Proteins: Crucial Components of All Body Tissues

What Are Proteins?

Proteins are large, complex molecules essential for the structure and function of all living cells. They play critical roles in metabolism, immunity, fluid balance, and nutrient transport, and can serve as an energy source under certain conditions. Proteins are unique among macronutrients because they contain nitrogen in a form readily usable by the human body.

Amino Acids: Building Blocks of Proteins

Amino acids are nitrogen-containing molecules that combine to form proteins. There are 20 amino acids in the human body, classified as essential or nonessential:

• Essential amino acids: Cannot be synthesized by the body and must be obtained from food (9 out of 20).

• Nonessential amino acids: Can be synthesized by the body.

Each amino acid consists of a central carbon atom, an amine group, an acid group, a hydrogen atom, and a unique side chain (R group) that determines its properties.

Transamination: Amino Acid Modification

Transamination is the process by which the amine group from one amino acid is transferred to a different acid group and side chain, allowing the body to synthesize nonessential amino acids.

Protein Synthesis: Formation and Organization

Proteins are formed by linking amino acids together via peptide bonds:

• Dipeptide: Two amino acids joined together.

• Polypeptide: Ten or more amino acids bonded together.

The process of protein synthesis involves two main steps:

• Transcription: Genetic information in DNA is used to make messenger RNA (mRNA), which carries the code to the ribosome.

• Translation: The ribosome reads the mRNA and assembles amino acids in the correct sequence to form a protein.

Protein Turnover: Synthesis and Breakdown

Protein turnover refers to the continuous process of protein synthesis and degradation in the body. Amino acids from food and cell breakdown enter the amino acid pool, which is used for various functions including synthesis of new proteins, energy production, and conversion to other compounds.

Levels of Protein Structure

The function of a protein is determined by its structure, which has four levels:

• Primary structure: Sequential order of amino acids.

• Secondary structure: Spiral shape due to chemical bonding between amino acids.

• Tertiary structure: Further folding into a unique three-dimensional shape.

• Quaternary structure: Association of multiple polypeptide chains.

Protein Shape and Function

Proteins lose their shape (denaturation) when exposed to heat, acids, bases, heavy metals, or alcohol, resulting in an irreversible loss of function. The shape of a protein is critical for its biological activity.

Protein Quality: Complete and Incomplete Proteins

Protein synthesis can be limited by the availability of essential amino acids:

• Incomplete protein: Lacks one or more essential amino acids; considered "low-quality" and may compromise growth and health.

• Complete protein: Contains all nine essential amino acids in sufficient amounts; considered "high-quality." Examples include animal proteins and some plant proteins (quinoa, soy).

Mutual supplementation involves combining two incomplete proteins to make a complete protein. Complementary proteins are two sources that together supply all essential amino acids (e.g., beans and rice).

Functions of Proteins in the Body

Proteins serve numerous functions, including:

• Cell growth, repair, and maintenance

• Enzymes and hormones

• Fluid and electrolyte balance

• pH balance

• Antibodies for immunity

• Energy source

• Transport and storage of nutrients

• Formation of compounds such as neurotransmitters, fibrin, and collagen

Role of Proteins in Fluid and Electrolyte Balance

Proteins help maintain fluid balance by attracting water and keeping it within blood vessels. Insufficient protein can lead to edema, a condition characterized by fluid accumulation in tissues.

Proteins also regulate electrolyte balance by transporting ions across cell membranes, which is essential for nerve and muscle function.

Protein Digestion and Absorption

Protein digestion begins in the stomach, where acids and enzymes break proteins into short polypeptides. Digestion continues in the small intestine, where proteases from the pancreas further break down polypeptides into single amino acids for absorption.

Protein digestibility affects protein quality. Animal proteins, soy products, and legumes are highly digestible, while plant proteins (grains, vegetables) are less digestible due to fiber content.

Recommended Protein Intake

Nitrogen balance describes the relationship between protein consumed and excreted. Positive nitrogen balance is needed for growth, pregnancy, and recovery. The Recommended Dietary Allowance (RDA) for protein is:

• 0.8 grams per kilogram of body weight per day

• Acceptable Macronutrient Distribution Range (AMDR): 10–35% of total energy intake

Most Americans meet or exceed the RDA for protein, but certain athletes and groups may be at risk for inadequate intake.

Protein Content of Common Foods

Protein is found in a variety of foods, including meat, poultry, seafood, dairy, soy products, beans, and nuts. The following table summarizes the protein content of commonly consumed foods:

Vegetarian Diets and Protein

Vegetarianism involves restricting the diet mostly or entirely to foods of plant origin. There are several types of vegetarian diets, each with varying restrictions:

Health Benefits and Challenges of Vegetarianism

Benefits include lower intake of fat and energy, reduced risk of heart disease, lower blood pressure, fewer digestive problems, and reduced risk of certain cancers, kidney stones, and gallstones. Challenges include potential deficiencies in iron, calcium, zinc, vitamins D and B12, and the need for careful dietary planning to ensure adequate protein and micronutrient intake.

Nutrients of Concern in Vegan Diets

Vegans must pay special attention to the following nutrients:

Consequences of High Protein Intake and Protein Deficiency

Excessive protein intake can increase the risk of kidney disease (especially in susceptible individuals), heart disease (due to high saturated fat in animal proteins), and bone loss (though higher protein intake may benefit bone health in healthy populations). Protein-energy malnutrition results from inadequate intake of protein and energy, with two serious forms:

• Marasmus: Severe deficiency of protein, energy, and nutrients; characterized by extreme tissue wasting and stunted growth.

• Kwashiorkor: Extremely low protein intake; symptoms include weight loss, muscle wasting, edema (swollen belly), and retarded growth.