Based on...what? Humans lived on much lower calories for thousands of years. Only in the past two centuries have Westerners been told they need "2,860 calories per day" or what have you, by the USDA or other incompetent authorities who were likely lobbied to do so. And obestity rates EXPLODED. It's all a lie. We do not need this many calories at all, and intermittent fasting is critical to good health as well.
IF is good, particularly down to a 4-6 hr feeding window. But 800 is obviously unsustainable past the rapid weight loss phrase. Even Okinawans pre 1960 averaged 1500-1800 a day. The longest lived people on earth and aren’t vegetarians by any stretch.