In patients at risk for re‑feeding syndrome (e.g., severely malnourished, chronically ill, alcoholic, postoperative), should thiamine (vitamin B1) be given prophylactically, and what dose, route, timing, and duration are recommended?

Thiamine for Refeeding Syndrome

In patients at risk for refeeding syndrome, administer 300 mg IV thiamine immediately before initiating any nutrition, then continue 200–300 mg IV daily for at least 3 days. 1

Risk Identification

Patients requiring prophylactic thiamine include those with:

• **BMI <16 kg/m²** or unintentional weight loss >15% in 3–6 months 1, 2
• No nutritional intake for >10 days (or >5 days in some guidelines) 1, 2
• Chronic alcoholism – 30–80% have thiamine deficiency 1
• Low baseline electrolytes (potassium, phosphate, or magnesium) before feeding 1, 2
• Post-bariatric surgery with prolonged vomiting or poor intake 1
• Critical illness (sepsis, major surgery, severe burns) – >90% are thiamine deficient 1
• Chronic diuretic therapy or continuous renal replacement therapy 1

Thiamine Dosing Protocol

Pre-Feeding Phase (Critical)

• 300 mg IV thiamine as a single dose before any nutrition begins 1, 2
• Must be given before glucose-containing fluids – glucose loading in thiamine-depleted patients precipitates Wernicke's encephalopathy and acute heart failure 1, 2, 3

During Refeeding (Days 1–3 minimum)

• 200–300 mg IV daily for at least 3 days 1, 2
• Continue throughout the high-risk refeeding period 1
• Full B-complex vitamins IV should be given simultaneously 1, 2

Route Selection

• IV route is mandatory in refeeding syndrome prevention 1, 2
• Oral thiamine is inadequate for acute situations due to poor absorption in malnourished patients 1
• The FDA label confirms IV thiamine is indicated when rapid restoration is necessary and for patients who cannot take oral thiamine due to severe anorexia, nausea, vomiting, or malabsorption 3

Nutritional Reintroduction Strategy

Start nutrition at 5–10 kcal/kg/day in very high-risk patients (BMI <16, chronic alcoholism, prolonged starvation) and increase gradually over 4–7 days 1, 2. Standard high-risk patients can start at 10–20 kcal/kg/day 1, 2.

Macronutrient Distribution

• 40–60% carbohydrate, 30–40% fat, 15–20% protein 1, 2
• Protein intake should be ≥1.2 g/kg ideal body weight 2

Special Populations

• Severe acute pancreatitis: limit to 15–20 non-protein kcal/kg/day 1, 2
• Anorexia nervosa: start at the lower end (5–10 kcal/kg/day) with very slow progression 2

Concurrent Electrolyte Replacement

Thiamine alone is insufficient – aggressive electrolyte supplementation is mandatory:

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

The biochemical features of refeeding syndrome result from feeding-induced hormonal shifts regardless of baseline stability, so prophylactic replacement is essential even if pre-feeding electrolytes appear normal 2.

Monitoring Protocol

First 72 Hours (Critical Period)

• Daily electrolyte monitoring (phosphate, potassium, magnesium, calcium) 1, 2
• Strict glucose monitoring to avoid hyperglycemia 2
• Clinical surveillance for edema, arrhythmias, confusion, respiratory failure 1, 2
• Volume status and fluid balance monitoring 2

If Hypophosphatemia Develops

• Measure electrolytes 2–3 times daily 2
• Restrict energy to 5–10 kcal/kg/day for 48 hours before gradually increasing 2
• Intensify phosphate replacement while continuing thiamine 2

Critical Pitfalls to Avoid

1. Never start feeding without thiamine – carbohydrate loading precipitates Wernicke's encephalopathy, Korsakoff syndrome, acute heart failure, and sudden death 1, 2

2. Never use low-dose thiamine (10–100 mg) in high-risk patients – this is inadequate for refeeding syndrome prevention 1

3. Never stop thiamine prematurely – continue for the full minimum 3-day period even if symptoms improve 2

4. Never correct electrolytes alone before feeding – this provides false security without correcting massive intracellular deficits 2

5. Never stop feeding abruptly – taper gradually if necessary to avoid rebound hypoglycemia 1, 2

Duration of Thiamine Therapy

• Minimum 3 days of high-dose IV thiamine (200–300 mg daily) 1, 2
• Transition to oral thiamine 50–100 mg daily once stable oral intake is established 1
• Continue for 2–3 months in patients with chronic alcoholism or ongoing risk factors 1
• Lifelong supplementation (50–100 mg daily) may be necessary for post-bariatric surgery patients or those with chronic malabsorption 1

Evidence Quality Note

The benefit-risk ratio for prophylactic thiamine is overwhelmingly favorable – thiamine has no established upper toxicity limit, excess is excreted in urine, and high IV doses rarely cause anaphylaxis 1. Doses >400 mg may cause mild nausea, anorexia, or mild ataxia 1. The guideline recommendations are consistent across multiple societies despite limited high-quality randomized trials, reflecting the life-threatening nature of untreated deficiency and the safety of treatment 1, 2.