In a 19-year-old woman with anorexia nervosa, BMI of 16.3, history of restrictive eating and purging behaviors, presenting with hypokalemia and hypophosphatemia, which electrolyte abnormality should be prioritized in management to prevent refeeding syndrome?

Prioritize Hypophosphatemia Over Hypokalemia in Refeeding Syndrome Prevention

In this 19-year-old woman with severe anorexia nervosa (BMI 16.3), hypophosphatemia must be prioritized as the primary electrolyte abnormality to prevent refeeding syndrome, though both require aggressive simultaneous correction. 1

Why Hypophosphatemia Takes Priority

Hypophosphatemia is the most frequent and clinically significant electrolyte disturbance in refeeding syndrome and directly causes the life-threatening complications. 1 The biochemical cascade of refeeding syndrome begins when carbohydrate reintroduction triggers insulin release, driving phosphate (along with potassium and magnesium) intracellularly, causing severe plasma depletion regardless of baseline levels. 1

Life-Threatening Complications of Hypophosphatemia

Severe hypophosphatemia (<0.32 mmol/L) causes:

• Respiratory failure requiring mechanical ventilation 1
• Cardiac arrhythmias and sudden cardiac death (up to 20% mortality in severe cases) 1
• Cardiac dysfunction and congestive heart failure 1, 2
• Encephalopathy, delirium, and seizures 1
• Muscle weakness and rhabdomyolysis 1

Critical Timing Consideration

Hypophosphatemia develops within the first 72 hours of refeeding, with symptoms typically appearing within 4 days of nutritional reintroduction. 1 This patient's BMI of 16.3 places her at very high risk, as BMI <16 kg/m² is a categorical high-risk criterion. 1

Why Hypokalemia is Secondary (But Still Critical)

While hypokalemia contributes to cardiac arrhythmias and neuromuscular complications 1, patients with chronic purging anorexia nervosa demonstrate remarkable adaptation to chronic hypokalemia, tolerating levels as low as 1.6 mmol/L without physical symptoms. 3 This adaptation does not occur with acute phosphate depletion during refeeding. 1

Hypokalemia in anorexia nervosa is primarily a marker of purging behavior and chronic depletion, whereas hypophosphatemia represents the acute metabolic crisis of refeeding itself. 4

The Correct Management Algorithm

Pre-Feeding Protocol (Mandatory Before Any Nutrition)

1. Administer thiamine 200-300 mg IV daily before initiating any feeding 1, 5

   • Thiamine deficiency precipitates Wernicke's encephalopathy, Korsakoff's syndrome, and acute heart failure when carbohydrates are introduced 1, 5
   • Continue for minimum 3 days 1

2. Provide full B-complex vitamins IV simultaneously 1

3. Check baseline electrolytes (phosphate, potassium, magnesium, calcium) 6, 1

Aggressive Electrolyte Replacement (Start Immediately with Feeding)

Both electrolytes require simultaneous aggressive replacement, but phosphate dosing must be prioritized:

• Phosphate: 0.3-0.6 mmol/kg/day IV 1
• Potassium: 2-4 mmol/kg/day 1
• Magnesium: 0.2 mmol/kg/day IV or 0.4 mmol/kg/day orally 1
• Calcium as needed 1

Nutritional Reintroduction Strategy

For this patient with BMI 16.3 (very high risk):

• Start at 5-10 kcal/kg/day 1, 2
• Increase gradually over 4-7 days until reaching 25-30 kcal/kg/day 1
• Macronutrient distribution: 40-60% carbohydrate, 30-40% fat, 15-20% protein 1

Intensive Monitoring Protocol

• Monitor electrolytes (phosphate, potassium, magnesium) 2-3 times daily when hypophosphatemia is present 1
• Daily electrolyte monitoring for first 72 hours minimum 1
• Continue monitoring beyond 3 days if abnormalities persist 1
• Monitor glucose strictly to avoid hyperglycemia 1
• Watch for clinical signs: edema, arrhythmias, confusion, respiratory failure 1

Critical Pitfall to Avoid

Never correct electrolytes alone before feeding without simultaneous nutritional support. 1 Severely malnourished patients have massive intracellular deficits that cannot be corrected without feeding to drive transmembrane transfer—correcting electrolytes alone provides false security. 1 The feeding-induced hormonal and metabolic derangements trigger the syndrome regardless of baseline stability. 1

Special Consideration for This Patient

This patient's purging behaviors increase her risk for both electrolyte abnormalities. 4 However, the acute mortality risk during refeeding comes from hypophosphatemia-induced cardiac and respiratory failure, not from the chronic hypokalemia she has likely adapted to. 3 Up to one-third of deaths in anorexia nervosa are cardiac, with many occurring during the first week of refeeding. 2

The answer is hypophosphatemia—it is the primary driver of refeeding syndrome mortality and must be the focus of prevention, though both require aggressive simultaneous correction from day one. 1