Controlled study shows the drop in metabolic rate as caloric intake drops. The intervention fed 60% of the energy of the ad libitum control - but equivalent essential nutrition (vitamins & minerals). After 6 weeks the intervention group’s total metabolic
rate and basal metabolic rate as a function of their lean body mass had dropped by 14% and 12% respectively.
This shows why Calories in : Calories out is not a simple function, but a mathematically chaotic one.