500 kcal is too strong a restriction. The body begins to adapt to new conditions, trying to survive: 1. The metabolic rate decreases, i.e. less kcal is spent on maintaining life support. 2. At every opportunity, "reserves" are made - special enzymes are produced that help speed up the process of processing food into fat. In order to part with fat reserves with maximum efficiency, it is recommended not to reduce the calorie content of food less than 200-400 kcal below the level of basal metabolic rate .
Because 500 kcal is super low, and the body freaks out from what is happening and turns on the mode of extreme survival, retains water and gives up fat extremely reluctantly