What is the primary effect of adding cysteine to parenteral nutrition?

• A. Improves solubility by lowering pH
• B. Improves solubility by raising pH
• C. Decreases solubility
• D. No effect on solubility

Answer: (A)

The key idea is how pH affects mineral solubility in parenteral nutrition. Calcium and phosphate tend to precipitate as calcium phosphate when the solution is neutral to slightly alkaline. Lowering the pH makes phosphate more protonated (forming species that stay in solution) and reduces the propensity for calcium phosphate precipitation. Cysteine is added as an acidifying amino acid, so it mildly lowers the PN solution’s pH and thus improves the solubility of calcium and phosphate salts. This allows higher mineral concentrations to be delivered without precipitation. If the pH were not lowered (or were raised), solubility would decrease and precipitation would be more likely.