But your basal bodily need (aka basal metabolic rate) does not really drop, as its bulk does not involve active digestive functions, but primarily circulatory processes, respiration, keeping your body's temperature up and so on - all the things that are non-negotiable to keep the body alive around the clock.

The number varies slightly with genetics and age, but does not vary notably between fasting and eating, nor between sedentary and active lifestyle, and averages at about 26 kcal per kilo body weight. This parameter is what makes up the resonable daily intake limit to aim at (or under) to let the body consume itself through catabolism (ideally lipolysis) in the safest way possible, without having to involve exercise.