Intermittent fasting lowers measurable inflammatory markers in healthy people and in people with metabolic conditions, and through a metabolic switch influences how immune cells respond. The evidence is reasonable for those two groups, but the idea of a complete immune reset remains an appealing theory without direct confirmation in humans. In autoimmune diseases the picture is mixed and guidance and further research are needed.
In healthy people, intermittent fasting measurably lowers inflammation in the blood. Two forms have been studied most thoroughly: time-restricted eating (all meals within a window of, for example, eight hours) and alternate-day fasting. Both reduce inflammatory markers such as CRP, a protein that indicates how active inflammation is in the body. This applies to healthy people; in people with inflammatory bowel diseases, good research was lacking, and the well-known Ramadan fast showed barely any effect1.
In people with overweight or type 2 diabetes the evidence is stronger. Intermittent fasting appears to reduce so-called metabolic inflammation and to improve blood sugar regulation, even when no weight is lost. This effect works through the interplay between the liver, fat tissue, muscles and immune cells, and is tissue- and time-dependent, meaning that not every form of fasting produces the same result2.
When the body runs through its sugar reserves during a fasting period, it switches to fatty acids and ketones. That metabolic switch influences how immune cells respond to infections and inflammation, and also alters the composition of gut bacteria. Researchers call this a plausible mechanism behind the immune effects of fasting, but they also emphasise that there are potential side effects that you should weigh up before starting a fasting regimen3,4.
For autoimmune diseases such as rheumatoid arthritis and lupus, there are early indications that fasting lowers inflammatory markers and modulates the immune system. The picture is mixed, however: for multiple sclerosis, thyroid diseases and psoriasis the results are contradictory or largely absent. There are as yet no good guidelines for the optimal duration or form of fasting for these conditions, and long-term effects have not been studied5.
An evolutionary idea also circulates suggesting that the modern habit of constant eating disrupts the gut microbiome and thereby over-stimulates the immune system, and that fasting offers a kind of 'reset' of the sort our bodies once knew. This is theoretically appealing, but there is no direct experimental confirmation in humans; it is a hypothesis, not a proven mechanism6. A small proteomic study involving fourteen people who fasted for thirty days from sunrise to sunset found more immune-related proteins in the blood, but it lacked a control group, so firm conclusions cannot be drawn from it7.
Seven sources used: two reviews with moderate evidence on metabolic inflammation and inflammatory markers in healthy people, four reviews/studies with limited or mixed evidence on autoimmune diseases, neurodegeneration and the evolutionary theory, and one small proteomic study (n=14) without a control group. No large RCTs or meta-analyses available in this set. Total number of participants not specified precisely; the only study with an explicit number had 14 subjects.