What happens to metabolism if calorie intake remains below BMR for a significant period of time?

BMR is short for the basal metabolic rate. It is the rate of expenditure of the energy per unit time by the body at the time of rest. In other words it can be said that it is the the total amount of calories required to keep the body to do the necessary vital functions even in rest. Some of the activities which are regulated in the body even the body in the rest are breathing, control of the body temperature, control of blood circulation, and contraction of muscles all these activities require the expenditure of calories.

If the calorie intake becomes severely low than the basal metabolic rate will also get lower the body will not function as it should be.