Unit 2 – Proteins
3.1 Introduction to Proteins
Proteins are complex organic molecules composed of amino acids and are essential macronutrients for all living organisms. They serve as the structural and functional building blocks of the body, vital for tissue repair, enzymatic activity, hormone production, immune function, and numerous biochemical reactions.
In the human diet, proteins must be consumed regularly as the body does not store excess proteins like carbohydrates (as glycogen) or fats (as adipose). Therefore, adequate intake is essential to maintain homeostasis, growth, and repair.
3.2 Sources and Functions of Proteins
Sources
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| Bowl loaded with full of Protein-rich food |
- Animal Sources: Eggs, milk, meat, poultry, fish, cheese
- Plant Sources: Legumes (lentils, chickpeas, soybeans), cereals (wheat, rice), nuts, and seeds
Animal proteins are generally of higher biological value because they contain all essential amino acids in the right proportions. Plant proteins, though valuable, may lack one or more essential amino acids but can be combined to form a complete profile.
Functions
- Structural Role: Proteins form structural components like muscles, skin, hair, and nails (keratin, collagen, actin, myosin).
- Enzymatic Functions: Enzymes such as amylase, lipase, and DNA polymerase are proteins that catalyze biological reactions.
- Transport and Storage: Hemoglobin transports oxygen; ferritin stores iron.
- Hormonal Functions: Many hormones (like insulin, glucagon) are proteins.
- Immune Function: Antibodies or immunoglobulins defend against pathogens.
- Repair and Maintenance: Proteins repair cells and tissues damaged by wear and tear.
3.3 Essential and Non-Essential Amino Acids
Proteins are made up of 20 amino acids, categorized into:
- Essential Amino Acids: Cannot be synthesized by the human body and must be obtained from the diet. Examples include: Leucine, Isoleucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, Valine, and Histidine (in infants).
- Non-Essential Amino Acids: Can be synthesized by the body. Examples include: Alanine, Glutamic acid, Aspartic acid, Serine, Cysteine.
A well-planned diet, especially for vegetarians, should ensure the intake of all essential amino acids through food combinations (e.g., rice and dal, bread and peanut butter).
3.4 Incomplete and Complete Proteins
Based on the amino acid profile, dietary proteins are classified as:
- Complete Proteins: Contain all 9 essential amino acids in sufficient quantity and proportion. Mostly found in animal sources (milk, eggs, meat) and soy (a plant exception).
- Incomplete Proteins: Lack one or more essential amino acids. Most plant-based proteins fall under this category.
For example, cereals are low in lysine, while pulses are low in methionine. However, combining different plant sources can yield a complete protein profile—a principle called “mutual supplementation.”
3.5 Supplementary Foods
Supplementary foods are formulations provided to individuals or groups with increased protein needs or those suffering from protein deficiency. These include:
- Ready-to-use therapeutic foods (RUTF) for malnourished children
- Protein powders for pregnant women or athletes
- Fortified blended foods in public health programs
In many government-run nutrition programs (like ICDS in India), protein-energy supplements are provided to children, pregnant and lactating women to prevent malnutrition.
3.6 Protein-Energy Malnutrition (PEM) and the Eye
Protein-Energy Malnutrition is a major public health problem in developing countries and occurs due to inadequate intake of protein and calories. The two severe clinical forms are:
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| Kwashiorkor and Marasmus |
- Marasmus: Severe calorie deficiency; children appear emaciated with visible bones, dry skin, and growth failure.
- Kwashiorkor: Severe protein deficiency; features include edema, enlarged liver, dermatitis, and fatty liver. Children often appear plump due to fluid retention.
Impact on Eye Health
- PEM can lead to:
- Xerophthalmia: Dryness of conjunctiva and cornea due to vitamin A deficiency
- Bitot's Spots: Foamy patches on the conjunctiva
- Night Blindness: Early symptom of vitamin A deficiency
- Corneal Ulcers and Keratomalacia: Severe protein and vitamin A deficiency can result in corneal melting, leading to blindness
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| Xerophthalmia |
Optometrists play a vital role in detecting such signs early and referring patients for nutritional management, especially children in rural or economically weaker sections.
3.7 Nitrogen Balance
Nitrogen balance refers to the difference between nitrogen intake (from dietary proteins) and nitrogen excretion (via urine, feces, sweat).
- Positive Nitrogen Balance: Intake exceeds loss. Seen in growth, pregnancy, recovery from illness.
- Negative Nitrogen Balance: Loss exceeds intake. Occurs in malnutrition, trauma, infection, and chronic diseases.
- Neutral Balance: Intake equals loss. Found in healthy adults.
Nitrogen balance is an important indicator of protein metabolism and overall nutritional status. In hospitals, it is often monitored in patients with severe burns, sepsis, or during total parenteral nutrition (TPN).
3.8 Changes in Protein Requirement
Protein requirements are dynamic and vary with:
- Age: Children and adolescents require more protein per kg body weight due to rapid growth.
- Physiological states: Pregnancy and lactation increase protein demands to support fetal and infant development.
- Physical activity: Athletes and laborers need more protein to maintain muscle repair and synthesis.
- Disease conditions: Infections, trauma, surgery, burns, or chronic illnesses demand increased protein intake.
ICMR Recommendations (per kg body weight)
| Age Group | Protein Requirement (g/kg/day) |
|---|---|
| Infants (0–6 months) | 1.52 |
| Children (1–3 years) | 1.08 |
| Adolescents | 1.00 |
| Adult Male | 0.83 |
| Adult Female | 0.83 |
| Pregnant Woman | +23g/day |
| Lactating Woman | +19g/day (first 6 months) |
3.9 Clinical Relevance for Optometrists
Conclusion
Proteins are indispensable to human health, acting as the fundamental molecules for growth, repair, immunity, and physiological regulation. An understanding of their dietary sources, functions, and clinical implications is crucial in all healthcare domains, including optometry.
For optometrists, awareness of protein-energy malnutrition and its impact on eye health is essential. Nutritional interventions—whether through education, referrals, or supplementation—can significantly reduce preventable visual impairment in vulnerable populations.
In summary, ensuring adequate and high-quality protein intake across the life span, especially in children, pregnant women, and the elderly, contributes not only to general well-being but also to the prevention of ocular diseases and maintenance of visual function.
