During starvation, body materials are necessarily used up to supply the energy requisite for the vital functions. Some of the organs, like the heart and the respiratory muscles, are maintained in continued activity independent of the intake of food, while life lasts. The result is well known. The body loses weight during inanition, owing to the consumption of some of its own tissue substances. This process of nutrition at the expense of one's own tissues may go on not alone for days, but even for weeks. Fasts exceeding, a month in duration are not at all uncommon in the literature of physiology.5
Whether or not all tissues participate to a comparatively equal degree in the depletion brought about by starvation is a question which has long interested investigators. Statistics on the relative losses of weight in the case of different organs were collected more than half a century ago.
THE LOSSES OF WEIGHT DURING STARVATION. JAMA. 1915;LXV(22):1920. doi:10.1001/jama.1915.02580220060026