Questions: Clinical Nutrition Support: Enteral and Parenteral Feeding

Question 1

A critically ill patient on mechanical ventilation has a functioning gastrointestinal tract but cannot eat. Which nutrition route is most appropriate and why?

• A. Parenteral nutrition, because it bypasses aspiration risk and delivers nutrients with greater precision
• B. Enteral nutrition, because it maintains gut barrier integrity, gut-associated immune function, and prevents bacterial translocation
• C. Parenteral nutrition, because it is faster to initiate and requires no tube placement
• D. Either route is equivalent as long as caloric and protein targets are met

Answer: (B)

When the GI tract is functional, enteral nutrition is always preferred. The gut is not merely a passive conduit — it is an active immune and endocrine organ. Luminal nutrition maintains the intestinal epithelium, stimulates gut-associated lymphoid tissue, and prevents the bacterial translocation that can occur when the mucosa atrophies during prolonged fasting. Option A reflects a common clinical misconception; parenteral nutrition is reserved for when the gut is non-functional or inaccessible.

Question 2

A 24-year-old woman with severe anorexia nervosa is admitted after weeks of near-total starvation. On day 3 of enteral refeeding at a modest caloric rate, she develops cardiac arrhythmias and acute respiratory weakness. The most likely explanation is:

• A. Aspiration pneumonia from the nasogastric tube, causing hypoxia and cardiac irritability
• B. Hyperglycemia from the carbohydrate load causing osmotic fluid shifts into cells
• C. Hypophosphatemia from intracellular phosphate shift driven by insulin release upon carbohydrate reintroduction
• D. Central line-associated bloodstream infection causing septic arrhythmia

Answer: (C)

This is classic refeeding syndrome. During starvation, intracellular phosphate stores are depleted while serum levels are maintained near-normal through compensatory shifts. When carbohydrate is reintroduced, insulin surges and drives glucose into cells for metabolism — taking phosphate (needed for glycolysis and ATP synthesis) with it. Serum phosphate plummets, causing hypophosphatemia severe enough to impair cardiac muscle, respiratory muscle, and erythrocyte function. Option D is wrong because she is on enteral, not parenteral, nutrition.

Question 3

Pre-refeeding serum electrolyte levels in a severely malnourished patient are often near-normal, suggesting that refeeding syndrome risk is low.

• T. True
• F. False

Answer: False

This is the critical misconception that makes refeeding syndrome so dangerous. During starvation, cells release phosphate, potassium, and magnesium into the bloodstream, and the kidneys conserve these minerals — keeping serum levels apparently normal despite massively depleted intracellular stores. The 'normal' serum value is a compensated equilibrium, not a reflection of true body stores. When refeeding begins and insulin drives these minerals back into cells, serum levels can fall precipitously.

Question 4

Parenteral nutrition bypassing the gut means that every nutrient that would normally be absorbed from food must be explicitly included in the formulation.

• T. True
• F. False

Answer: True

Unlike enteral nutrition, which can rely on normal digestive and absorptive processes to extract nutrients from formula, parenteral nutrition requires the clinician to prescribe each component individually: glucose, lipid emulsion, crystalline amino acids, electrolytes (sodium, potassium, calcium, magnesium, phosphate), vitamins, and trace elements. This provides precise control but also means that any omission or error in compounding is directly delivered to the bloodstream with no absorptive buffering.

Question 5

Why does refeeding syndrome cause dangerous drops in serum phosphate, and why do near-normal serum phosphate levels before refeeding begins give a false sense of security?

Model answer: During starvation, cells release phosphate into the bloodstream and the kidneys retain it, maintaining serum levels near-normal despite depleted intracellular stores. This compensated state collapses when carbohydrates are reintroduced: insulin secretion surges, driving glucose into cells for glycolysis and ATP synthesis — a process that consumes large amounts of phosphate. Phosphate rushes into cells faster than the depleted stores can buffer the shift, and serum levels plummet. The pre-refeeding 'normal' serum value is an artifact of starvation physiology, not a reflection of total body phosphate.

The practical implication is that risk stratification for refeeding syndrome must rely on clinical history (duration of starvation, degree of weight loss, underlying conditions) rather than serum electrolyte levels. High-risk patients require electrolyte repletion before refeeding begins, slow initiation of nutrition, and close monitoring during the first week regardless of their pre-refeeding lab values.