|Nutrition and Nutrients|
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Proteins and Amino Acids
Food supplies the building materials to permit continuous cellular renewal and growth. Proteins form a major part of our structure. Proteins are chains of amino acids, specified in the DNA code. Proteins are amino acids linked together like beads on a necklace. The individual amino acids fall into two groups: the essential AA's, which must be ingested, and the non-essential AA's, which can be synthesized in the body and need not appear in the food. A total of 9 amino acids are considered essential, while another 11 or so can be synthesized from the essential amino acids.
Most of our body protein is recycled and we do well by ingesting very little protein. About 3% of the total body protein is recycled every day (approximately 200 grams). In a healthy adult, net protein loss in a day may be as low as 2 grams. Dietary requirements for protein increase with activity, growth, and protein losses, especially following injury or during illness. The average American diet supplies 11-14% of total calories as protein, or 25-300 gms/day. Protein digestion and absorption are generally efficient. A minimum average protein intake is approximately 25 grams.
Since all amino acids contain a nitrogen atom (N), protein balance is synonymous with nitrogen balance. When nitrogen intake exceeds nitrogen loss, there is net protein synthesis. Anabolism, or tissue construction, prevails. When nitrogen losses exceed intake, protein tissue is being broken down and catabolism prevails. Loss of protein-tissues occurs with malnutrition, following surgery, injury, and chronic illness. Adequate intake of energy molecules, both carbohydrate and fats, is said to "spare protein", permitting a small protein intake to maintain positive nitrogen balance. In metabolic studies, the total amount of nitrogen intake is compared with the total excretion of nitrogen to assess protein balance. Excess amino acids may be converted to fuel.
When amino acids are "burned" as a fuel, ammonia (NH3] is the waste product. Ammonia must be carried to the liver, converted to urea and excreted by the kidneys. One of the penalties of amino acid excess is ammonia excess, a potential cause of body malfunction following a high protein meal. The blood measurement of urea nitrogen (BUN) shows the balance between urea production by the liver and excretion by the kidneys. The BUN rises in kidney failure and serves as a measure of ammonia or nitrogen. In liver disease, reduced ability to synthesize urea leads to ammonia accumulation. Ammonia is neurotoxic and contributes to the syndrome of brain dysfunction in liver failure, hepatic encephalopathy. Patients with reduced kidney or liver function are required to restrict protein, since their ability to handle the nitrogen waste of oxidized amino acids is limited. Fluctuating levels of ammonia should be considered whenever brain function is abnormal. Some children are born with metabolic abnormalities in the handling of amino acids and ammonia. They often present with malfunctioning brains.
Proteins and Disease In the popular imagination, proteins are the safe and desirable components of food. Food products are often promoted by boasting of high protein content and protein is connected falsely with increased energy. In body- building fantasies increased protein turns into bulky muscles. Standard nutritional recommendations in Canada and the USA promoted meat, milk and egg consumption as protein sources. RDA levels for protein intakes tend to be excessive. The idea that proteins are agents of disease is foreign to popular nutrition and most physicians are unaware of protein disease.
Protein diseases can be divided into a six categories:
1 Immune mediated disease, proteins act as antigens
Molecular assembly and molecular disassembly are essential procedures of life. The synthesis of complex structures from a finite set of raw materials underlies the prodigious complexity of life on earth. Control of molecular behavior is achieved by an elite class of protein molecules. Enzymes know how to grab other molecules and break them apart or stick them together, according to the very specific blueprint, contained in DNA molecules. Enzymes themselves are made by other enzymes from the amino acids which food proteins provide. There are about two thousand different enzymes. The largest enzyme populations live inside cells where they are attached to molecular assembly structures. Other enzymes are secreted into body spaces as mobile chemical workers.
Many enzymes become popular after someone writes an article praising their wonderful abilities to manipulate molecular behavior - ingestion of the enzyme is usually recommended. Superoxide dysmutase is one popular enzyme which cannot be delivered by oral intake to the intercellular sites where it does its useful things. Ingested proteins tend to get digested, losing their information as shape and function, or, if they are not digested, tend to cause allergic reactions rather than functioning normally. The next development of molecular engineering will be vehicles to deliver enzymes to intracellular sites where they will be useful. Delivery vehicles may be physical structures or carrier molecules that protect the enzymes while directing them through the GIT, circulation, and filtering systems such as the liver and lungs.
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