The biochemistry of stress related starvation is quite different from simple starvation when looked at from a clinician’s point of view. There are many events that are not seen as clinical events since every organ is affected and changes occur at cellular and sub-cellular level. There are significant differences between stressed and simple starvation. If humans are not stressed they use fat for energy but during stressed states endogenous fat is a poor source of energy, protein is broken down at alarming rates.
A look at what proteins do reveals that they make up the critical cell structure in viscera, red cells and connective tissue while also being present in the muscles. Enzymes and antibodies are also made up of proteins. During starvation, less essential proteins for example skeletal muscles are degraded and gluconeogenesis is needed to meet obligatory demand of 150g/day. A simple calculation shows us that 3.5gm of glucose or 14 Kcal is derived from 6.25 gm of protein. So 150gm of glucose (minimum requirement to provide substrate to WBC’s) is derived from 270gm of protein and since 60% of muscle mass is water therefore 270gm of protein represents an actual loss of 675gm of protein per day. This represents a lot of protein being lost during stressed times such as during treatment after surgery etc.
Glycogen which is also a form of protein is produced in muscles and in liver. But the glycogen produced in muscles can only be used by the muscles. Hence people who are going for some strenuous activity like a marathon usually take in a lot of rice and spaghetti before it. The liver glycogen on the other hand can be used by the entire body but it depletes itself in 24 hrs. The excess glucose present in the body does not reduce gluconeogenesis and if not immediately metabolized is stored as glycogen or converted to fatty acids and stored as triglycerides.
Dietary fats like SFA, MUFA, PUFA and dietary cholesterol provides the body with fatty acids. The essential fatty acids are Linoleic acid and alpha-linoleic acid. During fatty acid metabolism triglycerides are broken into fatty acids and glycerol in capillaries. During lipid metabolism there is increased lipolysis but also increases re-esterification, the net effect being ineffective utilization of endogenous fat as an energy source. One of the problems is that most of the infused triglycerides are hydrolysed by lipoprotein lipase by endothelium in extra-hepatic sites. Parenteral fat emulsion is safe in liver disease as human hepatocytes are poor in lipoprotein lipase.
There are significant differences in normal starvation and stressed starvation. The problem of stressed starvation can be taken care of if the treatment for the ailment for which the patient is in ICU is undertaken closely.