Proteins are biological molecules that serve various functions, including muscle structure and contraction, molecular transportation, enzymatic reactions, and as an energy source. Proteins form three-fourth of the total body solids and are the second most abundant molecule in the body after water. Amino acids are the building blocks of proteins. Amino acids contain an acidic group and a nitrogen-containing amino group. There are 20 different amino acids found in proteins of the human body. Of these amino acids, 10 are termed essential amino acids, which cannot be synthesized, and thus must be ingested. Sources of protein include meats, eggs, milk, nuts, legumes, whole grains, and various fruits and vegetables.
As proteins are digested, the component amino acids are absorbed into the bloodstream. These free amino acids in the blood act as a small reservoir for the various intracellular locations of protein utilization. There is a reversible equilibrium between plasma amino acids and intracellular proteins. A low concentration of plasma amino acids will cause intracellular protein catabolism and release of amino acids back into the bloodstream.
Amino acids enter into cell via either facilitated diffuse or active transport, as they are too large to passively diffuse through cellular membranes. Once inside of a cell, the amino acids are combined via cellular machinery to form intracellular proteins with various functions. The liver is a primary location for synthesis of plasma proteins which are released into the bloodstream. Examples include albumin, fibrinogen, and globulins. Albumin provides colloid osmotic pressure, retaining plasma in the capillaries. Failure to produce albumin (malnutrition, liver disease) or inability to retain albumin (renal disease) can result in hypoalbuminemia.
Once the body’s protein stores are maximized, excess amino acids can be broken down for energy or converted into storage entities like fat or glycogen. Protein release 4.1 kcal/g when oxidized. A byproduct of protein breakdown is ammonia, which is converted into urea by the liver. Urea is then excreted via the kidneys to remove nitrogenous waste. Liver failure can cause a decreased ability to convert ammonia into urea, leading to excess ammonia levels, ultimately resulting in hepatic encephalopathy.
The human body has a basal metabolic rate of approximately 20 grams of protein per day. Nitrogen balance is the measure of nitrogen intake minus nitrogen output. A positive nitrogen balance is required in times of growth (birth through adolescence, pregnancy) or to balance out losses. A negative nitrogen balance occurs during malnutrition or excessive wasting states, such as in septic or burn patients.
The body’s primary choice for energy under normal circumstances is carbohydrates. In a fasting state, stored energy sources of glycogen and fat are used. Once stores are depleted, proteins become a final source of energy. Degradation of proteins leads to rapid deterioration of cellular function and impending morbidity.
Protein metabolism is under multifactorial hormonal control. Anabolic hormones include growth hormone, insulin, ...