In normal man, almost all the energy comes from carbohydrate and fat as long as they are present in sufficient amounts in the diet.  With a carbohydrate or fat deficit or a protein excess, the amino acids are metabolized to produce calories.  Approximately 4 calories are generated per gram of protein.  Normally for every 300 calories, 1 gram of nitrogen (6.25 grams of protein) is ingested in the normal diet, the majority of the nitrogen being used for protein synthesis.  With surgical trauma, or infection and its altered endocrine and inflammatory environment, more than 20 percent of the total energy comes from the use of protein as fuel. The RQ value for protein is 0.8.  The three branched chain amino acids (valine, leucine, and isoleucine) can be utilized directly by tissues for energy via the Krebs cycle and mitochondria.  The remaining amino acids require deamination in the liver with conversion to keto-acids which can be transported to all tissues for burning using the Krebs cycle. Protein synthesis is vital in the maintenance of the integrity of all cells, in particular those that have a rapid protein turnover.  Deamination of amino acids in the liver leads to liver urea production.  Adequate renal clearance is necessary to avoid a rapid increase in the blood urea concentration, especially during a catabolic state.  The rate of protein synthesis depends on the available amino acid substrate, tissue demands and the hormonal environment, especially the activity of anabolic hormones.

Sufficient protein intake in the form of amino acids is necessary to keep up with necessary new protein formation and net losses seen in the surgical patient.  In a normal uninjured man this usually requires about 1 gram of nitrogen (6.25 grams of protein) for every 300 calories – less than 1 gram of protein per kilogram of body weight.  With increased tissue demands and the increased use of protein for energy, as seen in injured patients, protein needs, increase to approximately 1.5 grams per kilogram of body weight, with a ratio of calories to nitrogen of 100:1 to as low as 80:1.  Growth hormone,  androgens, and insulin increase protein synthesis, whereas glucocorticoids and the catecholamines increase protein breakdown.

D: MICRONUTRIENT METABOLISM

Micronutrients are essential for cellular function.  They are called nutrients because of their key role in metabolism, but these of their key role in metabolism, but these compounds and elements also are involved in many other aspects of homeostasis, including wound healing, antioxidant protection, and immune function.  The term micro is used because of the extremely small amounts found in the circulation tissues.  Their concentration is critical to cellular function.

Micronutrients usually are divided into the organic compounds (vitamins) and inorganic compounds (trace minerals).  The micro-minerals are used in a variety of metabolic pathways often used as co-factors for enzymatic reactions.  Both need to be provided and both are utilized and lost in increased quantities to metabolic response to stress seen in the surgical patient.    Deficiency states therefore can occur easily.  Because measurement of adequate levels is difficult, if not impossible, prevention of a deficiency often is accomplished only by increasing intake.

 Vitamins are organic substances that are essential in humans for growth and homeostasis  A few of their characteristics are:

• Essential organic micronutrients involved in fundamental body functions.

• Supplied mainly by food.

• Each vitamin has multiple, unrelated functions.

• No chemical relationship among the group.

The term Vitamin was first used by a biochemist who discovered one of the first of these essential elements, which happened to be an amine, thus “vitamin”.  Vitamins are found in very small quantities in the body.  Each has a name defined by a letter as well as a chemical name.  These compounds play a key role in metabolism, growth, and homeostasis and therefore are especially important in surgical critical illness, in which hypermetabolism, healing, and immune function are so important for survival.  A deficiency state, which can occur readily, clearly will amplify  the magnitude of disease. 

It is important to point out that trace elements are absorbed from food and mineralized water, which means an intact, functioning gut and food intake are critical.  Tube feeding solutions and TON contain insufficient quantities of these nutrients for the critically ill.  The addition of trace elements in increased amounts to enteral and parenteral feeding regimens is of major importance in managing the surgical patient considering their importance in metabolism, healing, and immune defenses.