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Understanding Protein: Structure, Function, and Dietary Importance

Protein Structure and Composition

Amino Acids and Peptide Bonds

Proteins are polymers composed of amino acids linked by peptide bonds. These amino acids can be arranged into primary, secondary, tertiary, and quaternary structures, but from a nutritional perspective, the primary sequence is most important. The primary structure is determined by the DNA sequence of the gene encoding the protein. There are 20 canonical amino acids encoded in DNA, plus five others that play significant roles in the body.

Essential and Non-Essential Amino Acids

Of the 20 amino acids, nine are considered nutritionally indispensable (essential) because the human body cannot synthesize their carbon skeletons. These essential amino acids include leucine, valine, isoleucine, histidine, lysine, methionine, threonine, tryptophan, and phenylalanine. Additionally, cysteine and tyrosine are synthesized from methionine and phenylalanine, respectively. Some amino acids, such as glutamine, glutamate, glycine, proline, and arginine, may be conditionally indispensable under specific physiological or pathological conditions.

Dietary Protein Intake and Human Health

Recommended Dietary Allowance (RDA)

The RDA for protein for a healthy adult with minimal physical activity is 0.8 grams per kilogram of body weight per day. However, to meet functional needs such as promoting skeletal-muscle protein accretion and physical strength, higher intakes are recommended: 1.0, 1.3, and 1.6 grams per kilogram of body weight per day for individuals with minimal, moderate, and intense physical activity, respectively. Long-term consumption of protein at 2 grams per kilogram of body weight per day is considered safe for healthy adults, with a tolerable upper limit of 3.5 grams per kilogram per day for well-adapted individuals.

Health Implications of Protein Undernutrition

Protein undernutrition can lead to severe health issues, including stunting, anemia, physical weakness, edema, vascular dysfunction, and impaired immunity. Therefore, adequate consumption of high-quality proteins is essential for optimal growth, development, and overall health.

Sources of Dietary Protein

Animal vs. Plant Proteins

Animal proteins are considered complete proteins as they contain all essential amino acids. However, they often come with concerns about saturated fat content. On the other hand, plant proteins, while generally lacking one or more essential amino acids, can provide similar benefits when consumed in proper combinations. Plant proteins are also seen as a sustainable alternative to animal proteins, especially in the context of a growing global population .

Emerging Protein Sources

In addition to traditional animal and plant proteins, emerging sources such as insects, yeast, and microalgae are being explored for their nutritional and sustainability benefits . These sources offer potential solutions to meet the increasing demand for protein while addressing environmental and social concerns.

Protein Function in the Body

Roles of Proteins

Proteins play a variety of roles in the body, including structural (cytoskeleton), mechanical (muscle), biochemical (enzymes), and cell signaling (hormones) functions . They are involved in the synthesis of specific non-protein products such as nitric oxide, polyamines, creatine, glutathione, nucleotides, glucosamine, hormones, and neurotransmitters. The diversity in protein function is due to the vast array of possible amino acid sequences and conformations.

Conclusion

Proteins are indispensable macronutrients essential for numerous physiological functions and overall health. Understanding the structure, dietary sources, and recommended intake of proteins can help in making informed dietary choices. Both animal and plant proteins have their unique benefits and challenges, and emerging protein sources offer promising alternatives for sustainable nutrition. Adequate and balanced protein intake is crucial for maintaining health and preventing protein undernutrition-related issues.