The primary thing that needs to be considered is the basal metabolic rate that regulates the amount of calorie expended by the body for existence. Definitely as the net body mass becomes low on shrinkage with lowered bone density and body fat, the lowered calorie intake induces the body to burn lesser energy. Lowering of BMR is somewhat a self induced adjustment due to the metabolic changes associated with shrinkage helping the body to function as an efficient energy saver, thereby burning lesser calories. (United Nations University, 2008)
The next important adaptation related to the shrinkage must be a lower cardiac output due to the lower aerobic capacity. The lowered cardiac output that is the extent of oxygen transport from the heart to the cells of the body is related to the overall cardiovascular requirements of the body. A low cardiac output is definitely coupled with the lowered tidal volume of respiration in a reduced body with a decrease in the cardiac tissue mass.
There is a formidable relationship between body mass, body size and blood pressure level. In fact blood pressure has been found to vary directly to the body height. The body being smaller in size hardly requires much pumping pressure to transmit blood to the rest of the body. Hence, lowered blood pressure is an essential physiological adjustment the body makes in order to go with the small stature after the shrinkage.
Due to the lowered blood pressure and cardiac output proper blood circulation demands more heart beats for a smaller sized body as compared to a larger one. This is, in fact, seen in case of smaller animals that have a faster heartbeat rate than humans. As a matter of fact the heartbeat rate in mammals varies inversely to the fourth root of the body weight measured in kilograms.
Amino Acid Metabolism
The protein - energy relationship is a significant parameter that adjusts itself with the other physiologic conditions during phenomenon of shrinkage. Amino acids required as essential substrates for protein synthesis are necessary for energy production along with increased body weight and stature. Therefore shrinkage essentially involves an adaptable lowered rate of amino acid metabolism, thereby curbing the extent of fat production leading to protein synthesis at a lower profile than in regular sized individuals.
Ammonia and Urea Metabolism
With a smaller stature the dietary requirements become lower and result in lowered amino acid intake thereby reducing the production of ammonia and urea in the liver. Moreover due to a lowered extent of protein synthesis, the essential enzymes like urease and protease are synthesized accordingly in sparse quantities compared to the normal sized individuals decreasing the production of ammonia to perfectly go with the smaller stature after shrinkage.
1. "Low BMIs, BMRs and Energy Requirements", 2008, United Nations University, retrieved from: http://www.unu.edu/unupress/food2/UID10E/uid10e08.htm#changes%20in%20body%20weights%20and%20stature%20and%20their%20influence%20on%20bmi%20and%20energy%20requir