Parenteral nutrition (PN) serves as a lifeline for patients who cannot meet their nutritional needs through enteral intake. The provision of adequate macronutrients — carbohydrates, nitrogen (amino acids), and lipids — requires careful clinical consideration to optimize patient outcomes while minimizing metabolic complications.
Carbohydrate
Carbohydrates in PN are delivered through glucose, available in concentrations from 5% to 70%. As the primary energy substrate for the central nervous system (CNS), glucose typically accounts for up to 60% of total daily energy provision. Clinicians must carefully titrate glucose delivery to avoid exceeding the maximum oxidation rate of 4–7 mg/kg/min/day, as excessive administration may trigger hyperglycemia, hepatic steatosis, and increased CO2 production with consequent respiratory compromise.
Critically ill patients, particularly those with systemic inflammation, often exhibit impaired glucose tolerance. In such cases, concurrent insulin therapy may be required to maintain glycemic control. It is important to note that insulin should be administered as a separate infusion rather than added directly to the PN admixture, ensuring both stability and precise dosage adjustment. Given that hyperglycemia in hospitalized patients is independently associated with increased morbidity and mortality, vigilant monitoring and tailored carbohydrate prescription are essential.
Nitrogen (Amino Acids)
Nitrogen provision is fundamental to maintaining lean body mass and supporting metabolic function. Amino acid solutions available today offer a range of nitrogen content, typically 6–20g per formulation, with recommended dosages of 0.17–0.3g/kg/day depending on the degree of metabolic stress and catabolism.
A significant advancement has been the development of glutamine-enriched solutions. Although glutamine is classified as a non-essential amino acid under normal conditions, it becomes conditionally essential during periods of severe illness or trauma. However, formulating stable PN solutions with glutamine presents technical challenges due to its inherent instability and limited aqueous solubility.
Lipid Emulsions
Lipids play a multifaceted role in PN. They provide a concentrated energy source that reduces glucose infusion requirements, mitigate respiratory and metabolic stress, and prevent essential fatty acid deficiency. Lipid emulsions are available in 10%, 20%, and 30% concentrations, generally contributing 20–30% of daily energy intake.
However, caution is warranted. Lipid accumulation in the reticuloendothelial system may impair bacterial clearance and increase infection risk, with current guidelines recommending a maximum of 1–1.5g/kg/day. Cholestatic complications have also been associated with higher lipid content.
Soybean oil emulsions (e.g., Intralipid®) are the most established lipid preparations, providing essential fatty acids and fat-soluble vitamins. However, their high proportion of n-6 polyunsaturated fatty acids (≥60%) may exert pro-inflammatory and immunosuppressive effects under stress conditions. This has driven the development of third-generation lipid emulsions incorporating medium-chain triglycerides (MCT), olive oil, or fish oil to modulate inflammation and improve outcomes. Olive oil-based preparations may reduce PUFA intake and offer greater resistance to lipid peroxidation, while n-3 fatty acids from fish oil demonstrate anti-inflammatory properties.
Current evidence suggests these novel emulsions are safe and may offer advantages over soybean oil alone. However, definitive recommendations regarding differential clinical endpoints require further robust investigation.
Conclusion
The composition of macronutrients in parenteral nutrition is not a one-size-fits-all formula. Optimal PN therapy demands individualized prescribing that considers disease severity, metabolic status, and evolving clinical evidence. At Beijing L&Z, we remain committed to advancing nutritional support through research and collaboration with healthcare professionals to improve patient outcomes.
Post time: Jul-16-2026