In short
A fasting-mimicking diet (FMD) is a structured, typically five-day, low-calorie and low-protein eating protocol designed to trigger metabolic changes similar to water-only fasting while still providing some nutrition.
The best-known FMD protocol was developed by Valter Longo's research group at USC and is commercially available as ProLon. It provides a specific, plant-based sequence of pre-packaged low-calorie meals (roughly 1,090 kcal on day one, tapering to about 725 kcal on days two through five) formulated to keep blood glucose and IGF-1 (a growth-signaling hormone) low enough to trigger fasting-associated cellular responses, while avoiding the more severe complications of complete fasting.
The largest published human evidence is a 2017 randomized controlled trial in Science Translational Medicine by Wei, Longo, and colleagues involving 100 generally healthy participants who did three monthly FMD cycles. The FMD group showed reductions in body weight, blood pressure, fasting glucose, and IGF-1, plus a reduction in a composite marker the researchers called a "biological age" score, compared to controls eating a normal diet.
This is real human RCT evidence for changes in cardiometabolic biomarkers over a period of months — it is not evidence of extended lifespan or reduced disease incidence over years, which the trial was not designed or powered to measure. Longer, larger trials with hard clinical endpoints (disease incidence, mortality) have not been completed, and, as with most nutrition intervention research, adherence and diet-quality differences between the FMD and control arms make it difficult to fully separate the effect of the specific fasting-mimicking formulation from the effect of simply eating fewer calories for five days.