Proteins are broken down into amino acids during digestion. Unlike fats and carbohydrates, which are stored for later use, proteins are not. Instead, amino acids are either used to produce ATP through oxidation or contribute to the creation of new proteins for the growth and repair of the body. Any surplus amino acids from the diet are converted into glucose or triglycerides rather than excreted.

Amino acids play various roles in the body once they are absorbed into cells. They are restructured into proteins, which can serve multiple functions such as enzymes, transporters, antibodies, or hormones

Protein breakdown occurs daily in the body as part of normal physiological processes. While cortisol can promote protein catabolism, other factors such as exercise, growth cycles, aging, and cellular turnover also contribute significantly. Proteins from dying or expired cells are broken down into amino acids, some of which are reused to form new proteins or converted into other amino acids. Additionally, some amino acids are metabolized by liver cells (hepatocytes) into fatty acids, ketone bodies, or glucose, depending on the body's energy needs.

Before amino acids can be utilized for ATP production, they need to be converted into molecules that can either join the Krebs cycle or become a part of it, such as acetyl CoA. However, this conversion requires the removal of the amino group (NH2), a process known as deamination, which occurs in hepatocytes. Deamination produces ammonia, a highly toxic substance that is quickly converted into urea by liver cells. Urea, which is a relatively harmless substance, is then excreted through urine.

Protein anabolism, the process of creating new proteins by forming peptide bonds between amino acids, happens in the ribosomes of nearly every cell in the body, directed by the cell's DNA and RNA. Several hormones stimulate protein synthesis, including growth hormone, insulin-like growth factors, thyroid hormones, insulin, estrogen, and testosterone. Given that proteins are a primary component of most cell structures, adequate dietary protein is vital, especially during growth periods, pregnancy, and times of tissue repair following disease or injury.

Among the 20 amino acids found in the human body, 10 are considered essential, as they must be obtained through diet because the body can't produce them in sufficient amounts. Complete proteins, which contain all essential amino acids, can be found in food like beef, fish, poultry, eggs, and milk. Incomplete proteins, which lack some essential amino acids, are found in leafy green vegetables, legumes, and grains. However, some plant-based foods, such as quinoa and soy, are exceptions and contain complete proteins with all essential amino acids.

The body can synthesize nonessential amino acids through a process called transamination, where an amino group is transferred from an amino acid to pyruvic acid or an acid in the Krebs cycle. Once the necessary essential and nonessential amino acids are present in the cells, protein synthesis can proceed rapidly.

Disorders of Protein Absorption, Catabolism and Anabolism:

Protein-Energy Malnutrition (PEM): A condition typically associated with insufficient protein and calorie intake, PEM can lead to severe health problems, including muscle wasting, stunted growth, and a compromised immune system. Two primary forms of PEM are kwashiorkor (protein malnutrition) and marasmus (calorie malnutrition).

Muscle Atrophy: This disorder is characterized by the wasting away of muscle tissue, often due to extended periods of inactivity, aging, or diseases such as ALS (Amyotrophic Lateral Sclerosis) or muscular dystrophy. These conditions lead to an imbalance where protein catabolism exceeds protein anabolism, causing muscle degradation.

Chronic Kidney Disease (CKD): In CKD, the kidneys' ability to filter waste products from the blood is impaired. This leads to a buildup of urea, a waste product of protein catabolism, causing symptoms like nausea, fatigue, and loss of appetite.

Growth Disorders: Conditions like acromegaly (excessive growth hormone in adults) and gigantism (excessive growth hormone in children) result from the overproduction of growth hormone. As a result, protein anabolism is over-stimulated, causing abnormal growth of tissues and organs.

Cancers: Uncontrolled cell growth in cancers, is primarily driven by mutations in oncogenes and tumor suppressor genes, leading to dysregulated cell cycle control and unchecked cell division. While protein synthesis may be upregulated to support rapid cell proliferation, cancer primarily stems from metabolic and genetic dysregulation rather than simple overactive protein anabolism.

Proteinuria: This condition involves the presence of an abnormal amount of protein in the urine, typically indicating a problem with kidney function rather than protein metabolism. It occurs when the kidneys' filtering capacity is compromised, allowing proteins to pass into the urine. While proteinuria is commonly associated with chronic kidney disease (CKD), it can also occur in other kidney disorders. It is important to note that not all cases of CKD present with proteinuria, and the two conditions, though related, are not synonymous.

Amino Acid Metabolism Disorders are a group of disorders that affect the body's ability to metabolize certain amino acids. Examples include phenylketonuria (PKU) and maple syrup urine disease. In PKU, the inability to break down phenylalanine leads to toxic buildup in the blood, causing intellectual disability and other complications. In maple syrup urine disease, certain branched-chain amino acids cannot be broken down, resulting in neurological damage if untreated.

Lysinuric Protein Intolerance: This is a rare genetic disorder characterized by the body's inability to digest and use certain amino acids. Symptoms can include failure to thrive, muscle weakness, and intellectual disability.

Disorders of Protein Digestion: These occur when protein digestion processes are impaired. Examples include exocrine pancreatic insufficiency, where insufficient digestive enzymes are produced, and celiac disease, where an autoimmune reaction to gluten damages the intestinal lining, impairing protein absorption.

Kwashiorkor and Marasmus: These are severe forms of malnutrition associated with protein deficiency. Kwashiorkor primarily results from protein deficiency despite adequate or near-adequate caloric intake. It is characterized by edema, poor growth, and loss of muscle mass. In contrast, Marasmus arises from severe caloric deficiency, including both protein and energy deprivation, leading to extreme wasting of muscle and fat tissues. Both conditions highlight the importance of balanced nutrition but differ in their underlying dietary deficiencies and clinical presentation.