2.2 Proteins: Quality, requirements, and metabolism

Proteins are crucial macronutrients that play vital roles in our bodies. They're made up of amino acids, some of which are essential and must come from our diet. Understanding protein quality, sources, and requirements is key to maintaining optimal health and body composition.

Protein metabolism is a complex process involving digestion, absorption, and the balance between synthesis and breakdown. This balance affects our lean body mass and overall health. Factors like age, activity level, and physiological state influence our protein needs and how our bodies use this important nutrient.

Protein sources: Amino acid composition

Complete and incomplete proteins

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• Complete proteins provide all nine essential amino acids in adequate proportions to support growth and tissue maintenance
  • Animal-based foods (meat, poultry, fish, eggs, and dairy) are complete protein sources
  • Few plant-based sources (soy, quinoa, and hemp) are also complete proteins
• Incomplete proteins lack one or more essential amino acids or contain them in insufficient quantities
  • Most plant-based foods (legumes, grains, nuts, and vegetables) are incomplete protein sources

Complementary proteins

• Complementary proteins are two or more incomplete protein sources that, when combined, provide all essential amino acids in adequate amounts
  • Examples include rice and beans, hummus and pita bread, or peanut butter and whole-grain bread
• Combining complementary proteins helps ensure a balanced intake of essential amino acids for vegetarians and vegans

Protein quality and digestibility

• Protein quality is determined by the essential amino acid composition and digestibility of the protein source
• High-quality proteins contain all nine essential amino acids in sufficient amounts to support growth and tissue maintenance
• The digestibility of a protein source affects its quality
  • Animal-based proteins generally have higher digestibility than plant-based proteins
  • Anti-nutritional factors (phytates, tannins) in plant-based proteins can reduce their digestibility

Protein requirements: Life stages and activity

Age-specific protein needs

• Protein requirements vary depending on age, sex, body weight, physical activity level, and physiological state
• Infants and children have higher protein requirements per unit of body weight compared to adults due to their rapid growth and development
  • RDA for protein ranges from 1.52 g/kg/day for infants aged 0-6 months to 0.95 g/kg/day for children aged 4-13 years
• Older adults (>65 years) may benefit from protein intakes slightly above the RDA (1.0-1.2 g/kg/day) to counteract age-related muscle loss (sarcopenia) and maintain functional status

Protein needs for pregnancy and lactation

• Pregnant and lactating women require additional protein to support fetal growth, maternal tissue expansion, and milk production
  • RDA for protein increases by 25 grams per day during pregnancy and 20 grams per day during lactation
• Adequate protein intake is crucial for optimal fetal development and maternal health

Physical activity and protein requirements

• Athletes and individuals engaging in regular physical activity have higher protein needs to support muscle repair, growth, and maintenance
  • Endurance athletes may require 1.2-1.4 g/kg/day
  • Strength athletes may need 1.6-2.0 g/kg/day
• Timing of protein intake (before and after exercise) can optimize muscle protein synthesis and recovery

Recommended Dietary Allowance (RDA) for protein

• The RDA for protein is 0.8 grams per kilogram of body weight per day for adults
  • This is the minimum amount required to prevent deficiency in most healthy individuals
• Individual protein requirements may be higher based on specific factors (age, physical activity, physiological state)

Protein metabolism: Maintaining lean mass

Protein digestion and absorption

• Protein digestion begins in the stomach, where pepsin initiates the breakdown of proteins into smaller polypeptides
• Further digestion occurs in the small intestine by pancreatic enzymes (trypsin, chymotrypsin, and carboxypeptidase) and brush border enzymes (aminopeptidases and dipeptidases)
  • This process results in free amino acids and small peptides that can be absorbed
• Absorbed amino acids enter the bloodstream and are transported to various tissues for protein synthesis, energy production, or conversion into other compounds (neurotransmitters, hormones)

Protein synthesis and breakdown

• Protein metabolism involves the continuous synthesis (anabolism) and breakdown (catabolism) of proteins in the body
  • The balance between these processes determines net protein balance and influences lean body mass
• Protein synthesis occurs in the ribosomes and involves the assembly of amino acids into polypeptide chains based on the genetic information in mRNA
  • This process is essential for the growth, repair, and maintenance of tissues, including skeletal muscle
• Protein breakdown (proteolysis) is catalyzed by proteolytic enzymes called proteases
  • This process is important for removing damaged or misfolded proteins, generating amino acids for new protein synthesis, and providing substrates for energy production during periods of fasting or energy deficit

Factors influencing net protein balance

• Maintaining a positive net protein balance (protein synthesis exceeding protein breakdown) is crucial for preserving lean body mass
  • Factors that promote a positive protein balance include adequate protein and energy intake, resistance exercise, and anabolic hormones (insulin, testosterone, growth hormone)
• Negative net protein balance (protein breakdown exceeding protein synthesis) can lead to muscle wasting and loss of lean body mass
  • This may occur during prolonged fasting, severe calorie restriction, immobilization, or catabolic conditions (trauma, sepsis, cancer cachexia)
• Optimizing protein intake, engaging in resistance exercise, and managing catabolic stressors can help maintain lean body mass over time