Nitrogen Balance & Limiting Amino Acids

1. Nitrogen Balance — Definition

N balance = Protein intake (g) / 6.25 − (UUN + 4 g)

• Urea (main form in urine)
• Creatinine, NH₄⁺, uric acid (minor urinary forms)
• Undigested protein, shed intestinal epithelial cells (fecal)
• Small amounts via sweat, skin, hair

2. States of Nitrogen Balance

Positive Nitrogen Balance

• Growing children and adolescents
• Pregnancy
• Recovery from illness, emaciating disease, or trauma
• Reflects net protein synthesis exceeding breakdown

Negative Nitrogen Balance

• Inadequate total dietary protein
• Lack of one or more essential amino acids (EAAs) — even with adequate total protein, if a single EAA is absent, protein synthesis halts and unused amino acids are deaminated and excreted as nitrogen
• Physiologic stress: trauma, burns, surgery, infections, cancer, hyperthyroidism, starvation
• During these catabolic states, cytokines and glucocorticoids drive increased tissue protein breakdown; up to 6–7% of total body protein may be lost over 10 days following major trauma

Note: A high protein intake does NOT produce positive nitrogen balance in healthy adults — it increases both synthesis and catabolism proportionally, maintaining equilibrium at a higher turnover rate (with increased diet-induced thermogenesis).

3. Protein Requirements from N Balance Studies

• Average daily requirement: 0.66 g protein/kg body weight (reference intake ~0.825 g/kg/day allowing for individual variation), roughly 55 g/day
• RDA (US): 0.8 g/kg/day for adults
  • Sedentary 70-kg adult: ~56 g/day
  • Athletes/regular vigorous exercise: ~1 g/kg/day
  • Pregnancy/lactation: +30 g/day above baseline
  • Infants: 2 g/kg/day (for growth)
  • Kidney disease: may require restriction
  • Burns: require increased intake

4. The Requirement Is for Specific Amino Acids, Not Just Total Protein

Essential (Indispensable) Amino Acids — the 9 that cannot be synthesized in humans:

• Cysteine ← from methionine
• Tyrosine ← from phenylalanine

5. Limiting Amino Acid — The "Barrel Stave" Concept

• If any one EAA is absent or deficient, protein synthesis stops at that point, and the remaining amino acids are deaminated and their nitrogen excreted → negative nitrogen balance results.
• The first-limiting amino acid is the EAA that becomes deficient first.

Common Limiting Amino Acids by Food Source

6. Protein Quality & Scoring Systems

Biological Value (BV)

PDCAAS (Protein Digestibility-Corrected Amino Acid Score)

• Standard adopted by most governments
• Based on EAA profile corrected for protein digestibility
• Maximum score = 1.0 (casein, whey, egg, soy)

DIAAS (Digestible Indispensable Amino Acid Score)

• Proposed by FAO in 2013 to replace PDCAAS
• Uses true ileal digestibility of individual EAAs (more accurate) vs. total tract crude protein digestibility used in PDCAAS
• More precisely reflects actual bioavailability

7. Complementary Proteins — Practical Application of Limiting AA Concept

• Wheat (lysine-deficient, methionine-rich) + Kidney beans (methionine-poor, lysine-rich) → combined BV approaches complete protein

8. Clinical Significance

Gastric HCl: Synthesis and Functions

1. Site of Production

• Has an extensive intracellular canalicular system — deep branching invaginations of the apical membrane
• Is packed with mitochondria (providing ATP for active transport)
• HCl is formed at the villus-like projections inside these canaliculi and conducted outward into the gastric lumen

2. Mechanism of HCl Synthesis

Step 1 — H⁺ Generation via Carbonic Anhydrase

CO₂ (from blood or cell metabolism) + H₂O → H₂CO₃ → H⁺ + HCO₃⁻

• Catalyzed by carbonic anhydrase in the parietal cell cytoplasm
• CO₂ diffuses across the basolateral membrane from blood capillaries
• This is the source of H⁺ ions

Step 2 — H⁺ Secretion via H⁺/K⁺-ATPase (Proton Pump)

• H⁺/K⁺-ATPase on the apical canalicular membrane actively pumps H⁺ into the canalicular lumen in exchange for K⁺ (entering the cell)
• This is the primary driving force — against a massive concentration gradient
• K⁺ recycled back into the lumen via K⁺ channels, keeping the pump running

Step 3 — Cl⁻ Secretion & HCO₃⁻ Exchange

• Cl⁻ is transported into the canalicular lumen via Cl⁻ channels
• On the basolateral membrane, HCO₃⁻ generated in Step 1 is exchanged for plasma Cl⁻ via a HCO₃⁻/Cl⁻ antiporter → this is the "alkaline tide" (post-meal rise in blood pH)
• Na⁺/K⁺-ATPase on the basolateral membrane maintains low intracellular Na⁺ and drives the overall electrochemical gradient

Final Result

H⁺ + Cl⁻ → HCl in the canalicular lumen → secreted into the gastric gland lumen

3. Stimulation of HCl Secretion

The ECL Cell Axis (Primary Pathway)

• G cells (antrum) → secrete gastrin (stimulated by dietary proteins, stomach distension, vagal input)
• Gastrin → stimulates ECL (enterochromaffin-like) cells in the oxyntic glands
• ECL cells → release histamine directly onto parietal cells
• Histamine → activates H₂ receptors → ↑cAMP → stimulates H⁺/K⁺-ATPase

Three Phases of Gastric Secretion

Inhibition of HCl Secretion

• Somatostatin from D cells (↑ when antral pH falls below 3) → inhibits G cells and parietal cells directly
• Enterogastric reflex: duodenal distension, acidic chyme, fat, or hyperosmolarity → inhibits gastric secretion
• Secretin (from S cells) and CCK (from I cells) → inhibit gastric emptying and secretion

4. Functions of HCl

(i) Protein Digestion — Pepsin Activation

• HCl denatures dietary proteins, unfolding them and making peptide bonds accessible
• HCl converts pepsinogen (inactive, MW ~42,500) → pepsin (active, MW ~35,000) by autocatalytic cleavage
• Pepsin has optimal proteolytic activity at pH 1.8–3.5; inactivated above pH 5
• HCl is thus as necessary as pepsin for gastric protein digestion

(ii) Antimicrobial Barrier

• The highly acidic environment (pH ~1–2) kills most ingested pathogens
• Acts as a defense against enteric pathogens including Salmonella, Giardia lamblia, helminths
• Patients with achlorhydria (loss of acid secretion — from surgery, PPIs, H₂ blockers, chronic gastritis) have significantly increased risk of enteric colonization and infections

(iii) Iron Absorption

• HCl maintains iron in its soluble ferrous (Fe²⁺) form, which is the absorbable form in the duodenum
• Achlorhydria impairs iron absorption → iron-deficiency anemia

(iv) Calcium & Mineral Absorption

• Acid environment promotes dissolution of calcium salts, facilitating Ca²⁺ absorption in the proximal small intestine

(v) Intrinsic Factor Secretion

• Intrinsic factor (IF) is a 44-kDa glycoprotein secreted by parietal cells alongside HCl
• IF binds vitamin B₁₂ in the stomach/duodenum → IF–B₁₂ complex is absorbed in the terminal ileum
• Destruction of parietal cells (e.g., chronic gastritis, autoimmune) → loss of both acid AND intrinsic factor → pernicious anemia (megaloblastic) due to B₁₂ deficiency

(vi) Regulation of Gastric Emptying

• Acid pH of gastric contents entering the duodenum triggers secretin and CCK release, which regulate the rate of gastric emptying

(vii) Stimulus for Pancreatic Secretion

• Acid chyme in the duodenum stimulates S cells → secrete secretin → stimulates pancreatic bicarbonate secretion (to neutralize acid)
• Also stimulates CCK → pancreatic enzyme secretion and bile release

5. Clinical Correlates