Human Biochemistry, Volume 2 - Murray R. 1993

Special Topics
Nutrition, Digestion, and Absorption
Amino Acid Nitrogen and the Requirement for Specific Amino Acids

Dietary protein normally meets the body's requirement for amino acid nitrogen and the Amino Acids themselves. All dietary protein undergoes Digestion and enters the bloodstream as individual amino acids. The body requires 20 amino acids to synthesize specific Proteins and other nitrogenous compounds, such as Purines, Pyrimidines, and heme. Humans cannot synthesize 9 of these; these are the Essential Amino Acids, which must be obtained from the diet (Table 53.1). Two Other Amino Acids, Cysteine and Tyrosine, can be formed from the essential amino acids Methionine and phenylalanine, respectively. When dietary methionine and phenylalanine are deficient, cysteine and tyrosine become essential amino acids. Conversely, when cysteine and tyrosine are present in the diet in adequate amounts, they help spare methionine and phenylalanine. As long as the diet supplies sufficient essential amino acids, the remaining 9 amino acids required for Protein Synthesis AND other purposes can be synthesized via Transamination reactions.

Nitrogen balance (see also Chapter 30)

Under conditions of metabolic equilibrium in adult animals, dietary protein is required to replace the loss of essential Amino Acids and amino nitrogen during their metabolic turnover. Nitrogen is lost through urine, feces, saliva, sloughed Skin, Hair, and Nails. Data on the daily requirements for total protein and essential amino acids in humans are presented in Table 53.4. When calculated per unit of body weight, it becomes quite evident that these requirements are markedly elevated in infants and children. They also increase during Pregnancy, Lactation, wound healing, convalescence, and periods of high physical activity. For most situations, a diet in which 12% of the energy comes from protein is adequate.

The efficiency of dietary protein utilization determines the required amount. In addition, this amount depends on the following factors: protein quality, energy intake, and physical activity.

A. Protein quality. The quality (nutritional value) of a protein is determined by how closely the proportion of essential amino acids in the food matches the requirements for adequate Nutrition. The closer the two values, the higher the protein quality. Egg and milk proteins have high nutritional value, are efficiently utilized by the body, and serve as a standard for evaluating other proteins. High-quality protein is found in meat, whereas many plant proteins used as staple foods exhibit relative deficiencies in certain essential amino acids—for example, Tryptophan and Lysine (corn, grains), lysine (wheat), and methionine (certain beans). In a mixed diet, the deficiency of a given amino acid in one protein is compensated for by its Abundance in another. Such proteins are considered complementary; for example, combining wheat and bean proteins provides a complete balance of consumed amino acids. Under such conditions, total protein intake must be increased to meet nutritional requirements. Amino acids that are not incorporated into newly synthesized protein and are not required for the immediate needs of the body cannot be stored; instead, they are rapidly degraded and excreted as urea and other products.

Class="center">Table 53.4. Data on PROTEIN AND AMINO acid requirements and dietary intake. (Data from Recommended Dietary Allowances, 9th ed. Food and Nutrition Board, National Research Council - National Academy of Sciences, 1980.)

B. Energy intake. Energy derived from CARBOHYDRATES and fats influences the protein requirement by exerting a protein-sparing effect. To use "expensive" (high-value) dietary protein efficiently and minimize requirements, it is essential to ensure an adequate intake of non-protein Energy Sources, particularly carbohydrates, which spare protein from being catabolized via Gluconeogenesis.

C. Physical activity. Physical activity enhances the retention of nitrogen from dietary protein.

Protein-energy malnutrition

Protein-energy malnutrition encompasses a spectrum of disorders caused by starvation, in which deficiencies of other nutrients, such as Vitamins AND MINERALS, play a role alongside Protein deficiency. The acute form of this condition is frequently observed in young children (typically under 5 years of age) in developing countries of Asia, Africa, and South America. Its most pronounced manifestations are marasmus and kwashiorkor, though various intermediate forms are also described. Marasmus presents as generalized wasting resulting from a deficiency of both energy and protein, whereas kwashiorkor, characterized by edema, is associated with quantitative and qualitative protein deficiency despite potentially adequate energy intake.