The Physiology of Starvation

It is important to understand that the body will regulate metabolism differently depending on the macromolecules stores that are available. Important changes in metabolism occur in both skeletal muscle and the brain when the body is starving or fasting for a couple of days, compared to when this occurs for a couple weeks.

During the 1-2 day mark, the skeletal muscle will switch from glucose consumption to free fatty acid and ketone metabolism, while the brain will still try and use glucose whenever possible. The liver is also a major player during this phase, because it can activate the Cori cycle to store lactate from glucose and create a lactate reserve that can go through gluconeogenesis to form glucose once again for immediate energy needs (Sherwood, Parris, & Cahill, 1970). During this period of lower glucose levels, insulin levels are also decreased because of less of a need to put glucose into the cells. This will stimulate lipolysis and proteolysis and breakdown triglycerides and proteins. Although the skeletal muscle will use a variety of macromolecules during this initial starvation phase, the brain will continue to use glucose as its primary energy source.

As starvation continues into a week and then to two weeks the rate of proteolysis declines, and the skeletal muscle and brain begin to change their metabolism pattern. If starvation continues long enough the body will begin proteolysis once again but cannot sustain this for long before death. During this phase, the brain will switch from using glucose to using primarily ketone bodies as a source of fuel. At the same point protein breakdown is no longer as efficient because the brain is not in immediate need of glucose made by gluconeogenesis from amino acids.

The use of ketone bodies by the brain is a useful adaptation; however it is important to note that prolonged use of ketone bodies can cause ketoacidosis if the mechanisms to reduce this are not working. The kidney is a prime location where H+ ions can be excreted by converting an ammonia to an ammonium ion and thereby protect the pH balance in the body. Thus the body has useful methods for surviving through starvation, but like any organism there is a limit to how long survival is possible.

Ferrier, D. R. (2014). Biochemistry. Philadelphia: Wolters Kluwer Health/Lippincott Williams &                Wilkins.

Sherwood, L. M., Parris, E. E., & Cahill, G. F. (1970). Starvation in Man. New England Journal of Medicine, 282(12), 668–675. https://doi.org/10.1056/NEJM197003192821209