Does fasting reset your immune system?

Fasting demonstrably lowers inflammatory markers in the blood. Time-restricted eating (for example, eating only within an 8-hour window) and alternate-day fasting reduce CRP, a protein that signals inflammation in the body, in healthy people. The effect appears to be greater the more consecutive days you fast. Ramadan fasting showed hardly any difference, probably because that protocol is fundamentally different.

The evidence is strongest when it comes to weight, blood pressure and blood sugar: an analysis of 56 studies shows with high certainty that modified alternate-day fasting reduces weight by an average of just over 5 kg and blood pressure by nearly 8 points. These are not direct measures of the immune system, but chronic inflammation is closely linked to excess weight and poor blood sugar regulation, so these effects count indirectly.

Fasting also appears to influence gut bacteria favourably and restores the daily rhythms of genes that regulate the gut. Because the gut microbiome strongly drives the immune system, this is a plausible pathway. Part of that evidence, however, comes from animal studies and has not yet been fully confirmed in humans.

Whether fasting truly 'resets' the immune system in autoimmune diseases remains unclear. There are tentative indications of a beneficial effect in rheumatoid arthritis and lupus, but for conditions such as multiple sclerosis and psoriasis the evidence is too thin to draw any conclusions. Optimal fasting schedules and long-term effects in this group are unknown.

Long-term safety has not yet been well studied. If you have diabetes, an autoimmune disease, or another chronic condition, discuss a fasting schedule with your doctor before starting.