The 800 calories number indicates that this might be related to the Fasting Mimicking Diet of Valter Longo. Since actual fasting has a "scary" reputation among some patients and doctors, Valter Longo developed a 5 day fasting like protocol with severely restricted calories that appears to have similar benefits to fully fasting for 5 days. It looks like this modified version of complete fasting is gaining popularity among the medical establishment.
https://en.wikipedia.org/wiki/Valter_Longo#Fasting_mimicking...
https://www.marksdailyapple.com/does-the-fasting-mimicking-d...