BIOCHEMISTRY: A TEXTBOOK FOR HIGHER EDUCATION - E. S. Severin - 2004

SECTION 9. AMINO ACID METABOLISM AND FUNCTIONS

II. Biological Value of Proteins

A. Nitrogen balance

Amino Acids (both free and incorporated into Proteins) account for nearly 95% of total body nitrogen, which is why they are the primary regulators of the body's nitrogen balance. Nitrogen balance is the difference between the intake of nitrogen via diet and its output (predominantly as urea and ammonium salts). When nitrogen intake equals output, the body achieves nitrogen equilibrium. This state is typical of a healthy individual on A balanced diet. A positive nitrogen balance (where nitrogen intake exceeds output) is observed in children and in patients recovering from severe illnesses. A negative nitrogen balance (where nitrogen excretion exceeds intake) occurs during Aging, starvation, and severe diseases.

A protein-free diet leads to a negative nitrogen balance. Maintaining such a diet for a week causes nitrogen excretion to stop increasing and stabilize at approximately 4 g/day. This amount of nitrogen is contained in 25 g of protein. Consequently, protein starvation forces the body to catabolize about 25 g of its own tissue proteins per day. The minimum Dietary Protein Intake required to maintain nitrogen equilibrium is 30 — 50 g/day, while the optimal intake for an individual with moderate physical activity is 100 — 120 g/day.

B. Adequacy of protein Nutrition

Over the course of evolution, humans have lost The ability to synthesize nearly half of the twenty amino acids that make up proteins. These include amino acids whose synthesis involves numerous steps and requires a large set of Enzymes encoded by multiple genes. Consequently, amino acids whose synthesis is complex and metabolically costly for the Organism are best obtained from the diet. Such Amino acids are termed essential. They include phenylalanine, Methionine, Threonine, Tryptophan, valine, Lysine, leucine, and isoleucine.

Two amino acids—Arginine and Histidine—are produced in sufficient quantities in adults, but growing children require an additional dietary supply of these amino acids. Therefore, they are classified as conditionally essential (semi-essential). Two Other Amino Acids—Tyrosine and Cysteine—are conditionally dispensable because their synthesis depends on Essential Amino Acids. Tyrosine is synthesized from phenylalanine, whereas The formation of cysteine requires the sulfur atom from methionine.

The remaining amino acids are readily synthesized within Cells and are classified as non-essential. These include Glycine, aspartic acid, asparagine, glutamic acid, glutamine, Serine, Proline, and Alanine.

As shown above, dietary proteins serve as the primary source of amino acids for the body's cells. Protein content varies widely across different food products (Table 9-1).

Class="center">Table 9-1. Protein content in selected food products

The table shows that common plant-based foods contain low amounts of protein (with the exception of peas and soybeans). Animal-derived foods are the richest sources of protein (meat, fish, cheese). Proteins themselves are not strictly essential dietary factors; rather, they serve as vehicles for the essential amino acids they contain, which are required for proper nutrition.

The nutritional value of a protein depends on its Amino Acid Composition and digestibility by the body. Proteins vary significantly in their amino acid profiles. Some contain a full Complement of essential amino acids in optimal ratios, while others lack one or more essential amino acids. Plant proteins, particularly those from wheat and other cereals, are not fully digestible because they are encased in a protective outer layer made of Cellulose and other Polysaccharides that resist Hydrolysis by digestive enzymes. Certain proteins have an amino acid composition similar to human body proteins but cannot be utilized as food sources due to their fibrous Structure, poor solubility, and resistance to gastrointestinal proteases. Examples include Hair, wool, and feather proteins. When a protein contains all essential amino acids in the required proportions and is readily digested by proteases, its biological value is arbitrarily set at 100, and it is considered a complete protein. Egg and milk proteins fall into this category. Beef proteins have a biological value of 98. Plant proteins are generally inferior to animal proteins in biological value because they are harder to digest and deficient in lysine, methionine, and tryptophan. However, a strategic combination of plant proteins can provide the body with a complete and balanced amino acid mixture. For instance, corn proteins (biological value 36) are low in lysine but contain adequate amounts of tryptophan, whereas bean proteins are rich in lysine but deficient in tryptophan. While each of these proteins is incomplete on its own, a mixture of beans and corn provides all the essential amino acids required by humans.

C. Dietary protein allowances

To maintain nitrogen equilibrium, consuming 30 — 50 g of protein per day is sufficient. However, this amount does not ensure optimal health and working capacity. Standard protein allowances for adults and children take into account climatic conditions, profession, working conditions, and other factors. An adult with moderate physical activity should consume 100 — 120 g of protein daily. For heavy physical labor, this requirement increases to 130 — 150 g. For children under 12 years of age, 50 — 70 g of protein per day is adequate. This assumes that the diet includes a varied mix of animal and plant proteins.

D. Protein deficiency

It is well known that even prolonged exclusion of fats or CARBOHYDRATES from the human diet does not cause severe health disorders. Conversely, a protein-free diet (especially a prolonged one) causes profound metabolic disruptions and invariably leads to death. Omitting even a single essential amino acid from the diet impairs the utilization of other Amino Acids and results in a negative nitrogen balance, wasting, growth retardation, and neurological dysfunction.

Specific manifestations of individual amino acid deficiencies have been demonstrated in rats fed diets lacking specific amino acids. For instance, the absence of cysteine (or cystine) resulted in acute hepatic necrosis, while a lack of histidine caused cataracts; methionine deficiency led to anemia, fatty Liver, cirrhosis, alopecia, and renal Hemorrhage. The exclusion of lysine from the diet of young rats led to anemia and sudden death (this syndrome was absent in adult animals).

Protein malnutrition leads to a disease known in Central Africa as "kwashiorkor," which translates to "golden (or red) boy." Today, this term is widely used in other PARTS OF THE world to describe similar clinical symptoms. The disease typically develops in children who are deprived of milk and other animal proteins and fed an exclusively plant-based diet consisting of bananas, taro, millet, and most commonly, corn. Kwashiorkor is characterized by growth retardation, anemia, hypoproteinemia (often accompanied by edema), and fatty degeneration of the liver. In individuals of African descent, the hair takes on a reddish-brown hue. This condition is frequently accompanied by Atrophy of the pancreatic acinar cells, which impairs the secretion of pancreatic enzymes and prevents the Digestion of even the small amounts of dietary protein consumed. Renal damage ensues, leading to a sharp increase in the urinary excretion of free amino acids. Without Treatment, the mortality rate among children reaches 50 — 90%. Even if children survive, prolonged protein deficiency causes irreversible damage not only to physiological Functions but also to cognitive development. The disease can be reversed by promptly switching the patient to a protein-rich diet containing large amounts of meat and dairy products. One approach to solving this problem is fortifying foods with lysine supplements.