Refeeding syndrome occurs when carbohydrates are reintroduced too quickly to a chronically starved patient. During starvation the body shifts to fat/ketone metabolism and depletes intracellular phosphate, potassium and magnesium while total-body stores fall — serum levels can look deceptively normal because the losses are mostly intracellular.
Carbohydrate intake triggers insulin secretion. Insulin activates the Na⁺/K⁺-ATPase pump and drives glucose, phosphate, potassium and magnesium out of the serum and into cells to support glycolysis and glycogen/protein synthesis. In a starved patient with empty intracellular stores, this uptake is exaggerated and can drop serum phosphate precipitously — the defining feature of refeeding syndrome.
Insulin drive: I(t) ∝ carb_fraction × refeeding_rate
Cellular uptake: dP_serum/dt = −k · I(t) · severity · P_serum
+ repletion_rate − renal_clearance·(P_serum − P0)
Danger thresholds: P_serum < 2.5 mg/dL → mild/moderate
P_serum < 1.0 mg/dL → severe (cardiac/respiratory failure risk)
- Refeeding rate — total calories per kg per day; higher rate → larger, faster insulin surge.
- Carbohydrate fraction — the share of calories from carbs; carbs (not protein/fat) are the dominant insulin trigger.
- Starvation severity — how depleted intracellular stores are before feeding starts; this scales how avidly cells pull ions in (k above).
- IV phosphate supplementation — a repletion term that offsets the intracellular shift, mirroring real ASPEN/NICE refeeding protocols (start low, replete electrolytes proactively, advance calories slowly).
Clinically this is why guidelines recommend starting malnourished patients at ~10–20 kcal/kg/day, correcting electrolytes before and during feeding, and advancing slowly over 4–7 days rather than feeding to full caloric target immediately.