrated the profound effects of fasting on the immune system[2,3,4,5,6], however findings by some studies suggest that fasting could also have detrimental effects. For instance, one study found that fasting for 48 hours led to significantly slower wound healing in mice, as well as impaired immune response to infection. The effects of fasting may also differ depending on a person’s age and health status, making results hard to predict. As such, individuals are advised to consult their medical provider before beginning any fasting diet.
A summary of research findings on the potential benefits and drawbacks of fasting and calorie restriction diets has been published by the National Institute on Aging, a division of the U.S. National Institutes of Health.
The claim may be technically correct but misleading. Fasting increases the level and function of long-term hematopoietic stem cells (LT HSCs) in mice, which in fact can be responsible for the regeneration of the immune system and as far as I know the “entire immune system”. However, this observation was made in mice, not in humans. In humans, we only know that fasting decreases the level of immune cells while re-feeding restores the level of immune cells. This suggests but does not prove that similar regenerative effects will take place in humans.
The claim that fasting regenerates the entire immune system is NOT justified.
The study’s authors discovered that fasting has a pro-regenerative effect on stem cells after chemotherapy. The claim sounds as if one would get a brand new immune system through fasting. It is important to note that prolonged fasting in healthy adults can actually have negative effects on the immune response to infection and after tissue injury, as we reported in our recent study published in Cell.
- 1 – Cheng et al. (2014) Prolonged Fasting Reduces IGF-1/PKA to Promote Hematopoietic-Stem-Cell-Based Regeneration and Reverse Immunosuppression. Cell Stem Cell.
- 2 – Jordan et al. (2019) Dietary Intake Regulates the Circulating Inflammatory Monocyte Pool. Cell.
- 3 – Wang et al. (2016) Opposing Effects of Fasting Metabolism on Tissue Tolerance in Bacterial and Viral Inflammation. Cell.
- 4 – Nagai et al. (2019) Fasting-Refeeding Impacts Immune Cell Dynamics and Mucosal Immune Responses. Cell.
- 5 – Collins et al. (2019) The Bone Marrow Protects and Optimizes Immunological Memory during Dietary Restriction. Cell.
- 6 – Zenz et al. (2019) Intermittent Fasting Exacerbates the Acute Immune and Behavioral Sickness Response to the Viral Mimic Poly(I:C) in Mice. Frontiers in Neuroscience.