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Protein Metabolism in Health and Disease

written by: Emma Lloyd•edited by: Paul Arnold•updated: 10/12/2011

Proteins are an essential part of the human body—making up 45% of the body by weight—but despite this, protein can actually be a symptom of disease in certain circumstances.

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    Healthy Protein Metabolism

    Dietary protein can be obtained from many foods, including both plant and animal sources. During the metabolism of dietary protein, these macromolecules are broken up into constituent molecules, and from there into even smaller components.

    Protein is made up of amino acids, which contain carbon, hydrogen, oxygen, nitrogen, and sulfur. During the metabolism of protein, several things happen.

    1. Protein is broken down into amino acids, which are absorbed into the blood stream.
    2. Amino acids are broken down into smaller molecules. This process, which is called catabolism, involves deamination of the amino acids. This is the removal of the nitrogen-containing amino group.
    3. During deamination, a single amino acid is broken down into a ‘carbon skeleton,’ which contains only carbon, hydrogen, and oxygen. Ammonia is a byproduct of the deamination process.
    4. The ammonia produced by catabolism of amino acids is converted into urea, processed by the kidneys, and excreted in urine.
    5. The carbon skeleton is converted into glucose, used for synthesis of new proteins, or used in the production of energy (ATP).
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    Protein in Urine as a Marker of Disease

    The kidneys act as filtration units for the blood. As blood passes through the kidneys, they filter out waste, leaving behind essential molecules which then recirculate through the body.

    Most proteins are too large to filter through into the urine; hence in a healthy person protein is not present in urine. However, if the kidneys are damaged, proteins are able to filter through into urine. The protein most commonly found in urine is albumin, which is found in the blood, and helps to retain fluids.

    In a healthy metabolic state, urine sometimes contains trace amounts of protein. For example, this can happen after a strenuous physical workout. Other factors such as extreme temperatures, fever, and emotional stress can temporarily increase protein levels in urine by a small ammount. However, significant amounts of protein in the urine over the long-term can indicate that something is wrong.

    When the kidneys become damaged—typically through some sort of inflammation—the resulting symptoms often include protein in urine. Many infections and diseases can cause this type of inflammation to occur, including hypertension, diabetes, and kidney disease.

    The presence of protein in urine is, therefore, a signal that the kidneys have been sufficiently damaged by inflammation to allow larger molecules to pass into the urine.

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    Causes of Protein in Urine

    Many medical conditions can cause protein levels in urine to be consistently high. These include:

    • Amyloidosis
    • Kidney failure
    • Diabetes
    • Heart disease and heart failure
    • High blood pressure
    • Certain types of cancer, including leukemia and Hodgkin's lymphoma
    • Lupus
    • Malaria
    • Pericarditis
    • Rheumatoid arthritis
    • Sarcoidosis
    • Sickle cell anemia
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    Symptoms, Diagnosis, and Treatment of Proteinuria

    Perhaps the most easily distinguished symptom of proteinuria (protein in urine) is simply the appearance of the urine—it can appear foamy when large amounts of protein are presenct. In addition, the face, abdomen, hands, or feet may swell (due to the loss of albumin in the blood, and the reduced ability of the blood to draw fluid from those areas).

    However, most people with mild proteinuria will actually have no symptoms—in any case these often do not appear until there are large amounts of protein passing in the urine. Diagnosis of proteinuria is typically made via a urine test that measures the amount of protein present in the urine. If protein is found in the urine a doctor may then test for urea nitrogen and creatine, two molecules which can indicate impaired kidney function if found in high levels in the urine.

    Treatments for proteinuria depend largely on the underlying disease causing the symptoms. If a symptom is caused by high blood pressure or diabetes, for example, medication is often given to control the blood pressure or blood sugar.

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