The reson it decreases is because your body must conserve as many calories as possible to maintain the functioning of its vital organs. read more
I think people throw around the ‘starvation mode’ thing too freely though. If you steadily decrease calories in any manner, as opposed to crash dieting, you likely won’t see a DRASTIC change in BMR, hormone levels like leptin, etc. read more
The ratio of N excretion (as an index of protein oxidation) to BMR is variable during short-term starvation, but calculations based on the results of several studies suggest a tendency for this ratio to transiently increase during the first 2-3 days of starvation. read more