Undernutrition in Acute and Sub-Acute Care, Enteral Nutrition, Parenteral Nutrition, Post-Surgical Nutrition, and Refeeding Syndrome.
Adil Abbasi, MD FACP FACN
Learning Objectives
After completing this chapter, the reader should be able to:
Introduction
Undernutrition is one of the most common, yet under-recognized conditions encountered in hospitalized patients. Studies suggest that 20–50% of patients admitted to acute care hospitals exhibit evidence of malnutrition or are at high risk for developing malnutrition. The prevalence is even higher among elderly patients, those with chronic diseases, cancer patients, critically ill individuals, and patients recovering from major surgery.
Malnutrition adversely affects virtually every organ system. It impairs wound healing, compromises immune function, reduces muscle strength, prolongs hospitalization, increases hospital readmission rates, and contributes significantly to morbidity, mortality, and healthcare expenditures.
The transition from acute hospitalization to sub-acute rehabilitation or skilled nursing care represents a particularly vulnerable period during which nutritional deficits may worsen if not identified and treated appropriately.
Definitions
Undernutrition: Undernutrition refers to inadequate intake or assimilation of nutrients resulting in:
Hospital Malnutrition: Hospital malnutrition may be:
Disease-Related Malnutrition: Current guidelines recognize that inflammation plays a major role in disease-related malnutrition.
Examples include:
Epidemiology: Malnutrition prevalence is as follows:
Population | Prevalence |
Hospitalized patients | 20–50% |
ICU patients | 40–70% |
Elderly hospitalized patients | 35–60% |
Cancer patients | 30–80% |
Post-surgical patients | 20–60% |
Skilled nursing facility residents | 30–60% |
Despite its prevalence, malnutrition remains underdiagnosed and under coded in many healthcare systems.
Pathophysiology of Undernutrition: Acute illness triggers a complex metabolic response characterized by:
Inflammatory Cytokines
These mediators produce:
Catabolic State: The body responds by:
When prolonged, this process results in:
Consequences of Undernutrition:
Immune Dysfunction: Patients become susceptible to:
Impaired Wound Healing: Protein-energy malnutrition causes:
Muscle Loss: Results include:
Increased Mortality: Malnourished patients consistently demonstrate:
Risk Factors for Malnutrition:
Patient Factors
Medical Conditions
Surgical Conditions
Social Factors
Diagnosis of Malnutrition:
Nutritional Screening: All hospitalized patients should undergo screening within 24 hours of admission. Common tools include:
Malnutrition Screening Tool (MST)
Mini Nutritional Assessment (MNA)
Nutrition Risk Screening 2002 (NRS-2002)
MUST Score - (MUST = Malnutrition Universal Screening Tool)
ASPEN/Academy Diagnostic Criteria: Diagnosis requires at least two of the following:
1. Insufficient Energy Intake: Less than 75% of estimated needs.
2. Weight Loss: Examples:
3. Loss of Muscle Mass: Commonly observed in:
4. Loss of Subcutaneous Fat
Seen in:
5. Fluid Accumulation
6. Reduced Functional Status
Measured by:
Laboratory Assessment: No laboratory test alone diagnoses malnutrition. Common laboratory abnormalities include:
However, these markers are strongly influenced by inflammation. Laboratory studies that are more useful for identifying deficiencies include:
Indications for Nutritional Intervention: Nutrition support should be initiated when:
Existing Malnutrition: Moderate or severe malnutrition.
Inadequate Intake: Expected inability to meet nutritional requirements for:
Significant Weight Loss: Unintentional weight loss exceeding accepted thresholds.
High-Risk Surgical Patients: Patients undergoing:
Severe Dysphagia: Inability to safely swallow adequate nutrition.
Enteral Nutrition (EN)
Definition: Delivery of nutrients directly into the gastrointestinal tract.
Methods include:
Why Enteral Nutrition Is Preferred: The principle "If the gut works, use it.". Benefits include:
Indications for Enteral Nutrition: Patients with functioning GI tracts but unable to consume adequate nutrition orally. Examples include:
Contraindications to Enteral Nutrition:
Absolute
Relative
Enteral Nutrition in Post-Surgical Patients: Modern enhanced recovery protocols favor:
Early Enteral Feeding: Typically begin within 24–48 hours after surgery.
Benefits include:
Especially beneficial after:
Enteral Nutrition Monitoring: Monitor:
Daily
Weekly
Complications of Enteral Nutrition:
Gastrointestinal
Aspiration: Most serious complication.
Risk factors:
Mechanical
Metabolic
Total Parenteral Nutrition (TPN)
Definition: Intravenous administration of nutrients:
Usually delivered through:
Indications for TPN: TPN should be considered when adequate enteral nutrition is impossible. Examples include:
Gastrointestinal Failure
Mechanical Obstruction
Severe Ileus
High Output Enterocutaneous Fistula
Severe Pancreatitis
When enteral feeding is not tolerated.
Post-Surgical Complications
Timing of TPN:
Well-Nourished Patients: Generally considered after approximately 7 days if adequate nutrition cannot be provided enterally.
Malnourished Patients: Often initiated earlier.
Critically Ill Patients: Decision individualized according to nutritional risk and clinical status.
Composition of TPN:
Macronutrients
Dextrose: Primary energy source.
Amino Acids: Protein replacement.
Lipids: Essential fatty acids and calories.
Micronutrients: Vitamins, Trace minerals, Electrolytes.
Monitoring During TPN:
Daily Initially
Weekly
Refeeding Syndrome
Definition: Potentially life-threatening metabolic disturbance occurring when nutrition is reintroduced after prolonged starvation.
Hallmark Abnormalities
High-Risk Patients
Prevention
Complications of TPN:
Catheter-Related Complications:
Central Line Infection: May result in Bacteremia and Sepsis
Thrombosis: Upper extremity DVT and Central venous thrombosis
Metabolic Complications:
Hyperglycemia: Most common metabolic complication.
Hypoglycemia: Occurs if infusion interrupted abruptly.
Electrolyte Disturbances: Potassium, Phosphorus and Magnesium abnormalities
Hepatic Complications
Hepatic Steatosis
Cholestasis
Gallbladder Sludge
Long-Term Liver Dysfunction - More common with prolonged TPN.
Special Considerations in Elderly Patients
Older adults often have:
Goals include:
Protein requirements frequently exceed standard adult recommendations and may approach 1.2–1.5 g/kg/day, with higher needs in severe catabolic states.
Quality Measures and Documentation
Accurate documentation of malnutrition is important because it:
Documentation should specify:
When supported by clinical evidence.
Calculations and Recommendations for Enteral Nutrition and TPN:
The calculations and recommendations provided here are derived from major nutrition-support guidelines published by ASPEN, ESPEN, SCCM, and surgical nutrition societies. Importantly, most bedside formulas (25–30 kcal/kg/day, 1.2–2.0 g/kg/day protein, 30 mL/kg/day fluids, etc.) are estimation formulas used when indirect calorimetry or formal dietitian calculations are unavailable.
ASPEN/SCCM Critical Care Nutrition Guidelines: Reference: McClave SA, Taylor BE, Martindale RG, et al. Guidelines for the Provision and Assessment of Nutrition Support Therapy in the Adult Critically Ill Patient. JPEN. 2016;40(2):159-211.
Key recommendations:
ESPEN Guideline: Clinical Nutrition in Critical Illness: Reference: Singer P, Blaser AR, Berger MM, et al. ESPEN Guideline on Clinical Nutrition in the Intensive Care Unit. Clinical Nutrition. 2019;38:48-79.
Key recommendations:
ESPEN Surgery Guidelines: Reference: Weimann A, Braga M, Carli F, et al. ESPEN Practical Guideline: Clinical Nutrition in Surgery. Clinical Nutrition. 2021;40:4745-4761.
Key recommendations:
ASPEN Adult Nutrition Support Core Curriculum: Reference: Worthington P, Gilbert K, eds. ASPEN Adult Nutrition Support Core Curriculum, 4th Edition.
Widely regarded as the principal practical reference used by:
Provides detailed calculations for:
Protein Recommendations: Reference: Deutz NEP, Bauer JM, Barazzoni R, et al.
Protein Intake and Exercise for Optimal Muscle Function in Aging. Clinical Nutrition. 2014.
Recommendations:
Refeeding Syndrome: References:
National Institute for Health and Care Excellence (NICE).
Nutrition Support for Adults: Oral Nutrition Support, Enteral Tube Feeding and Parenteral Nutrition. Clinical Guideline 32.
Mehanna HM, Moledina J, Travis J. Refeeding Syndrome: What It Is and How to Prevent and Treat It. BMJ. 2008;336:1495-1498.
Key recommendations:
High-risk patients:
Start nutrition cautiously:
Fluid Requirement: Most adult maintenance fluid formulas are based on expert consensus and long-standing nutrition support practice rather than randomized trials.
References:
Madsen K, Burns B, et al.
The Hitchhiker's Guide to Parenteral Nutrition Management for Adult Patients. University of Virginia Nutrition Support Series.
Common recommendations:
TPN Macronutrient Calculations: Reference: ASPEN Adult Nutrition Support Core Curriculum.
Typical targets:
Protein: 1.2–2.0 g/kg/day
Dextrose: Maximum glucose infusion rate:
Target: <4–5 mg/kg/min
to minimize:
Lipids: Typically:20–30% of total calories AND Approximately 0.7–1.5 g/kg/day
Enteral Nutrition Advancement Rates Reference: ASPEN Safe Practices for Enteral Nutrition Therapy and ASPEN Core Curriculum.
Common practice:
Standard-risk patient:
High-risk refeeding patient:
These are guideline-based clinical practices rather than rigid evidence-derived formulas.
Summary
Undernutrition is highly prevalent in acute care hospitals, intensive care units, rehabilitation facilities, and skilled nursing settings. It is associated with increased morbidity, mortality, prolonged hospitalization, impaired wound healing, increased infection rates, functional decline, hospital readmissions, and increased healthcare costs. Early identification and treatment of malnutrition are therefore essential components of high-quality inpatient care.
The diagnosis of malnutrition should be based primarily on clinical assessment using established criteria such as the ASPEN and GLIM diagnostic frameworks. Important indicators include inadequate nutritional intake, significant unintentional weight loss, loss of muscle mass, loss of subcutaneous fat, fluid accumulation, and diminished functional status. Laboratory markers such as albumin and prealbumin may provide supportive information but should not be used in isolation to diagnose malnutrition because they are significantly influenced by inflammation and illness severity.
Nutritional screening should occur within 24 hours of hospital admission, particularly among elderly patients, patients with cancer, chronic organ dysfunction, neurologic disease, dysphagia, frailty, major trauma, and those undergoing major surgical procedures. Early intervention is especially important in post-surgical patients because nutritional deficits contribute directly to impaired wound healing, anastomotic complications, infections, delayed recovery, and prolonged rehabilitation.
When nutritional support is required, enteral nutrition remains the preferred route whenever the gastrointestinal tract is functional. Enteral feeding preserves gut integrity, supports mucosal immunity, reduces infectious complications, and is generally safer and less costly than parenteral nutrition. Current surgical and critical care guidelines support initiation of enteral nutrition within 24 to 48 hours in appropriate patients. Typical initiation rates range from 20 to 40 mL/hour in standard-risk patients, with gradual advancement every 4 to 12 hours as tolerated. Patients at risk for refeeding syndrome require substantially slower advancement and close monitoring.
Caloric requirements for hospitalized adults are commonly estimated at 25 to 30 kcal/kg/day, with higher requirements in patients experiencing major trauma, burns, severe infection, or significant malnutrition. Protein requirements generally range from 1.0 to 1.2 g/kg/day in stable hospitalized patients, 1.2 to 1.5 g/kg/day in malnourished patients, and 1.5 to 2.0 g/kg/day in severely catabolic states such as major surgery, trauma, and sepsis. Fluid requirements are often estimated at approximately 30 to 35 mL/kg/day and must be individualized according to cardiac, renal, hepatic, and fluid balance considerations.
Proper calculation of free water administration is an important component of enteral nutrition management. Clinicians must account for the intrinsic water content of enteral formulas and supplement additional free water flushes to achieve total daily fluid goals while avoiding dehydration or fluid overload.
Parenteral nutrition should be reserved for patients in whom adequate enteral nutrition is not feasible because of gastrointestinal failure, severe malabsorption, bowel obstruction, prolonged ileus, high-output fistulas, severe post-surgical complications, or other contraindications to enteral feeding. TPN formulation requires careful calculation of protein, carbohydrate, lipid, electrolyte, vitamin, and trace element requirements. Protein goals should be established first, followed by allocation of non-protein calories between dextrose and lipid administration. Dextrose infusion rates should generally remain below 4 to 5 mg/kg/min to reduce the risk of hyperglycemia, hepatic steatosis, and excessive carbon dioxide production.
Refeeding syndrome remains one of the most important complications encountered during nutritional rehabilitation. Patients with severe malnutrition, prolonged fasting, significant weight loss, alcoholism, or advanced chronic disease are at particularly high risk. Prevention requires cautious initiation of nutrition support, aggressive monitoring of phosphorus, potassium, and magnesium levels, and routine thiamine supplementation during refeeding.
Complications associated with enteral nutrition include aspiration, gastrointestinal intolerance, diarrhea, constipation, abdominal distention, tube malfunction, and metabolic abnormalities. Complications associated with TPN include catheter-related bloodstream infections, venous thrombosis, hyperglycemia, electrolyte disturbances, hepatobiliary dysfunction, and metabolic bone disease with prolonged use. Careful monitoring and multidisciplinary management involving physicians, dietitians, pharmacists, and nursing staff are essential to optimize outcomes.
In summary, nutritional assessment and intervention should be viewed as fundamental medical therapies rather than supportive measures alone. Early recognition and treatment of malnutrition, appropriate use of enteral and parenteral nutrition, and careful monitoring for complications can significantly improve clinical outcomes, accelerate recovery, reduce healthcare utilization, and enhance quality of life for hospitalized and post-surgical patients.
References:
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