01OverviewDefinition, clinical context and the essential points that orientate the chapter.
Malnutrition is deficiency or imbalance of energy, protein and micronutrients that impairs function and recovery. Causes include reduced access or appetite, dysphagia, cancer, chronic organ disease, malabsorption, depression, eating disorders, alcohol dependence and surgery. Oedema can conceal loss and a high BMI can coexist with profound muscle depletion, so recent trajectory and intake matter.
Refeeding syndrome is the dangerous metabolic response when carbohydrate stimulates insulin after prolonged undernutrition. Phosphate, potassium and magnesium shift into cells; thiamine demand rises; sodium and water are retained. The result can be arrhythmia, heart failure, respiratory-muscle weakness, rhabdomyolysis, seizures, delirium and Wernicke encephalopathy. A normal initial phosphate does not remove risk because the fall occurs after feeding begins.
Prevention is a controlled process rather than simply feeding less. Identify risk, prescribe a cautious starting energy amount, give thiamine before carbohydrate, replace electrolytes in parallel and monitor intensely while calories advance. Dietitians, pharmacists and nutrition-support teams should calculate full energy, protein, fluid, electrolyte and micronutrient needs; bedside staff must know the plan and escalation thresholds.
Key points
- Malnutrition can occur at any BMI, including obesity, oedema, pregnancy and after bariatric surgery; weight alone is not a nutritional assessment.
- NICE considers nutrition support when BMI is below 18.5, unintentional loss exceeds 10% in three to six months, or BMI below 20 accompanies more than 5% loss.
- Little or no intake for more than five days also creates nutrition-support need and refeeding considerations even when formal BMI criteria are not met.
- Use a validated screening tool such as MUST, then assess cause, trajectory, muscle, swallowing, gastrointestinal function, cognition, capacity and goals of care.
- High refeeding risk is present with one major criterion such as BMI below 16, loss above 15%, no intake above ten days or low potassium, phosphate or magnesium.
- Risk also exists with two moderate criteria: BMI below 18.5, loss above 10%, no intake above five days, or alcohol or relevant medicine history.
- Give thiamine and balanced vitamin supplementation before and during early feeding, and prescribe potassium, phosphate and magnesium replacement from the outset unless levels are already high.
- NICE starts high-risk feeding at no more than 10 kcal/kg/day, using 5 kcal/kg/day in extreme cases, then advances toward need over four to seven days.
- Pre-feeding correction of low electrolytes is not routinely necessary before any nutrition begins, but replacement and close monitoring must start immediately with expert oversight.
- Fluid and sodium can cause as much harm as calories; record balance, weight, oedema, heart rate, blood pressure and respiratory status.
- Oral, enteral and parenteral routes follow swallowing safety and gastrointestinal function, with informed consent, capacity assessment and a reviewable treatment goal.
- When malnutrition is linked to an eating disorder, alcohol dependence, neglect or inability to obtain food, combine medical stabilisation with mental-health, safeguarding and social support.
02AetiologyUnderlying causes, associations and risk factors, with why each one matters.
Reduced intake or access
Poor appetite, dysphagia, depression, eating disorder, alcohol dependence, poverty or limited support can prevent adequate energy, protein and micronutrient intake.
Increased demand or loss
Cancer, infection, chronic organ disease, surgery and inflammation raise requirements or catabolism while gastrointestinal disease can impair absorption.
High-risk nutritional restart
Rapid carbohydrate delivery after prolonged undernutrition triggers refeeding physiology even when baseline electrolytes appear normal or body mass is high.
03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.
- 1Body stores become depleted
Prolonged inadequate nutrition reduces fat, muscle, intracellular phosphate, potassium, magnesium and thiamine despite sometimes preserved serum measurements.
- 2Carbohydrate raises insulin
Refeeding shifts metabolism from catabolism to anabolism, increasing insulin and cellular uptake of glucose and electrolytes.
- 3Electrolytes move intracellularly
Phosphate, potassium and magnesium fall in plasma while thiamine demand rises, impairing ATP production, membrane stability and carbohydrate metabolism.
- 4Sodium and water are retained
Insulin-related renal effects expand extracellular volume, which can overwhelm a weakened heart or kidneys and worsen oedema.
04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.
Unintentional loss, reduced intake, temporal wasting, weak grip, poor wound healing, recurrent infection, oedema, brittle hair or functional decline suggest undernutrition even when BMI remains high.
Very low BMI, substantial recent loss, negligible intake, baseline low phosphate, potassium or magnesium, alcohol dependence and insulin, chemotherapy, antacid or diuretic exposure are NICE risk markers.
A falling phosphate, potassium or magnesium after nutrition starts with oedema, weakness, arrhythmia, respiratory difficulty or neurological change indicates clinically important refeeding physiology.
Confusion, ataxia, eye-movement abnormality, neuropathy, lactic acidosis or cardiac failure in an undernourished person warrants immediate thiamine before glucose where possible.
Coughing, wet voice, recurrent chest infection, neurological disease or prolonged mealtimes suggests aspiration risk and requires swallow assessment before an oral plan is assumed safe.
Rapid weight gain, oedema, rising respiratory rate, crackles, raised venous pressure or falling oxygenation after feeding suggests sodium-water retention and cardiac stress.
Bradycardia, hypotension, hypothermia, syncope, severe electrolyte change, suicidality or uncontrolled purging requires same-day eating-disorder and acute medical assessment under current NICE and local criteria.
05InvestigationsWhat to request, why it matters and how to interpret it.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
MUST or validated nutrition screeningFirst step - Why
- Identify risk from BMI, weight loss and acute intake reduction consistently.
- Interpretation and limitations
- A score triggers full assessment rather than replacing clinical judgement; oedema, amputation, inability to stand and previous obesity require alternative measurements and trajectory.
- 02
Documented weight and intake history - Why
- Quantify percentage loss, duration of negligible intake and likely requirements.
- Interpretation and limitations
- Calculate percentage from a credible previous weight and distinguish planned from unintentional loss; intake below five or ten days changes refeeding risk even if BMI is normal.
- 03
Phosphate, potassium and magnesium - Why
- Establish refeeding risk and guide proactive replacement.
- Interpretation and limitations
- Low baseline values meet a major NICE risk criterion, but normal values can fall sharply after insulin rises, so daily early monitoring remains essential.
- 04
Glucose, renal, calcium and liver profile - Why
- Assess metabolic reserve, organ function and fluid or prescription constraints.
- Interpretation and limitations
- Hyperglycaemia, renal failure and liver disease alter feed composition and electrolyte replacement; do not correct every abnormality with uncoordinated fluid.
- 05
Full blood count, CRP, iron, B12 and folate - Why
- Identify anaemia, inflammation and coincident micronutrient deficiency.
- Interpretation and limitations
- Inflammation changes ferritin and albumin; combined deficiencies can mask MCV changes, so laboratory data need dietary and disease context.
- 06
ECG and cardiorespiratory observations - Why
- Detect baseline or emerging arrhythmia, QT effects and fluid-related cardiac stress.
- Interpretation and limitations
- Extreme refeeding risk, electrolyte abnormality or cardiac disease may need continuous rhythm monitoring; tachycardia, ectopy or QT change demands urgent correction review.
- 07
Swallow and gastrointestinal assessment - Why
- Choose a route that is safe and physiologically usable.
- Interpretation and limitations
- Unsafe swallow with functioning gut may require enteral tube feeding, while obstruction, ileus or severe malabsorption may require specialist parenteral nutrition.
- 08
Capacity, consent and goals assessment - Why
- Ensure nutrition support follows lawful, person-centred decision-making.
- Interpretation and limitations
- A capacitous refusal must be respected after informed discussion; if capacity is lacking, make and document a best-interests decision with relevant representatives and review it as circumstances change.
06Differential diagnosisRealistic alternatives and the features that help distinguish them.
Cancer cachexia
Inflammatory catabolism can cause disproportionate muscle loss despite nutritional support, often alongside known malignancy and systemic inflammation.
Malabsorption
Chronic diarrhoea, steatorrhoea, micronutrient deficiencies or gastrointestinal surgery suggests failure to absorb nutrients rather than intake alone.
Fluid-related weight loss or gain
Diuresis, dehydration or oedema can conceal true tissue trajectory, so weight must be interpreted with examination and intake history.
Sepsis-related electrolyte change
Critical illness can lower phosphate, potassium and magnesium independently, but a recent nutritional restart and high-risk history support refeeding syndrome.
07ManagementImmediate care, first-line treatment, alternatives and escalation.
01ScreenFind malnutrition earlyFirst stepHospital admission, first relevant outpatient contact or clinical concern about intake or weight loss.+
- 1Use MUST or the local validated tool, obtaining recent weight trajectory and intake duration and adapting measurements for oedema, disability or inability to stand.
- 2Assess disease cause, swallowing, GI function, muscle and function, alcohol, medicines, mental health, food access, capacity and the person's treatment goals.
- 3Refer at-risk patients to dietetics or the nutrition-support team and begin a documented oral, enteral or parenteral plan matched to urgency and refeeding risk.
02High riskPrevent refeeding syndromeOne major or two moderate NICE high-risk criteria are present.+
- 1Obtain baseline electrolytes, glucose, renal and liver tests, fluid status and ECG as indicated, but do not postpone urgent thiamine and a controlled feeding plan.
- 2Give thiamine, vitamin B and multivitamin before carbohydrate, prescribe potassium, phosphate and magnesium support, and start at no more than 10 kcal/kg/day.
- 3Use 5 kcal/kg/day with continuous cardiac monitoring in extreme risk, then advance toward full needs over four to seven days as fluid, electrolytes and organ function permit.
03DeteriorationTreat established refeeding complicationsElectrolytes fall or cardiac, respiratory or neurological symptoms develop after nutrition begins.+
- 1EscalationEscalate to acute medicine and the nutrition team, obtain ECG and urgent electrolytes, assess airway, breathing, circulation, fluid overload, glucose and thiamine deficiency.
- 2Pause or reduce nutrition according to severity while giving immediate thiamine and controlled phosphate, potassium, magnesium and fluid treatment through the local protocol.
- 3Restart and advance only after stabilisation with a revised prescription, frequent laboratory monitoring and a clear response plan for recurrent abnormalities.
04Choose routeUse the gut when safe and functionalNutrition needs cannot be met through ordinary food alone.+
- 1Optimise food fortification and oral supplements first when swallow and intake are safe, involving speech and language therapy if aspiration is possible.
- 2Use enteral tube feeding when the gastrointestinal tract works but oral intake is unsafe or inadequate, confirming tube position and aspiration precautions.
- 3Reserve parenteral nutrition for an inaccessible or non-functional gut or inadequate enteral support, with specialist line, metabolic and infection monitoring.
05Complex refusalIntegrate capacity, mental health and safeguardingA patient declines nutrition or neglect, coercion, eating disorder or impaired capacity may be present.+
- 1Assess decision-specific capacity without assuming incapacity from diagnosis or body size, and explore communication, trauma, culture, symptoms and reversible delirium.
- 2Involve eating-disorder, liaison psychiatry, safeguarding, social care and advocates as relevant, while treating immediate medical instability within lawful emergency powers.
- 3Document the person's wishes, risks, best-interests reasoning and review date, ensuring nutrition is neither imposed nor withheld solely for staff convenience.
Key medicines and prescribing safety4 treatments · regimens, roles and cautions+
Thiamine and vitamin B for high refeeding risk
Give oral thiamine 200 to 300 mg daily plus vitamin B co-strong 1 or 2 tablets three times daily immediately before and during the first 10 days of feeding; use intravenous thiamine when oral absorption is unreliable or neurological deficiency is suspected.Administer before glucose or feed where possible and never delay treatment for a thiamine level. Use the local parenteral regimen with resuscitation capability because rare hypersensitivity can occur.
Balanced multivitamin and trace-element supplement
Give a complete once-daily oral or intravenous multivitamin and trace-element preparation from before feeding and for at least the first 10 days, then continue according to nutritional cause and route.A generic product does not replace targeted thiamine or electrolyte therapy. Adjust for renal or liver disease, pregnancy, parenteral compatibility and known deficiency; avoid unplanned duplicate fat-soluble vitamin dosing.
Potassium, phosphate and magnesium supplementation
NICE likely requirements during high-risk feeding are potassium 2 to 4 mmol/kg/day, phosphate 0.3 to 0.6 mmol/kg/day and magnesium 0.2 mmol/kg/day intravenously or 0.4 mmol/kg/day orally, adjusted to serial levels and organ function.Use pharmacy and local electrolyte protocols; renal failure, severe baseline elevation, arrhythmia and intravenous concentration or rate require specialist adjustment and ECG monitoring. Never give large uncoordinated doses from an estimated requirement alone.
Oral nutritional supplement
Select a dietitian-prescribed energy- and protein-dense formulation and daily volume that fits swallowing, fluid, renal, glycaemic and refeeding requirements, divided across the day rather than replacing all ordinary food automatically.Count its calories and carbohydrate within the refeeding start, check allergens and aspiration risk, and monitor tolerance, diarrhoea, glucose and adherence; do not use as an unreviewed lifelong prescription.
08ComplicationsImportant consequences, why they occur and why they matter clinically.
Cardiac failure and arrhythmia
Fluid retention, low phosphate, potassium and magnesium can impair contractility and destabilise conduction in an already depleted heart.
Respiratory muscle failure
Phosphate depletion reduces ATP available to the diaphragm, causing weakness and difficulty maintaining or weaning ventilation.
Wernicke encephalopathy
Rapid carbohydrate metabolism consumes limited thiamine, risking confusion, ataxia and eye-movement abnormalities followed by persistent amnesia if treatment is delayed.
Rhabdomyolysis and seizure
Severe intracellular electrolyte depletion disrupts muscle and neuronal function, causing tissue breakdown, delirium, seizure or coma.
09Monitoring and follow-upTreatment response, safety checks and longer-term review.
- In high-risk refeeding, check phosphate, potassium, magnesium, renal function and glucose at least daily initially, increasing frequency when values are unstable or replacement is intravenous.
- Record pulse, blood pressure, respiratory rate, oxygenation, temperature and neurological state, with continuous ECG in extreme risk or significant electrolyte abnormality.
- Measure fluid input, urine output, daily weight and oedema because rapid sodium-water retention can precede overt heart or respiratory failure.
- Document actual feed delivered rather than prescribed volume only, including oral food, supplements, intravenous glucose and propofol or other non-obvious calories.
- Advance energy and protein toward calculated needs over four to seven days only when electrolytes, fluid status and clinical physiology remain safe.
- Re-screen hospital patients weekly and outpatients when clinical concern changes, preserving continuity at transfer between ward, care home and community.
- Review route, goals, capacity and benefit regularly; stop tube or parenteral support when adequate safe oral intake returns or goals no longer justify burden.
10Special situationsVariants, exceptions and circumstances that change the usual approach.
Normal phosphate is temporary reassurance
Total-body depletion can be profound while serum concentration remains normal before insulin drives phosphate intracellularly, making post-feed monitoring indispensable.
Every calorie source counts
Dextrose infusion, oral supplements and lipid-containing sedatives contribute substrate; refeeding risk follows total delivery, not the dietetic feed bag alone.
Oedema hides loss
Fluid accumulation can maintain or raise measured weight while muscle and micronutrient stores fall, so clinical examination and trajectory outperform one scale value.
Correction occurs with feeding
NICE does not require every low electrolyte to normalise before nutrition starts; controlled feeding and replacement proceed together under close monitoring.
Capacity is decision-specific
A person may understand one nutrition decision but not another, and capacity can improve after delirium or metabolic stabilisation; document and reassess rather than applying a permanent label.
11Common pitfallsFrequent interpretation and management errors.
- 01
Excluding malnutrition because BMI is high or oedema preserves weight.
- 02
Beginning unrestricted carbohydrate before thiamine and a refeeding prescription.
- 03
Relying on a normal pre-feed phosphate to declare no risk.
- 04
Correcting electrolytes in isolation while overlooking fluid and sodium overload.
- 05
Counting only enteral feed and forgetting intravenous glucose or oral supplements.
- 06
Using oral feeding despite an unsafe swallow without speech and language assessment.
- 07
Assuming a patient with an eating disorder lacks capacity automatically.
- 08
Starting parenteral nutrition merely because oral intake is difficult while a usable gut remains.
- 09
Advancing calories on schedule despite falling electrolytes or respiratory deterioration.