Your metabolism adapts to what you eat, you were built to survive, you are a miraculous meat suit! But are these adaptations detrimental to your goals? Your overall quality of life? Basal metabolic rate is what you (scientifically calculated) expend in energy just laying on the floor breathing, the energy required simply just to survive. If you are eating 1500 cal and you have a BMR of 1300 but you are also expanding 300 cal through activity, that leaves you with only 1200 cal to contribute to your BMR, your body will adapt and match those 1200 cal by decreasing its BMR. Understanding your basal metabolic rate and how it adapts to your environment can help you achieve your goals and improve your overall quality of life!