Proteins Health Article

Digestion, metabolism, and elimination

Food in the human diet consists of proteins, carbohydrates, fats, vitamins, and minerals. The majority of minerals and vitamins pass through to the bloodstream without the need for further digestive changes, but other nutrient molecules must be broken down to simpler substances before they can be absorbed and used. Foods in the stomach are broken down by the action of the gastric juice containing hydrochloric acid and a protein-digesting enzyme called pepsin. Gastric juice is needed mainly for the digestion of protein by pepsin. If a hamburger and bun reach the stomach, there is no need for extra gastric juice for the bun (carbohydrate), but the hamburger (protein) will require a much greater supply of gastric juice. The gastric juice already present will begin the breakdown of the large protein molecules of the hamburger into smaller molecules: polypeptides and peptides.

From the time a protein-containing food is eaten, to its breakdown and subsequent use or excretion, many complicated processes and steps take place in the body. These processes are part of metabolism, in which a series of metabolic pathways are involved in the breakdown of the foods. Proteins are initially degraded into constituent amino acids, which may be converted to compounds called pyruvic acid or acetyl-CoA before being passed into the metabolic pathway known as the Krebs cycle; or they may enter the Krebs cycle directly after being converted into one of the metabolites of this metabolic pathway.

Proteins contain substantial amounts of nitrogen. When dietary protein is broken down into amino acids, nitrogen is produced and is eliminated in the urine in the form of urea, and in smaller amounts as uric acid, ammonia, and creatinine. Unabsorbed protein is excreted in the feces, but only about 10 grams per day because protein is used very efficiently in the body. Amino acids may be recycled many times for different functions. There are minute losses of protein as skin, or in menstrual blood, semen, and hair.

Function

After water, protein makes up the greatest percentage of human body weight. This key nutrient provides the building blocks children and adults need for growing, maintaining, and repairing worn-out cells. Without protein, human bodies could not regulate fluids and immune systems would shut down. In fact, if not for protein there would be no hormones or enzymes—the protein compounds that take part in every single physical function. The role of protein in the diet is mainly as a source of amino acids, some of which are essential because they cannot be produced in the body. Others are referred to as nonessential because they can be made in the body from simple precursors. Amino acids are central to every human bodily function with every chemical reaction that occurs. Some of the uses of amino acids are:

• Synthesis of substances called purines and pyrimidines, important for deoxyribonucleic acid (DNA).
• Producing creatine in skeletal muscle; creatine is needed for subsequent production of creatinine.
• Building and maintaining muscle and tissues.
• Maintaining proper cellular function.
• Controlling chemical reactions through enzymes.

There are also circulating proteins in the plasma of the blood that vary depending on the levels in the diet. Some examples of plasma proteins with important functions in the body are: serum albumin, retinal binding protein, fibrinogen, etc. If the protein intake is low, these proteins will be reduced in the blood and therefore their functions in the body may be affected.

Approximately 300 grams of protein is produced per day in the body with a dietary intake of about 100 grams. Some of the protein needed is acquired from endogenous sources (in the body from protein breakdown) and is released into the intestinal lumen; it is estimated at about 70 grams per day.

Role in human health

The human body, minus water, is mostly composed of amino acids. Almost all of the hormones are amino acids. Regulation of protein metabolism is necessary to maintain proper bodily function; therefore, it is important to eat protein-rich foods. Protein is also important for building body tissue and synthesizing enzymes. Twenty amino acids are used for protein synthesis. Animals and plants are quick and available sources of what are termed "essential" amino acids; they are called essential because the body cannot internally build them. Normal growth and health are dependent upon these essential amino acids. These essential amino acids are histidine, tryptophan, lysine, methionine, phenylalanine, threonine, valine, leucine, isoleucine and possibly arginine. Tyrosine and cysteine are produced in the body from phenylalanine and methionine respectively. The "nonessential" amino acids include alanine, glycine, serine, glutamic acid, aspartic acid, asparagine, glutamine, proline. They are considered nonessential because the body can produce them with simple precursors.