Nutrition: Importance of Proteins

Protein Importance for Nutrition

• Proteins are crucial for various bodily functions, including muscle contraction, blood clotting, vision, and immune response.
• They are essential structural components of bones, skin, and hair.
• Proteins act as enzymes, regulators for fluid balance, acid-base balance, transporters of nutrients, and antibodies.
• Proteins provide energy and glucose.

Protein Definition

• Proteins are composed of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) atoms.
• All amino acids share a basic structure: a central carbon atom bonded to a hydrogen atom, an amino group (NH2), an acid group (COOH), and variable side groups.
• There are about 20 different amino acids used to construct proteins.
• Amino acids differ in size, shape, electrical charge, and other properties due to the differences in their side groups.

Protein Structure

• Amino acids are linked together by peptide bonds forming dipeptides, tripeptides, and ultimately polypeptides (proteins).
• Most proteins consist of tens to hundreds of amino acids.
• More than half of the amino acids are nonessential, meaning the body can synthesize them.
• Nine amino acids are essential and must be obtained through diet.

Essential Amino Acids

• A list of essential amino acids includes: Histidine, Isoleucine, Leucine, Lysine, Methionine, Cysteine, Phenylalanine, Threonine, Tryptophan, and Valine. Additional amino acids, although sometimes not considered essential, include Arginine, Glutamine, and Tyrosine.

Nonessential Amino Acids

• A list of nonessentiual amino acids includes: Alanine, Arginine, Asparagine, Aspartic acid, Glycine, Proline, Serine, Tyrosine.

Conjugated Proteins

• Conjugated proteins combine amino acids with other compounds called prosthetic groups.
  • Examples include nucleoproteins (DNA, RNA), lipoproteins (LDL, HDL), chromoproteins (hemoglobin), glycoproteins (egg white), and phosphoproteins (milk protein casein).

Protein Denaturation

• Proteins are sensitive to changes in chemical and physical conditions.
• Denaturation occurs when proteins lose their shape and function due to heat, acid, or other factors.
• Denaturation affects solubility, changes chemical, physical, and biological properties.
• Denaturation is often irreversible. Examples include boiling an egg, curdling milk, and whipping egg whites.
• Primary and secondary protein structure remains intact following denaturation. Nutritional value is retained.

Protein Digestion and Absorption

• Protein digestion begins in the mouth with crushing and moistening.
• In the stomach, proteins are partially broken down (hydrolyzed) by hydrochloric acid (denaturation) and pepsin, an activated enzyme.
• In the small intestine, pancreatic and intestinal proteases further break down polypeptides and shorter chains into amino acids.
• Peptidase enzymes in the intestinal cells break down the smaller chains to individual amino acids.
• Most proteins are completely digested into amino acids before absorption.

Protein Synthesis

• Proteins are more complex than carbohydrates and fats.
• Amino acid sequences in proteins are diverse. There are many possible combinations of amino acids to form various proteins in the body.
• Crucial for the body to have all required amino acids to facilitate protein production.
• If one essential amino acid is missing, the body has to break down existing proteins to obtain the missing ones.

Protein Quality

• The quality of proteins is important for nutritional health, especially for children in development.
• Foods with high-quality proteins contain all essential amino acids in the required amounts.
• Animal products (meat, seafood, poultry, eggs, dairy) typically provide high-quality protein.
• Plant proteins (vegetables, nuts, seeds, grains, pulses) may be lower in one or more essential amino acids.
• Biological value (BV) measures how easily digested protein can support protein synthesis in cells. Egg protein has a high BV.

Protein Intake

• Recommendations for protein intake vary.
• The recommendations generally include 10-15% of total daily energy from protein intake in a daily diet, 0.8 g/kg/day per ideal body weight, or 50g per day.
• Excessive protein intake can be harmful.
• The body can adapt to varying protein intake but extremes can damage kidney function and cause bone demineralization.

Protein and Amino Acid Supplements

• Protein and amino acid supplements are often marketed for various health benefits, but there is no scientific backing that supports their efficacy.
• These supplements are not needed for healthy individuals and can be harmful.
• Natural protein sources (lean meat, milk, eggs, pulses) offer the complete amino acid profile.
• Impure isolated amino acids are not beneficial and can be harmful.