Digestion and AA Absorption

The speaker expresses a sincere apology for failing to front load important information in lectures on protein metabolism.
Acknowledges that key points from the first lecture were not included in notes.
Recognizes the importance of prompting and reiterating key concepts during lectures.

Understanding Protein Demand:
- We have a significant protein demand for gene expression, yet we only consume about a third of what we need; the rest is recycled.
- Efficient amino acid sharing among tissues is crucial, especially during conditions like hospitalization, severe infection, or aging.
Amino Acid Sharing:
- Tissues share amino acids to meet protein demand; inefficient sharing can lead to severe health issues.
- Amino acids must be deaminated and transaminated to create nonessential amino acids for sharing.

Deamination: Process where amino acids lose their amine group, crucial for amino acid conversion.
- Allows for amino acids to be used in energy production or glucose production during fasting states.
Transamination: The process involved in creating nonessential amino acids; less critical specificity is required for understanding.

Storage Forms:
- Functional muscle tissue is the primary storage for amino acids; it is expensive energetically to deplete.
- Continuous amino acid needs exist due to constant gene expression without a dedicated storage form.

Urea Cycle:
- Detoxification of ammonia, produced during deamination; converted into urea for excretion.
Anabolic vs. Catabolic Uses of Amino Acids:
- Anabolic: Synthesis of proteins, blood proteins, membranes, DNA/RNA nucleotides.
- Catabolic: Energy production or glucose production (gluconeogenesis) during fasting states.
- Conversion to triglycerides and storage as fat in fed states when excess amino acids occur.

Main Sites:
- Digestion primarily occurs in the stomach and small intestine, with initial mechanical digestion occurring in the mouth.
Satiating Effects of Protein:
- Protein is the most satiating nutrient. Combined with lipids and fiber, it enhances feelings of fullness.
Digestion Process:
- In stomach: Hydrochloric acid denatures proteins; pepsin cleaves peptide bonds to form smaller peptides.
- In small intestine: Pancreatic enzymes (trypsinogen, chymotrypsinogen, etc.) activated by acidity of chyme.

Small peptides (di- and tri-peptides) can be absorbed directly, not exclusively as single amino acids.
Transport Mechanisms:
- Active transport for amino acids alongside sodium ions via cotransport mechanisms; sodium-potassium ATPase involved in charge balance within cells.
- Specific transporters exist for different amino acid classes (e.g., basic vs. acidic).

Plant vs. Animal Protein:
- Plant proteins have lower bioavailability (40-60%), whereas animal proteins have higher efficiency (85-100%).
- Plant proteins bound to fiber limit amino acid release.
Protein Digestibility-Corrected Amino Acid Score (PDCAAS):
- Measure of protein quality based on amino acid digestibility and profile for human needs.

Different proteolytic enzymes (e.g., trypsin, chymotrypsin) have specific targets for clipping amino acids based on side chains.
Branching amino acids are prioritized for absorption due to their use in muscle protein.

Essential vs. Nonessential:
- Essential amino acids are preferentially absorbed compared to nonessential ones.
Di and tri-peptides have higher absorption rates than free amino acids.
Impact of Amino Acid Supplements:
- Single amino acid supplementation can lead to competition among transporters, reducing overall absorption of other amino acids.

Enterocytes in the small intestine have high protein demand and utilize absorbed amino acids primarily for their own protein synthesis and energy needs.
The intestine prioritizes glucose, fat, and amino acids, with amino acids being key for rapid turnover and function.

Amino acids from the small intestine enter blood circulation and are directed to the liver, which has high protein synthesis needs due to its metabolic functions.
Liver Functions:
- Synthesizes blood proteins, adjusts to fasting states, engages in gluconeogenesis based on energy needs, and generally regulates amino acid levels in circulation.

Circulation Impact:
- Insulin supports muscle usage of amino acids, while glucagon aids in gluconeogenesis during fasting.
Understanding the metabolic pathways of amino acids is essential for optimizing dietary intake and protein timing for muscle maintenance and overall physiological health.

The substantial demands placed on protein from the small intestine, pancreas, and liver mean that optimal strategies for intake (e.g., larger meals at specific times) can help mitigate loss through these organs and support protein synthesis in other tissues such as muscles.